# Publications

 Authors: ALLAndersonBennieBillingtonFischerGroszekHelmersonJasperseJohnstoneKuopanporttiLevinsenMawsonParishRubenSimulaStarkeyTemponeTurnerWood Type: ALLJournal/magazine articleBook chapterPh.D. thesis

### 2017

• High efficiency, low cost holographic optical elements for ultracold atom trapping,
S. Tempone-Wiltshire, S. Johnstone, and K. Helmerson.
Optics Express 25, 296-304 (2017).
[DOI] [Bibtex] [Abstract]

We demonstrate a method of creating high efficiency, high fidelity, holographic optical elements for the generation of complex optical fields, in a low cost photopolymer, Bayfol HX. The desired optical field profile is generated by a spatial light modulator and written into an optically addressable photopolymer as a volume hologram. We demonstrate the utility of this approach by trapping a Bose-Einstein condensate of rubidium-87 atoms in the nodal plane of an HG0,1 mode generated by one of these holographic optical elements. We also extend this method to the generation holograms with twice the angular momentum per photon than can be generated with a given spatial light modulator.

@article{tempone-wiltshire_high_2017,
title = {High efficiency, low cost holographic optical elements for ultracold atom trapping},
volume = {25},
issn = {1094-4087},
url = {http://www.osapublishing.org/abstract.cfm?uri=oe-25-1-296},
doi = {10.1364/OE.25.000296},
abstract = {We demonstrate a method of creating high efficiency, high fidelity, holographic optical elements for the generation of complex optical fields, in a low cost photopolymer, Bayfol HX. The desired optical field profile is generated by a spatial light modulator and written into an optically addressable photopolymer as a volume hologram. We demonstrate the utility of this approach by trapping a Bose-Einstein condensate of rubidium-87 atoms in the nodal plane of an HG0,1 mode generated by one of these holographic optical elements. We also extend this method to the generation holograms with twice the angular momentum per photon than can be generated with a given spatial light modulator.},
language = {EN},
number = {1},
urldate = {2017-01-04TZ},
journal = {Optics Express},
author = {Tempone-Wiltshire, Sebastien and Johnstone, Shaun and Helmerson, Kristian},
month = jan,
year = {2017},
pages = {296--304}
}

### 2016

• Evolution of an isolated monopole in a spin-1 Bose–Einstein condensate,
K. Tiurev, P. Kuopanportti, A. M. Gunyhó, M. Ueda, and M. Möttönen.
Physical Review A 94, 53616 (2016).
[arXiv] [DOI] [Bibtex] [Abstract]

We simulate the decay dynamics of an isolated monopole defect in the nematic vector of a spin-1 Bose-Einstein condensate during the polar-to-ferromagnetic phase transition of the system. Importantly, the decay of the monopole occurs in the absence of external magnetic fields and is driven principally by the dynamical instability due to the ferromagnetic spin-exchange interactions. An initial isolated monopole is observed to relax into a polar-core spin vortex, thus demonstrating the spontaneous transformation of a point defect of the polar order parameter manifold to a line defect of the ferromagnetic manifold. We also investigate the dynamics of an isolated monopole pierced by a quantum vortex line with winding number κ. It is shown to decay into a coreless Anderson-Toulouse vortex if κ=1 and into a singular vortex with an empty core if κ=2. In both cases, the resulting vortex is also encircled by a polar-core vortex ring.

@article{tiurev_evolution_2016,
title = {Evolution of an isolated monopole in a spin-1 {Bose--Einstein} condensate},
volume = {94},
doi = {10.1103/PhysRevA.94.053616},
abstract = {We simulate the decay dynamics of an isolated monopole defect in the nematic vector of a spin-1 Bose-Einstein condensate during the polar-to-ferromagnetic phase transition of the system. Importantly, the decay of the monopole occurs in the absence of external magnetic fields and is driven principally by the dynamical instability due to the ferromagnetic spin-exchange interactions. An initial isolated monopole is observed to relax into a polar-core spin vortex, thus demonstrating the spontaneous transformation of a point defect of the polar order parameter manifold to a line defect of the ferromagnetic manifold. We also investigate the dynamics of an isolated monopole pierced by a quantum vortex line with winding number κ. It is shown to decay into a coreless Anderson-Toulouse vortex if κ=1 and into a singular vortex with an empty core if κ=2. In both cases, the resulting vortex is also encircled by a polar-core vortex ring.},
number = {5},
urldate = {2016-11-18TZ},
journal = {Physical Review A},
author = {Tiurev, Konstantin and Kuopanportti, Pekko and Gunyhó, András Márton and Ueda, Masahito and Möttönen, Mikko},
month = nov,
year = {2016},
eprint = {1609.02506},
pages = {053616}
}
• Measurement and extinction of vector light shifts using interferometry of spinor condensates,
A. A. Wood, L. D. Turner, and R. P. Anderson.
Physical Review A 94, 52503 (2016).
[arXiv] [DOI] [Bibtex]
@article{wood_measurement_2016,
title = {Measurement and extinction of vector light shifts using interferometry of spinor condensates},
volume = {94},
doi = {10.1103/PhysRevA.94.052503},
number = {5},
journal = {Physical Review A},
author = {Wood, A. A. and Turner, L. D. and Anderson, R. P.},
month = nov,
year = {2016},
eprint = {1607.06898},
pages = {052503}
}
• Ultrafast many-body interferometry of impurities coupled to a Fermi sea,
M. Cetina, M. Jag, R. S. Lous, I. Fritsche, J. T. M. Walraven, R. Grimm, J. Levinsen, M. M. Parish, R. Schmidt, M. Knap, and E. Demler.
Science 354, 96-99 (2016).
[DOI] [Bibtex] [Abstract]

Sluggish turmoil in the Fermi sea The nonequilibrium dynamics of many-body quantum systems are tricky to study experimentally or theoretically. As an experimental setting, dilute atomic gases offer an advantage over electrons in metals. In this environment, the heavier atoms make collective processes that involve the entire Fermi sea occur at the sluggish time scale of microseconds. Cetina et al. studied these dynamics by using a small cloud of 40K atoms that was positioned at the center of a far larger 6Li cloud. Controlling the interactions between K and Li atoms enabled a detailed look into the formation of quasiparticles associated with K “impurity” atoms. Science, this issue p. 96 The fastest possible collective response of a quantum many-body system is related to its excitations at the highest possible energy. In condensed matter systems, the time scale for such “ultrafast” processes is typically set by the Fermi energy. Taking advantage of fast and precise control of interactions between ultracold atoms, we observed nonequilibrium dynamics of impurities coupled to an atomic Fermi sea. Our interferometric measurements track the nonperturbative quantum evolution of a fermionic many-body system, revealing in real time the formation dynamics of quasi-particles and the quantum interference between attractive and repulsive states throughout the full depth of the Fermi sea. Ultrafast time-domain methods applied to strongly interacting quantum gases enable the study of the dynamics of quantum matter under extreme nonequilibrium conditions. Precise manipulation of interactions between impurity and majority atoms gives insight into polaron formation. Precise manipulation of interactions between impurity and majority atoms gives insight into polaron formation.

@article{cetina_ultrafast_2016,
title = {Ultrafast many-body interferometry of impurities coupled to a {Fermi} sea},
volume = {354},
issn = {0036-8075, 1095-9203},
url = {http://science.sciencemag.org/content/354/6308/96},
doi = {10.1126/science.aaf5134},
abstract = {Sluggish turmoil in the Fermi sea
The nonequilibrium dynamics of many-body quantum systems are tricky to study experimentally or theoretically. As an experimental setting, dilute atomic gases offer an advantage over electrons in metals. In this environment, the heavier atoms make collective processes that involve the entire Fermi sea occur at the sluggish time scale of microseconds. Cetina et al. studied these dynamics by using a small cloud of 40K atoms that was positioned at the center of a far larger 6Li cloud. Controlling the interactions between K and Li atoms enabled a detailed look into the formation of quasiparticles associated with K “impurity” atoms.
Science, this issue p. 96
The fastest possible collective response of a quantum many-body system is related to its excitations at the highest possible energy. In condensed matter systems, the time scale for such “ultrafast” processes is typically set by the Fermi energy. Taking advantage of fast and precise control of interactions between ultracold atoms, we observed nonequilibrium dynamics of impurities coupled to an atomic Fermi sea. Our interferometric measurements track the nonperturbative quantum evolution of a fermionic many-body system, revealing in real time the formation dynamics of quasi-particles and the quantum interference between attractive and repulsive states throughout the full depth of the Fermi sea. Ultrafast time-domain methods applied to strongly interacting quantum gases enable the study of the dynamics of quantum matter under extreme nonequilibrium conditions.
Precise manipulation of interactions between impurity and majority atoms gives insight into polaron formation.
Precise manipulation of interactions between impurity and majority atoms gives insight into polaron formation.},
number = {6308},
urldate = {2016-11-14TZ},
journal = {Science},
author = {Cetina, Marko and Jag, Michael and Lous, Rianne S. and Fritsche, Isabella and Walraven, Jook T. M. and Grimm, Rudolf and Levinsen, Jesper and Parish, Meera M. and Schmidt, Richard and Knap, Michael and Demler, Eugene},
month = oct,
year = {2016},
pages = {96--99}
}
• Skyrmionic vortex lattices in coherently coupled three-component Bose–Einstein condensates,
N. V. Orlova, P. Kuopanportti, and M. Milošević.
Physical Review A 94, 23617 (2016).
[arXiv] [DOI] [Bibtex] [Abstract]

We show numerically that a harmonically trapped and coherently Rabi-coupled three-component Bose-Einstein condensate can host unconventional vortex lattices in its rotating ground state. The discovered lattices incorporate square and zig-zag patterns, vortex dimers and chains, and doubly quantized vortices, and they can be quantitatively classified in terms of a skyrmionic topological index, which takes into account the multicomponent nature of the system. The exotic ground-state lattices arise due to the intricate interplay of the repulsive density-density interactions and the Rabi couplings as well as the ubiquitous phase frustration between the components. In the frustrated state, domain walls in the relative phases can persist between some components even at strong Rabi coupling, while vanishing between others. Consequently, in this limit the three-component condensate effectively approaches a two-component condensate with only density-density interactions. At intermediate Rabi coupling strengths, however, we face unique vortex physics that occurs neither in the two-component counterpart nor in the purely density-density-coupled three-component system.

@article{orlova_skyrmionic_2016,
title = {Skyrmionic vortex lattices in coherently coupled three-component {Bose--Einstein} condensates},
volume = {94},
doi = {10.1103/PhysRevA.94.023617},
abstract = {We show numerically that a harmonically trapped and coherently Rabi-coupled three-component Bose-Einstein condensate can host unconventional vortex lattices in its rotating ground state. The discovered lattices incorporate square and zig-zag patterns, vortex dimers and chains, and doubly quantized vortices, and they can be quantitatively classified in terms of a skyrmionic topological index, which takes into account the multicomponent nature of the system. The exotic ground-state lattices arise due to the intricate interplay of the repulsive density-density interactions and the Rabi couplings as well as the ubiquitous phase frustration between the components. In the frustrated state, domain walls in the relative phases can persist between some components even at strong Rabi coupling, while vanishing between others. Consequently, in this limit the three-component condensate effectively approaches a two-component condensate with only density-density interactions. At intermediate Rabi coupling strengths, however, we face unique vortex physics that occurs neither in the two-component counterpart nor in the purely density-density-coupled three-component system.},
language = {EN},
number = {2},
journal = {Physical Review A},
author = {Orlova, Natalia V. and Kuopanportti, Pekko and Milošević, Milorad},
month = aug,
year = {2016},
eprint = {1603.05813},
keywords = {Condensed Matter - Quantum Gases, Superfluid, Vortex},
pages = {023617}
}
• Observation of Attractive and Repulsive Polarons in a Bose–Einstein Condensate,
N. B. Jørgensen, L. Wacker, K. T. Skalmstang, M. M. Parish, J. Levinsen, R. S. Christensen, G. M. Bruun, and J. J. Arlt.
Physical Review Letters 117, 55302 (2016).
[DOI] [Bibtex] [Abstract]

The problem of an impurity particle moving through a bosonic medium plays a fundamental role in physics. However, the canonical scenario of a mobile impurity immersed in a Bose-Einstein condensate (BEC) has not yet been realized. Here, we use radio frequency spectroscopy of ultracold bosonic K39 atoms to experimentally demonstrate the existence of a well-defined quasiparticle state of an impurity interacting with a BEC. We measure the energy of the impurity both for attractive and repulsive interactions, and find excellent agreement with theories that incorporate three-body correlations, both in the weak-coupling limits and across unitarity. The spectral response consists of a well-defined quasiparticle peak at weak coupling, while for increasing interaction strength, the spectrum is strongly broadened and becomes dominated by the many-body continuum of excited states. Crucially, no significant effects of three-body decay are observed. Our results open up exciting prospects for studying mobile impurities in a bosonic environment and strongly interacting Bose systems in general.

@article{jorgensen_observation_2016,
title = {Observation of Attractive and Repulsive Polarons in a {Bose--Einstein} Condensate},
volume = {117},
doi = {10.1103/PhysRevLett.117.055302},
abstract = {The problem of an impurity particle moving through a bosonic medium plays a fundamental role in physics. However, the canonical scenario of a mobile impurity immersed in a Bose-Einstein condensate (BEC) has not yet been realized. Here, we use radio frequency spectroscopy of ultracold bosonic K39 atoms to experimentally demonstrate the existence of a well-defined quasiparticle state of an impurity interacting with a BEC. We measure the energy of the impurity both for attractive and repulsive interactions, and find excellent agreement with theories that incorporate three-body correlations, both in the weak-coupling limits and across unitarity. The spectral response consists of a well-defined quasiparticle peak at weak coupling, while for increasing interaction strength, the spectrum is strongly broadened and becomes dominated by the many-body continuum of excited states. Crucially, no significant effects of three-body decay are observed. Our results open up exciting prospects for studying mobile impurities in a bosonic environment and strongly interacting Bose systems in general.},
number = {5},
urldate = {2016-08-05TZ},
journal = {Physical Review Letters},
author = {Jørgensen, Nils B. and Wacker, Lars and Skalmstang, Kristoffer T. and Parish, Meera M. and Levinsen, Jesper and Christensen, Rasmus S. and Bruun, Georg M. and Arlt, Jan J.},
month = jul,
year = {2016},
pages = {055302}
}
• Measurement and extinction of vector light shifts using interferometry of spinor condensates,
A. A. Wood, L. D. Turner, and R. P. Anderson.
[arXiv] [Bibtex] [Abstract]

We use differential Ramsey interferometry of ultracold atoms to characterize the vector light shift (VLS) from a far-off resonance optical dipole trap at λ=1064nm. The VLS manifests as a fictitious’ magnetic field, which we perceive as a change in the Larmor frequency of two spinor condensates exposed to different intensities of elliptically polarized light. We use our measurement scheme to diagnose the light-induced magnetic field and suppress it to 2.1(8)×10{\textasciicircum}−4 of its maximum value, by making the trapping light linearly polarized with a quarter-wave plate in each beam. Our sensitive measurement of the VLS-induced field demonstrates high-precision, in-vacuo interferometric polarimetry of dipole trapping light and can be adapted to measure vector shifts from other lasers, advancing the application of optically trapped atoms to precision metrology.

@article{wood_measurement_2016,
title = {Measurement and extinction of vector light shifts using interferometry of spinor condensates},
url = {http://arxiv.org/abs/1607.06898},
abstract = {We use differential Ramsey interferometry of ultracold atoms to characterize the vector light shift (VLS) from a far-off resonance optical dipole trap at λ=1064nm. The VLS manifests as a fictitious' magnetic field, which we perceive as a change in the Larmor frequency of two spinor condensates exposed to different intensities of elliptically polarized light. We use our measurement scheme to diagnose the light-induced magnetic field and suppress it to 2.1(8)×10{\textasciicircum}−4 of its maximum value, by making the trapping light linearly polarized with a quarter-wave plate in each beam. Our sensitive measurement of the VLS-induced field demonstrates high-precision, in-vacuo interferometric polarimetry of dipole trapping light and can be adapted to measure vector shifts from other lasers, advancing the application of optically trapped atoms to precision metrology.},
urldate = {2016-07-23TZ},
journal = {arXiv:1607.06898},
author = {Wood, A. A. and Turner, L. D. and Anderson, R. P.},
month = jul,
year = {2016},
eprint = {1607.06898},
keywords = {Astrophysics - Instrumentation and Methods for Astrophysics, General Relativity and Quantum Cosmology, Physics - Instrumentation and Detectors}
}
• Precise engineering of the Bose–Einstein condensate wavefunction using magnetic resonance control,
L. M. Starkey, PhD Thesis.
[DOI] [Bibtex] [Abstract]

This thesis develops a new way to sculpt the coldest matter in the universe, a Bose-Einstein condensate, using methods inspired by the medical diagnostic technique of magnetic resonance imaging. This superfluid matter, first predicted by Bose and Einstein in 1925, manifests quantum behaviour at the macroscopic scale. We demonstrate how to shape the finest structures into this matter with unprecedented control, with the future goal of observing structures which have never been created before in quantum fluids, and causing this matter to mimic other physical phenomena.

@phdthesis{starkey_precise_2016,
type = {{PhD} thesis},
title = {Precise engineering of the {Bose--Einstein} condensate wavefunction using magnetic resonance control},
url = {http://dx.doi.org/1959.1/1278937},
abstract = {This thesis develops a new way to sculpt the coldest matter in the universe, a Bose-Einstein condensate, using methods inspired by the medical diagnostic technique of magnetic resonance imaging.
This superfluid matter, first predicted by Bose and Einstein in 1925, manifests quantum behaviour at the macroscopic scale.
We demonstrate how to shape the finest structures into this matter with unprecedented control, with the future goal of observing structures which have never been created before in quantum fluids, and causing this matter to mimic other physical phenomena.},
school = {Monash University},
author = {Starkey, L. M.},
month = jul,
year = {2016},
doi = {1959.1/1278937}
}
• Sub-kilohertz laser linewidth narrowing using polarization spectroscopy,
J. S. Torrance, B. M. Sparkes, L. D. Turner, and R. E. Scholten.
Optics Express 24, 11396-11406 (2016).
[DOI] [Bibtex] [Abstract]

We identify several beneficial characteristics of polarization spectroscopy as an absolute atomic reference for frequency stabilization of lasers, and demonstrate sub-kilohertz laser spectral linewidth narrowing using polarization spectroscopy with high-bandwidth feedback. Polarization spectroscopy provides a highly dispersive velocity-selective absolute atomic reference based on frequency-dependent birefringence in an optically pumped atomic gas. The pumping process leads to dominance of the primary closed transition, suppressing closely-spaced subsidiary resonances which reduce the effective capture range for conventional atomic references. The locking signal is based on subtraction of two orthogonal polarization signals, reducing the effect of laser intensity noise to the shot noise limit. We measure noise-limited servo bandwidth comparable to that of a high-finesse optical cavity without the frequency limit or complexity imposed by optical modulation normally associated with high bandwidth laser frequency stabilization. We demonstrate narrowing to 600&\#x000B1;100 Hz laser linewidth using the beatnote between two similarly locked external cavity diode lasers.

@article{torrance_sub-kilohertz_2016,
title = {Sub-kilohertz laser linewidth narrowing using polarization spectroscopy},
volume = {24},
url = {http://www.opticsexpress.org/abstract.cfm?URI=oe-24-11-11396},
doi = {10.1364/OE.24.011396},
abstract = {We identify several beneficial characteristics of polarization spectroscopy as an absolute atomic reference for frequency stabilization of lasers, and demonstrate sub-kilohertz laser spectral linewidth narrowing using polarization spectroscopy with high-bandwidth feedback. Polarization spectroscopy provides a highly dispersive velocity-selective absolute atomic reference based on frequency-dependent birefringence in an optically pumped atomic gas. The pumping process leads to dominance of the primary closed transition, suppressing closely-spaced subsidiary resonances which reduce the effective capture range for conventional atomic references. The locking signal is based on subtraction of two orthogonal polarization signals, reducing the effect of laser intensity noise to the shot noise limit. We measure noise-limited servo bandwidth comparable to that of a high-finesse optical cavity without the frequency limit or complexity imposed by optical modulation normally associated with high bandwidth laser frequency stabilization. We demonstrate narrowing to 600\&\#x000B1;100 Hz laser linewidth using the beatnote between two similarly locked external cavity diode lasers.},
number = {11},
journal = {Optics Express},
author = {Torrance, Joshua S. and Sparkes, Ben M. and Turner, Lincoln D. and Scholten, Robert E.},
month = may,
year = {2016},
doi = {10.1364/OE.24.011396},
keywords = {Laser stabilization, Spectroscopy, atomic},
pages = {11396--11406}
}
• Optical vortex knots – one photon at a time,
S. J. Tempone-Wiltshire, S. P. Johnstone, and K. Helmerson.
Scientific Reports 6, 24463 (2016).
[DOI] [Bibtex] [Abstract]

Feynman described the double slit experiment as “a phenomenon which is impossible, absolutely impossible, to explain in any classical way and which has in it the heart of quantum mechanics”. The double-slit experiment, performed one photon at a time, dramatically demonstrates the particle-wave duality of quantum objects by generating a fringe pattern corresponding to the interference of light (a wave phenomenon) from two slits, even when there is only one photon (a particle) at a time passing through the apparatus. The particle-wave duality of light should also apply to complex three dimensional optical fields formed by multi-path interference, however, this has not been demonstrated. Here we observe particle-wave duality of a three dimensional field by generating a trefoil optical vortex knot – one photon at a time. This result demonstrates a fundamental physical principle, that particle-wave duality implies interference in both space (between spatially distinct modes) and time (through the complex evolution of the superposition of modes), and has implications for topologically entangled single photon states, orbital angular momentum multiplexing and topological quantum computing.

@article{tempone-wiltshire_optical_2016,
title = {Optical vortex knots – one photon at a time},
volume = {6},
issn = {2045-2322},
url = {http://www.nature.com/articles/srep24463},
doi = {10.1038/srep24463},
abstract = {Feynman described the double slit experiment as “a phenomenon which is impossible, absolutely impossible, to explain in any classical way and which has in it the heart of quantum mechanics”. The double-slit experiment, performed one photon at a time, dramatically demonstrates the particle-wave duality of quantum objects by generating a fringe pattern corresponding to the interference of light (a wave phenomenon) from two slits, even when there is only one photon (a particle) at a time passing through the apparatus. The particle-wave duality of light should also apply to complex three dimensional optical fields formed by multi-path interference, however, this has not been demonstrated. Here we observe particle-wave duality of a three dimensional field by generating a trefoil optical vortex knot – one photon at a time. This result demonstrates a fundamental physical principle, that particle-wave duality implies interference in both space (between spatially distinct modes) and time (through the complex evolution of the superposition of modes), and has implications for topologically entangled single photon states, orbital angular momentum multiplexing and topological quantum computing.},
urldate = {2016-04-18TZ},
journal = {Scientific Reports},
author = {Tempone-Wiltshire, Sebastien J. and Johnstone, Shaun P. and Helmerson, Kristian},
month = apr,
year = {2016},
pages = {24463}
}
• Onsager vortex formation in Bose–Einstein condensates in two-dimensional power-law traps,
A. J. Groszek, T. P. Simula, D. M. Paganin, and K. Helmerson.
Physical Review A 93, 43614 (2016).
[arXiv] [DOI] [Bibtex] [Abstract]

We study computationally dynamics of quantized vortices in two-dimensional superfluid Bose-Einstein condensates confined in highly oblate power-law traps. We have found that the formation of large-scale Onsager vortex clusters prevalent in steep-walled traps is suppressed in condensates confined by harmonic potentials. However, the shape of the trapping potential does not appear to adversely affect the evaporative heating efficiency of the vortex gas. Instead, the suppression of Onsager vortex formation in harmonic traps can be understood in terms of the energy of the vortex configurations. Furthermore, we find that the vortex-antivortex pair annihilation that underpins the vortex evaporative heating mechanism requires the interaction of at least three vortices. We conclude that experimental observation of Onsager vortices should be the most apparent in flat or inverted-bottom traps.

@article{groszek_onsager_2016,
title = {Onsager vortex formation in {Bose--Einstein} condensates in two-dimensional power-law traps},
volume = {93},
doi = {10.1103/PhysRevA.93.043614},
abstract = {We study computationally dynamics of quantized vortices in two-dimensional superfluid Bose-Einstein condensates confined in highly oblate power-law traps. We have found that the formation of large-scale Onsager vortex clusters prevalent in steep-walled traps is suppressed in condensates confined by harmonic potentials. However, the shape of the trapping potential does not appear to adversely affect the evaporative heating efficiency of the vortex gas. Instead, the suppression of Onsager vortex formation in harmonic traps can be understood in terms of the energy of the vortex configurations. Furthermore, we find that the vortex-antivortex pair annihilation that underpins the vortex evaporative heating mechanism requires the interaction of at least three vortices. We conclude that experimental observation of Onsager vortices should be the most apparent in flat or inverted-bottom traps.},
number = {4},
urldate = {2016-04-18TZ},
journal = {Physical Review A},
author = {Groszek, Andrew J. and Simula, Tapio P. and Paganin, David M. and Helmerson, Kristian},
month = apr,
year = {2016},
eprint = {1511.06552},
pages = {043614}
}
• Hamiltonian dynamics of two same-sign point vortices,
A. V. Murray, A. J. Groszek, P. Kuopanportti, and T. Simula.
Physical Review A 93, 33649 (2016).
[arXiv] [DOI] [Bibtex] [Abstract]

We have studied numerically the Hamiltonian dynamics of two same-sign point vortices in an effectively two-dimensional, harmonically trapped Bose-Einstein condensate. We have found in the phase space of the system an impenetrable wall that divides the dynamics into two distinct and exhaustive types. In the two-dimensional position-coordinate space, the first type corresponds to intersecting single-vortex orbits and the second type to orbits that have no points in common. The two types are also easily distinguished in the two-dimensional space spanned by the radial and angular velocities of the vortices: In the first type, both single-vortex orbits are the same simple loop in this two-dimensional space, whereas in the second type the two orbits constitute two nonintersecting loops. The phase-space-dividing wall is distinct from the bifurcation curve of rigidly rotating states found by Navarro et al. [Phys. Rev. Lett. 110, 225301 (2013)].

@article{murray_hamiltonian_2016,
title = {Hamiltonian dynamics of two same-sign point vortices},
volume = {93},
doi = {10.1103/PhysRevA.93.033649},
abstract = {We have studied numerically the Hamiltonian dynamics of two same-sign point vortices in an effectively two-dimensional, harmonically trapped Bose-Einstein condensate. We have found in the phase space of the system an impenetrable wall that divides the dynamics into two distinct and exhaustive types. In the two-dimensional position-coordinate space, the first type corresponds to intersecting single-vortex orbits and the second type to orbits that have no points in common. The two types are also easily distinguished in the two-dimensional space spanned by the radial and angular velocities of the vortices: In the first type, both single-vortex orbits are the same simple loop in this two-dimensional space, whereas in the second type the two orbits constitute two nonintersecting loops. The phase-space-dividing wall is distinct from the bifurcation curve of rigidly rotating states found by Navarro et al. [Phys. Rev. Lett. 110, 225301 (2013)].},
number = {3},
urldate = {2016-04-18TZ},
journal = {Physical Review A},
author = {Murray, Anderson V. and Groszek, Andrew J. and Kuopanportti, Pekko and Simula, Tapio},
month = mar,
year = {2016},
eprint = {1512.02809},
pages = {033649}
}
• Interatomic Fe–H potential for irradiation and embrittlement simulations,
P. Kuopanportti, E. Hayward, C. Fu, A. Kuronen, and K. Nordlund.
Computational Materials Science 111, 525-531 (2016).
[arXiv] [DOI] [Bibtex] [Abstract]

The behavior of hydrogen in iron and iron alloys is of interest in many fields of physics and materials science. To enable large-scale molecular dynamics simulations of systems with Fe–H interactions, we develop, based on density-functional theory calculations, an interatomic Fe–H potential in the Tersoff–Brenner formalism. The obtained analytical potential is suitable for simulations of H in bulk Fe as well as for modeling small FeH molecules, and it can be directly combined with our previously constructed potential for the stainless steel Fe–Cr–C system. This will allow simulations of, e.g., hydrocarbon molecule chemistry on steel surfaces. In the current work, we apply the potential to simulating hydrogen-induced embrittlement in monocrystalline bulk Fe and in an Fe bicrystal with a grain boundary. In both cases, hydrogen is found to soften the material.

@article{kuopanportti_interatomic_2016,
title = {Interatomic Fe–H potential for irradiation and embrittlement simulations},
volume = {111},
issn = {0927-0256},
url = {http://www.sciencedirect.com/science/article/pii/S092702561500590X},
doi = {10.1016/j.commatsci.2015.09.021},
abstract = {The behavior of hydrogen in iron and iron alloys is of interest in many fields of physics and materials science. To enable large-scale molecular dynamics simulations of systems with Fe–H interactions, we develop, based on density-functional theory calculations, an interatomic Fe–H potential in the Tersoff–Brenner formalism. The obtained analytical potential is suitable for simulations of H in bulk Fe as well as for modeling small FeH molecules, and it can be directly combined with our previously constructed potential for the stainless steel Fe–Cr–C system. This will allow simulations of, e.g., hydrocarbon molecule chemistry on steel surfaces. In the current work, we apply the potential to simulating hydrogen-induced embrittlement in monocrystalline bulk Fe and in an Fe bicrystal with a grain boundary. In both cases, hydrogen is found to soften the material.},
language = {EN},
journal = {Computational Materials Science},
author = {Kuopanportti, Pekko and Hayward, Erin and Fu, Chu-Chun and Kuronen, Antti and Nordlund, Kai},
month = jan,
year = {2016},
eprint = {1604.02829},
keywords = {Iron, hydrogen embrittlement, interatomic potential, materials science, molecular dynamics, tensile testing},
pages = {525--531}
}
• Journey from Classical to Quantum in Two Dimensions,
M. Parish.
Physics 9, 10 (2016).
[DOI] [Bibtex] [Abstract]

Two separate groups have extracted the thermodynamic equation of state for a two-dimensional gas of fermionic atoms, revealing its peculiar quantum features.

@article{parish_journey_2016,
title = {Journey from Classical to Quantum in Two Dimensions},
volume = {9},
doi = {10.1103/Physics.9.10},
abstract = {Two separate groups have extracted the thermodynamic equation of state for a two-dimensional gas of fermionic atoms, revealing its peculiar quantum features.},
journal = {Physics},
author = {Parish, M.},
month = jan,
year = {2016},
pages = {10}
}

### 2015

• Observation of an Orbital Interaction-Induced Feshbach Resonance in \${\textasciicircum}\173\{\textbackslash}mathrm\{{Yb}\}\$,
M. Höfer, L. Riegger, F. Scazza, C. Hofrichter, D.  R. Fernandes, M.  M. Parish, J. Levinsen, I. Bloch, and S. Fölling.
Physical Review Letters 115, 265302 (2015).
[DOI] [Bibtex] [Abstract]

We report on the experimental observation of a novel interorbital Feshbach resonance in ultracold Yb173 atoms. This opens up the possibility of tuning the interactions between the S01 and P03 metastable state, both possessing zero total electronic angular momentum. The resonance is observed at experimentally accessible magnetic field strengths and occurs universally for all hyperfine state combinations. We characterize the resonance in the bulk via interorbital cross thermalization as well as in a three-dimensional lattice using high-resolution clock-line spectroscopy. Our measurements are well described by a generalized two-channel model of the orbital-exchange interactions.

@article{hofer_observation_2015,
title = {Observation of an Orbital Interaction-Induced {Feshbach} Resonance in \${\textasciicircum}\173\{\textbackslash}mathrm\{{Yb}\}\$},
volume = {115},
doi = {10.1103/PhysRevLett.115.265302},
abstract = {We report on the experimental observation of a novel interorbital Feshbach resonance in ultracold Yb173 atoms. This opens up the possibility of tuning the interactions between the S01 and P03 metastable state, both possessing zero total electronic angular momentum. The resonance is observed at experimentally accessible magnetic field strengths and occurs universally for all hyperfine state combinations. We characterize the resonance in the bulk via interorbital cross thermalization as well as in a three-dimensional lattice using high-resolution clock-line spectroscopy. Our measurements are well described by a generalized two-channel model of the orbital-exchange interactions.},
number = {26},
urldate = {2016-11-14TZ},
journal = {Physical Review Letters},
author = {Höfer, M. and Riegger, L. and Scazza, F. and Hofrichter, C. and Fernandes, D. R. and Parish, M. M. and Levinsen, J. and Bloch, I. and Fölling, S.},
month = dec,
year = {2015},
pages = {265302}
}
• Faraday Magnetic Resonance Imaging of Bose–Einstein Condensates,
M. Jasperse, PhD Thesis.
[DOI] [Bibtex] [Abstract]

This thesis demonstrates the first ever magnetic resonance image, or MRI, of the coldest matter in the Universe. This superfluid – predicted by Einstein in 1925 – exhibits the curious hallmarks of quantum physics that occur inside atoms, but on a much larger length scale. Nevertheless, traditional imaging methods using lenses and cameras fail to see the finest structure of this matter without destroying it. This thesis closes the circle of reciprocity between fundamental science and medicine, applying a diagnostic imaging technique borne out of nuclear physics 40 years ago to cutting-edge atomic physics.

@phdthesis{jasperse_faraday_2015,
type = {{PhD} thesis},
title = {Faraday Magnetic Resonance Imaging of {Bose--Einstein} Condensates},
url = {http://dx.doi.org/1959.1/1231988},
abstract = {This thesis demonstrates the first ever magnetic resonance image, or MRI, of the coldest matter in the Universe. This superfluid – predicted by Einstein in 1925 – exhibits the curious hallmarks of quantum physics that occur inside atoms, but on a much larger length scale. Nevertheless, traditional imaging methods using lenses and cameras fail to see the finest structure of this matter without destroying it. This thesis closes the circle of reciprocity between fundamental science and medicine, applying a diagnostic imaging technique borne out of nuclear physics 40 years ago to cutting-edge atomic physics.},
school = {Monash University},
author = {Jasperse, M.},
month = dec,
year = {2015},
doi = {1959.1/1231988}
}
• Magnetism in Strongly Interacting One-Dimensional Quantum Mixtures,
P. Massignan, J. Levinsen, and M. M. Parish.
Physical Review Letters 115, 247202 (2015).
[arXiv] [DOI] [Bibtex] [Abstract]

We consider two species of bosons in one dimension near the Tonks-Girardeau limit of infinite interactions. For the case of equal masses and equal intraspecies interactions, the system can be mapped to a S=1/2 XXZ Heisenberg spin chain, thus allowing one to access different magnetic phases. Using a powerful ansatz developed for the two-component Fermi system, we elucidate the evolution from few to many particles for the experimentally relevant case of an external harmonic confinement. In the few-body limit, we already find clear evidence of both ferromagnetic and antiferromagnetic spin correlations as the ratio of intraspecies and interspecies interactions is varied. Furthermore, we observe the rapid emergence of symmetry-broken magnetic ground states as the particle number is increased. We therefore demonstrate that systems containing only a few bosons are an ideal setting in which to realize the highly sought-after itinerant ferromagnetic phase.

@article{massignan_magnetism_2015,
title = {Magnetism in Strongly Interacting One-Dimensional Quantum Mixtures},
volume = {115},
doi = {10.1103/PhysRevLett.115.247202},
abstract = {We consider two species of bosons in one dimension near the Tonks-Girardeau limit of infinite interactions. For the case of equal masses and equal intraspecies interactions, the system can be mapped to a S=1/2 XXZ Heisenberg spin chain, thus allowing one to access different magnetic phases. Using a powerful ansatz developed for the two-component Fermi system, we elucidate the evolution from few to many particles for the experimentally relevant case of an external harmonic confinement. In the few-body limit, we already find clear evidence of both ferromagnetic and antiferromagnetic spin correlations as the ratio of intraspecies and interspecies interactions is varied. Furthermore, we observe the rapid emergence of symmetry-broken magnetic ground states as the particle number is increased. We therefore demonstrate that systems containing only a few bosons are an ideal setting in which to realize the highly sought-after itinerant ferromagnetic phase.},
number = {24},
urldate = {2016-01-04TZ},
journal = {Physical Review Letters},
author = {Massignan, Pietro and Levinsen, Jesper and Parish, Meera M.},
month = dec,
year = {2015},
eprint = {1507.02814},
pages = {247202}
}
• Toward terrestrial detection of millihertz gravitational waves with magnetically assisted torsion pendulums,
E. Thrane, R. P. Anderson, Y. Levin, and L. D. Turner.
[arXiv] [Bibtex] [Abstract]

Current terrestrial gravitational-wave detectors operate at frequencies above 10Hz. There is strong astrophysical motivation to construct low-frequency gravitational-wave detectors capable of observing 10-1e4 mHz signals. However, there are numerous technological challenges. In particular, it is difficult to isolate test masses so that they are both seismically isolated and freely falling under the influence of gravity at mHz frequencies. We propose a Magnetically Assisted Gravitational-wave Pendulum Intorsion (MAGPI) suspension design for use in low-frequency gravitational-wave detectors. We construct a noise budget to determine the required specifications. In doing so, we identify what are likely to be a number of limiting noise sources for terrestrial mHz gravitational-wave suspension systems. We conclude that it may be possible to achieve the required seismic isolation and coupling to gravitational waves necessary for mHz detection, though, there are significant experimental challenges.

@article{thrane_toward_2015,
title = {Toward terrestrial detection of millihertz gravitational waves with magnetically assisted torsion pendulums},
url = {http://arxiv.org/abs/1512.03137},
abstract = {Current terrestrial gravitational-wave detectors operate at frequencies above 10Hz. There is strong astrophysical motivation to construct low-frequency gravitational-wave detectors capable of observing 10-1e4 mHz signals. However, there are numerous technological challenges. In particular, it is difficult to isolate test masses so that they are both seismically isolated and freely falling under the influence of gravity at mHz frequencies. We propose a Magnetically Assisted Gravitational-wave Pendulum Intorsion (MAGPI) suspension design for use in low-frequency gravitational-wave detectors. We construct a noise budget to determine the required specifications. In doing so, we identify what are likely to be a number of limiting noise sources for terrestrial mHz gravitational-wave suspension systems. We conclude that it may be possible to achieve the required seismic isolation and coupling to gravitational waves necessary for mHz detection, though, there are significant experimental challenges.},
urldate = {2015-12-11TZ},
journal = {arXiv:1512.03137},
author = {Thrane, Eric and Anderson, R. P. and Levin, Yuri and Turner, L. D.},
month = dec,
year = {2015},
eprint = {1512.03137},
keywords = {Astrophysics - Instrumentation and Methods for Astrophysics, General Relativity and Quantum Cosmology, Physics - Instrumentation and Detectors}
}
• Spinor Bose–Einstein condensates in magnetic field gradients,
A. A. Wood, PhD Thesis.
[DOI] [Bibtex] [Abstract]

This thesis develops a new kind of microscope for magnetic fields, using the coldest matter in the Universe. The medium is an exotic phase of matter – first predicted by Einstein in 1925 – exquisitely cold, and exquisitely sensitive to magnetic fields. The magnetic microscope can ‘see’ both real magnetic fields and fictitious ones produced by lasers, and does not require calibration or shielding from ambient fields. It may be applied to imaging cellular function, detecting vehicles and unexploded ordinance, and the fundamental physics of ultracold atoms in its own right.

@phdthesis{wood_spinor_2015,
type = {{PhD} thesis},
title = {Spinor {Bose--Einstein} condensates in magnetic field gradients},
url = {http://dx.doi.org/1959.1/1228163},
abstract = {This thesis develops a new kind of microscope for magnetic fields, using the coldest matter in the Universe. The medium is an exotic phase of matter - first predicted by Einstein in 1925 - exquisitely cold, and exquisitely sensitive to magnetic fields. The magnetic microscope can 'see' both real magnetic fields and fictitious ones produced by lasers, and does not require calibration or shielding from ambient fields. It may be applied to imaging cellular function, detecting vehicles and unexploded ordinance, and the fundamental physics of ultracold atoms in its own right.},
school = {Monash University},
author = {Wood, A. A.},
month = nov,
year = {2015},
doi = {1959.1/1228163}
}
• Magnetic tensor gradiometry using Ramsey interferometry of spinor condensates,
A. A. Wood, L. M. Bennie, A. Duong, M. Jasperse, L. D. Turner, and R. P. Anderson.
Physical Review A 92, 53604 (2015).
[arXiv] [DOI] [Bibtex] [Abstract]

We have realized a magnetic tensor gradiometer by interferometrically measuring the relative phase between two spatially separated Bose-Einstein condensates (BECs). We perform simultaneous Ramsey interferometry of the proximate Rb87 spin-1 condensates in free fall and infer their relative Larmor phase, and thus the differential magnetic-field strength, with a common-mode phase noise suppression exceeding 50dB. By appropriately biasing the magnetic field and separating the BECs along orthogonal directions, we measure in vacuo the magnetic-field-gradient tensor of ambient and applied magnetic fields with a nominal precision of 0.30nTmm−1 and a sensor volume of 2×10−5mm3. We predict a spin-projection noise-limited magnetic energy resolution of order ∼10ℏ for typical Zeeman coherence times of trapped condensates with this scheme, even with the low measurement duty cycle of current BEC experiments.

@article{wood_magnetic_2015,
title = {Magnetic tensor gradiometry using Ramsey interferometry of spinor condensates},
volume = {92},
doi = {10.1103/PhysRevA.92.053604},
abstract = {We have realized a magnetic tensor gradiometer by interferometrically measuring the relative phase between two spatially separated Bose-Einstein condensates (BECs). We perform simultaneous Ramsey interferometry of the proximate Rb87 spin-1 condensates in free fall and infer their relative Larmor phase, and thus the differential magnetic-field strength, with a common-mode phase noise suppression exceeding 50dB. By appropriately biasing the magnetic field and separating the BECs along orthogonal directions, we measure in vacuo the magnetic-field-gradient tensor of ambient and applied magnetic fields with a nominal precision of 0.30nTmm−1 and a sensor volume of 2×10−5mm3. We predict a spin-projection noise-limited magnetic energy resolution of order ∼10ℏ for typical Zeeman coherence times of trapped condensates with this scheme, even with the low measurement duty cycle of current BEC experiments.},
number = {5},
urldate = {2016-01-04TZ},
journal = {Physical Review A},
author = {Wood, A. A. and Bennie, L. M. and Duong, A. and Jasperse, M. and Turner, L. D. and Anderson, R. P.},
month = nov,
year = {2015},
eprint = {1408.0944},
pages = {053604}
}
• Quasiparticle Properties of a Mobile Impurity in a Bose–Einstein Condensate,
R. S. Christensen, J. Levinsen, and G. M. Bruun.
Physical Review Letters 115, 160401 (2015).
[arXiv] [DOI] [Bibtex] [Abstract]

We develop a systematic perturbation theory for the quasiparticle properties of a single impurity immersed in a Bose-Einstein condensate. Analytical results are derived for the impurity energy, effective mass, and residue to third order in the impurity-boson scattering length. The energy is shown to depend logarithmically on the scattering length to third order, whereas the residue and the effective mass are given by analytical power series. When the boson-boson scattering length equals the boson-impurity scattering length, the energy has the same structure as that of a weakly interacting Bose gas, including terms of the Lee-Huang-Yang and fourth order logarithmic form. Our results, which cannot be obtained within the canonical Fröhlich model of an impurity interacting with phonons, provide valuable benchmarks for many-body theories and for experiments.

@article{christensen_quasiparticle_2015,
title = {Quasiparticle Properties of a Mobile Impurity in a {Bose--Einstein} Condensate},
volume = {115},
doi = {10.1103/PhysRevLett.115.160401},
abstract = {We develop a systematic perturbation theory for the quasiparticle properties of a single impurity immersed in a Bose-Einstein condensate. Analytical results are derived for the impurity energy, effective mass, and residue to third order in the impurity-boson scattering length. The energy is shown to depend logarithmically on the scattering length to third order, whereas the residue and the effective mass are given by analytical power series. When the boson-boson scattering length equals the boson-impurity scattering length, the energy has the same structure as that of a weakly interacting Bose gas, including terms of the Lee-Huang-Yang and fourth order logarithmic form. Our results, which cannot be obtained within the canonical Fröhlich model of an impurity interacting with phonons, provide valuable benchmarks for many-body theories and for experiments.},
number = {16},
urldate = {2016-01-04TZ},
journal = {Physical Review Letters},
author = {Christensen, Rasmus Søgaard and Levinsen, Jesper and Bruun, Georg M.},
month = oct,
year = {2015},
eprint = {1503.06979},
pages = {160401}
}
• Impurity in a Bose–Einstein Condensate and the Efimov Effect,
J. Levinsen, M. M. Parish, and G. M. Bruun.
Physical Review Letters 115, 125302 (2015).
[arXiv] [DOI] [Bibtex] [Abstract]

We investigate the zero-temperature properties of an impurity particle interacting with a Bose-Einstein condensate (BEC), using a variational wave function that includes up to two Bogoliubov excitations of the BEC. This allows one to capture three-body Efimov physics, as well as to recover the first nontrivial terms in the weak-coupling expansion. We show that the energy and quasiparticle residue of the dressed impurity (polaron) are significantly lowered by three-body correlations, even for weak interactions where there is no Efimov trimer state in a vacuum. For increasing attraction between the impurity and the BEC, we observe a smooth crossover from atom to Efimov trimer, with a superposition of states near the Efimov resonance. We furthermore demonstrate that three-body loss does not prohibit the experimental observation of these effects. Our results thus suggest a route to realizing Efimov physics in a stable quantum many-body system for the first time.

@article{levinsen_impurity_2015,
title = {Impurity in a {Bose--Einstein} Condensate and the Efimov Effect},
volume = {115},
doi = {10.1103/PhysRevLett.115.125302},
abstract = {We investigate the zero-temperature properties of an impurity particle interacting with a Bose-Einstein condensate (BEC), using a variational wave function that includes up to two Bogoliubov excitations of the BEC. This allows one to capture three-body Efimov physics, as well as to recover the first nontrivial terms in the weak-coupling expansion. We show that the energy and quasiparticle residue of the dressed impurity (polaron) are significantly lowered by three-body correlations, even for weak interactions where there is no Efimov trimer state in a vacuum. For increasing attraction between the impurity and the BEC, we observe a smooth crossover from atom to Efimov trimer, with a superposition of states near the Efimov resonance. We furthermore demonstrate that three-body loss does not prohibit the experimental observation of these effects. Our results thus suggest a route to realizing Efimov physics in a stable quantum many-body system for the first time.},
number = {12},
urldate = {2016-01-04TZ},
journal = {Physical Review Letters},
author = {Levinsen, Jesper and Parish, Meera M. and Bruun, Georg M.},
month = sep,
year = {2015},
eprint = {1505.04530},
pages = {125302}
}
• Electron Singularities, Matter Wave Catastrophes, and Vortex Lattice Singularimetry,
T. C. Petersen, M. Weyland, D. M. Paganin, T. P. Simula, S. A. Eastwood, A. I. Bishop, and M. J. Morgan.
Microscopy and Microanalysis 21, 673-674 (2015).
[DOI] [Bibtex]
@article{petersen_electron_2015,
title = {Electron Singularities, Matter Wave Catastrophes, and Vortex Lattice Singularimetry},
volume = {21},
issn = {1435-8115},
url = {http://journals.cambridge.org/article_S143192761500416X},
doi = {10.1017/S143192761500416X},
number = {Supplement S3},
urldate = {2016-01-04TZ},
journal = {Microscopy and Microanalysis},
author = {Petersen, T. C. and Weyland, M. and Paganin, D. M. and Simula, T. P. and Eastwood, S. A. and Bishop, A. I. and Morgan, M. J.},
month = aug,
year = {2015},
pages = {673--674}
}
• Strong-coupling ansatz for the one-dimensional Fermi gas in a harmonic potential,
J. Levinsen, P. Massignan, G. M. Bruun, and M. M. Parish.
[DOI] [Bibtex] [Abstract]

A major challenge in modern physics is to accurately describe strongly interacting quantum many-body systems. One-dimensional systems provide fundamental insights because they are often amenable to exact methods. However, no exact solution is known for the experimentally relevant case of external confinement. We propose a powerful ansatz for the one-dimensional Fermi gas in a harmonic potential near the limit of infinite short-range repulsion. For the case of a single impurity in a Fermi sea, we show that our ansatz is indistinguishable from numerically exact results in both the few- and many-body limits. We furthermore derive an effective Heisenberg spin-chain model corresponding to our ansatz, valid for any spin-mixture, within which we obtain the impurity eigenstates analytically. In particular, the classical Pascal’s triangle emerges in the expression for the ground-state wave function. As well as providing an important benchmark for strongly correlated physics, our results are relevant for emerging quantum technologies, where a precise knowledge of one-dimensional quantum states is paramount. A near exact solution is revealed for a single impurity strongly interacting with a one-dimensional trapped quantum gas. A near exact solution is revealed for a single impurity strongly interacting with a one-dimensional trapped quantum gas.

@article{levinsen_strong-coupling_2015,
title = {Strong-coupling ansatz for the one-dimensional {Fermi} gas in a harmonic potential},
volume = {1},
copyright = {Copyright © 2015, The Authors. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.},
issn = {2375-2548},
abstract = {A major challenge in modern physics is to accurately describe strongly interacting quantum many-body systems. One-dimensional systems provide fundamental insights because they are often amenable to exact methods. However, no exact solution is known for the experimentally relevant case of external confinement. We propose a powerful ansatz for the one-dimensional Fermi gas in a harmonic potential near the limit of infinite short-range repulsion. For the case of a single impurity in a Fermi sea, we show that our ansatz is indistinguishable from numerically exact results in both the few- and many-body limits. We furthermore derive an effective Heisenberg spin-chain model corresponding to our ansatz, valid for any spin-mixture, within which we obtain the impurity eigenstates analytically. In particular, the classical Pascal’s triangle emerges in the expression for the ground-state wave function. As well as providing an important benchmark for strongly correlated physics, our results are relevant for emerging quantum technologies, where a precise knowledge of one-dimensional quantum states is paramount.
A near exact solution is revealed for a single impurity strongly interacting with a one-dimensional trapped quantum gas.
A near exact solution is revealed for a single impurity strongly interacting with a one-dimensional trapped quantum gas.},
number = {6},
urldate = {2016-11-14TZ},
author = {Levinsen, Jesper and Massignan, Pietro and Bruun, Georg M. and Parish, Meera M.},
month = jul,
year = {2015},
pages = {e1500197}
}
• Route to non-Abelian quantum turbulence in spinor Bose–Einstein condensates,
T. Mawson, G. Ruben, and T. Simula.
Physical Review A 91, 63630 (2015).
[arXiv] [DOI] [Bibtex] [Abstract]

We have studied computationally the collision dynamics of spin-2 Bose-Einstein condensates initially confined in a triple-well trap. Depending on the phase structure of the initial-state spinor wave function, the collision of the three condensate fragments produces one of many possible vortex-antivortex lattices, after which the system transitions to quantum turbulence. We find that the emerging vortex lattice structures can be described in terms of multiwave interference. We show that the three-fragment collisions can be used to systematically produce staggered vortex-antivortex honeycomb lattices of fractional-charge vortices, whose collision dynamics are known to be non-Abelian. Such condensate collider experiments could potentially be used as a controllable pathway to generating non-Abelian superfluid turbulence with networks of vortex rungs.

@article{mawson_route_2015,
title = {Route to non-Abelian quantum turbulence in spinor {Bose--Einstein} condensates},
volume = {91},
doi = {10.1103/PhysRevA.91.063630},
abstract = {We have studied computationally the collision dynamics of spin-2 Bose-Einstein condensates initially confined in a triple-well trap. Depending on the phase structure of the initial-state spinor wave function, the collision of the three condensate fragments produces one of many possible vortex-antivortex lattices, after which the system transitions to quantum turbulence. We find that the emerging vortex lattice structures can be described in terms of multiwave interference. We show that the three-fragment collisions can be used to systematically produce staggered vortex-antivortex honeycomb lattices of fractional-charge vortices, whose collision dynamics are known to be non-Abelian. Such condensate collider experiments could potentially be used as a controllable pathway to generating non-Abelian superfluid turbulence with networks of vortex rungs.},
number = {6},
urldate = {2016-01-04TZ},
journal = {Physical Review A},
author = {Mawson, Thomas and Ruben, Gary and Simula, Tapio},
month = jun,
year = {2015},
eprint = {1412.6186},
pages = {063630}
}
• Ground-state multiquantum vortices in rotating two-species superfluids,
P. Kuopanportti, N. V. Orlova, and M. V. Milošević.
Physical Review A 91, 43605 (2015).
[arXiv] [DOI] [Bibtex] [Abstract]

We show numerically that a rotating, harmonically trapped mixture of two Bose-Einstein-condensed superfluids can—contrary to its single-species counterpart—contain a multiply quantized vortex in the ground state of the system. This giant vortex can occur without any accompanying single-quantum vortices, may either be coreless or have an empty core, and can be realized in a Rb87−K41 Bose-Einstein condensate. Our results not only provide a rare example of a stable, solitary multiquantum vortex but also reveal exotic physics stemming from the coexistence of multiple, compositionally distinct condensates in one system.

@article{kuopanportti_ground-state_2015,
title = {Ground-state multiquantum vortices in rotating two-species superfluids},
volume = {91},
doi = {10.1103/PhysRevA.91.043605},
abstract = {We show numerically that a rotating, harmonically trapped mixture of two Bose-Einstein-condensed superfluids can—contrary to its single-species counterpart—contain a multiply quantized vortex in the ground state of the system. This giant vortex can occur without any accompanying single-quantum vortices, may either be coreless or have an empty core, and can be realized in a Rb87−K41 Bose-Einstein condensate. Our results not only provide a rare example of a stable, solitary multiquantum vortex but also reveal exotic physics stemming from the coexistence of multiple, compositionally distinct condensates in one system.},
number = {4},
urldate = {2016-01-06TZ},
journal = {Physical Review A},
author = {Kuopanportti, Pekko and Orlova, Natalia V. and Milošević, Milorad V.},
month = apr,
year = {2015},
eprint = {1504.02178},
pages = {043605}
}
• A Monte Carlo wavefunction method for semiclassical simulations of spin-position entanglement,
C. J. Billington, C. J. Watkins, R. P. Anderson, and L. D. Turner.
[arXiv] [Bibtex] [Abstract]

We present a Monte Carlo wavefunction method for semiclassically modeling spin-\${\textbackslash}frac12\$ systems in a magnetic field gradient in one dimension. Our model resolves the conflict of determining what classical force an atom should be subjected to when it is in an arbitrary superposition of internal states. Spatial degrees of freedom are considered to be an environment, entanglement with which decoheres the internal states. Atoms follow classical trajectories through space, punctuated by probabilistic jumps between spin states. We modify the conventional Monte Carlo wavefunction method to jump between states when population transfer occurs, rather than when population is later discarded via exponential decay. This results in a spinor wavefunction that is continuous in time, and allows us to model the classical particle trajectories (evolution of the environment variables) more accurately. The model is not computationally demanding and it agrees well with simulations of the full spatial wavefunction of an atom.

@article{billington_monte_2015,
title = {{A} Monte Carlo wavefunction method for semiclassical simulations of spin-position entanglement},
url = {http://arxiv.org/abs/1502.06674},
abstract = {We present a Monte Carlo wavefunction method for semiclassically modeling spin-\${\textbackslash}frac12\$ systems in a magnetic field gradient in one dimension. Our model resolves the conflict of determining what classical force an atom should be subjected to when it is in an arbitrary superposition of internal states. Spatial degrees of freedom are considered to be an environment, entanglement with which decoheres the internal states. Atoms follow classical trajectories through space, punctuated by probabilistic jumps between spin states. We modify the conventional Monte Carlo wavefunction method to jump between states when population transfer occurs, rather than when population is later discarded via exponential decay. This results in a spinor wavefunction that is continuous in time, and allows us to model the classical particle trajectories (evolution of the environment variables) more accurately. The model is not computationally demanding and it agrees well with simulations of the full spatial wavefunction of an atom.},
urldate = {2016-01-04TZ},
journal = {arXiv:1502.06674},
author = {Billington, C. J. and Watkins, C. J. and Anderson, R. P. and Turner, L. D.},
month = feb,
year = {2015},
eprint = {1502.06674},
keywords = {Physics - Atomic Physics, Quantum Physics}
}
• Strongly interacting two-dimensional fermi gases
J. Levinsen and M. M. Parish.
In Annual Review of Cold Atoms and Molecules. WORLD SCIENTIFIC, 2015, vol. Volume 3, pp. 1-75.
[Bibtex]
@incollection{levinsen_strongly_2015,
series = {Annual {Review} of {Cold} {Atoms} and {Molecules}},
title = {Strongly interacting two-dimensional fermi gases},
volume = {Volume 3},
isbn = {978-981-4667-73-9},
url = {http://www.worldscientific.com/doi/abs/10.1142/9789814667746_0001},
number = {Volume 3},
urldate = {2016-11-14TZ},
booktitle = {Annual Review of Cold Atoms and Molecules},
publisher = {WORLD SCIENTIFIC},
author = {Levinsen, Jesper and Parish, Meera M.},
month = feb,
year = {2015},
pages = {1--75}
}
• High-temperature limit of the resonant Fermi gas,
V. Ngampruetikorn, M. M. Parish, and J. Levinsen.
Physical Review A 91, 13606 (2015).
[DOI] [Bibtex] [Abstract]

We use the virial expansion to investigate the behavior of the two-component, attractive Fermi gas in the high-temperature limit, where the system smoothly evolves from weakly attractive fermions to weakly repulsive bosonic dimers as the short-range attraction is increased. We present a formalism for computing the virial coefficients that employs a diagrammatic approach to the grand potential and allows one to easily include an effective range R∗ in the interaction. In the limit where the thermal wavelength λ≪R∗, the calculation of the virial coefficients is perturbative even at unitarity and the system resembles a weakly interacting Bose-Fermi mixture for all scattering lengths a. By interpolating from the perturbative limits λ/{\textbar}a{\textbar}≫1 and R∗/λ≫1, we estimate the value of the fourth virial coefficient at unitarity for R∗=0 and we find that it is close to the value obtained in recent experiments. We also derive the equations of state for the pressure, density, and entropy, as well as the spectral function at high temperatures.

@article{ngampruetikorn_high-temperature_2015,
title = {High-temperature limit of the resonant {Fermi} gas},
volume = {91},
doi = {10.1103/PhysRevA.91.013606},
abstract = {We use the virial expansion to investigate the behavior of the two-component, attractive Fermi gas in the high-temperature limit, where the system smoothly evolves from weakly attractive fermions to weakly repulsive bosonic dimers as the short-range attraction is increased. We present a formalism for computing the virial coefficients that employs a diagrammatic approach to the grand potential and allows one to easily include an effective range R∗ in the interaction. In the limit where the thermal wavelength λ≪R∗, the calculation of the virial coefficients is perturbative even at unitarity and the system resembles a weakly interacting Bose-Fermi mixture for all scattering lengths a. By interpolating from the perturbative limits λ/{\textbar}a{\textbar}≫1 and R∗/λ≫1, we estimate the value of the fourth virial coefficient at unitarity for R∗=0 and we find that it is close to the value obtained in recent experiments. We also derive the equations of state for the pressure, density, and entropy, as well as the spectral function at high temperatures.},
number = {1},
urldate = {2016-11-14TZ},
journal = {Physical Review A},
author = {Ngampruetikorn, Vudtiwat and Parish, Meera M. and Levinsen, Jesper},
month = jan,
year = {2015},
pages = {013606}
}
• Persistent Gravitational Radiation from Glitching Pulsars,
A. Melatos, J. A. Douglass, and T. P. Simula.
The Astrophysical Journal 807, 132 (2015).
[DOI] [Bibtex] [Abstract]

Quantum mechanical simulations of neutron star rotational glitches, triggered by vortex avalanches in the superfluid stellar interior, reveal that vortices pin nonaxisymmetrically to the crust during the intervals between glitches. Hence a glitching neutron star emits a persistent current quadrupole gravitational wave signal at the star’s rotation frequency, whose interglitch amplitude is constant and determined by the avalanche history since birth. The signal can be detected in principle by coherent searches planned for the Laser Interferometer Gravitational Wave Observatory (LIGO), whether or not a glitch occurs during the observation, if the power-law distribution of glitch sizes extends up to \#\#IMG\#\# [http://ej.iop.org/images/0004-637X/807/2/132/apj515723ieqn1.gif] \${\textbackslash}rm{\textbackslash}Delta {\textbackslash}rmØmega _{\textbackslash}mathrmmax/{\textbackslash}rmØmega {\textbackslash}gtrsim 10{\textasciicircum}-6{\textbackslash}eta {\textasciicircum}-1({\textbackslash}rm{\textbackslash}Delta {\textbackslash}phi ){\textasciicircum}-1({\textbackslash}rmØmega /10{\textasciicircum}3{\textbackslash};{\textbackslash}mathrmrad{\textbackslash};{\textbackslash}rms{\textasciicircum}-1){\textasciicircum}-3(D/1{\textbackslash};{\textbackslash}mathrmkpc)\$ in the targeted object, where \#\#IMG\#\# [http://ej.iop.org/images/0004-637X/807/2/132/apj515723ieqn2.gif] \${\textbackslash}rm{\textbackslash}Delta {\textbackslash}rmØmega _{\textbackslash}mathrmmax\$ and \#\#IMG\#\# [http://ej.iop.org/images/0004-637X/807/2/132/apj515723ieqn3.gif] \${\textbackslash}rm{\textbackslash}Delta {\textbackslash}phi \$ are the largest angular velocity jump and avalanche opening angle, respectively, to have occurred in a glitch since birth, Ω is the angular velocity at present, η is the crustal fraction of the moment of inertia, and D is the distance from the Earth. A major caveat concerning detectability is whether the nonaxisymmetries observed in existing simulations with \#\#IMG\#\# [http://ej.iop.org/images/0004-637X/807/2/132/apj515723ieqn4.gif] \${\textbackslash}lesssim 10{\textasciicircum}3\$ vortices extrapolate to realistic neutron stars with \#\#IMG\#\# [http://ej.iop.org/images/0004-637X/807/2/132/apj515723ieqn5.gif] \${\textbackslash}gtrsim 10{\textasciicircum}15\$ vortices. The arguments for and against extrapolation are discussed critically in the context of avalanche dynamics in self-organized critical systems, but the issue cannot be resolved without larger simulations and tighter observational limits on \#\#IMG\#\# [http://ej.iop.org/images/0004-637X/807/2/132/apj515723ieqn6.gif] \${\textbackslash}eta {\textbackslash}rm{\textbackslash}Delta {\textbackslash}phi {\textbackslash}rm{\textbackslash}Delta {\textbackslash}rmØmega _{\textbackslash}mathrmmax\$ from future LIGO (non)detections and radio timing campaigns.

@article{melatos_persistent_2015,
title = {Persistent Gravitational Radiation from Glitching Pulsars},
volume = {807},
issn = {0004-637X},
url = {http://stacks.iop.org/0004-637X/807/i=2/a=132},
doi = {10.1088/0004-637X/807/2/132},
abstract = {Quantum mechanical simulations of neutron star rotational glitches, triggered by vortex avalanches in the superfluid stellar interior, reveal that vortices pin nonaxisymmetrically to the crust during the intervals between glitches. Hence a glitching neutron star emits a persistent current quadrupole gravitational wave signal at the star’s rotation frequency, whose interglitch amplitude is constant and determined by the avalanche history since birth. The signal can be detected in principle by coherent searches planned for the Laser Interferometer Gravitational Wave Observatory (LIGO), whether or not a glitch occurs during the observation, if the power-law distribution of glitch sizes extends up to \#\#IMG\#\# [http://ej.iop.org/images/0004-637X/807/2/132/apj515723ieqn1.gif] \${\textbackslash}rm{\textbackslash}Delta {\textbackslash}rmØmega \_{\textbackslash}mathrmmax/{\textbackslash}rmØmega {\textbackslash}gtrsim 10{\textasciicircum}-6{\textbackslash}eta {\textasciicircum}-1({\textbackslash}rm{\textbackslash}Delta {\textbackslash}phi ){\textasciicircum}-1({\textbackslash}rmØmega /10{\textasciicircum}3{\textbackslash};{\textbackslash}mathrmrad{\textbackslash};{\textbackslash}rms{\textasciicircum}-1){\textasciicircum}-3(D/1{\textbackslash};{\textbackslash}mathrmkpc)\$ in the targeted object, where \#\#IMG\#\# [http://ej.iop.org/images/0004-637X/807/2/132/apj515723ieqn2.gif] \${\textbackslash}rm{\textbackslash}Delta {\textbackslash}rmØmega \_{\textbackslash}mathrmmax\$ and \#\#IMG\#\# [http://ej.iop.org/images/0004-637X/807/2/132/apj515723ieqn3.gif] \${\textbackslash}rm{\textbackslash}Delta {\textbackslash}phi \$ are the largest angular velocity jump and avalanche opening angle, respectively, to have occurred in a glitch since birth, Ω is the angular velocity at present, η is the crustal fraction of the moment of inertia, and D is the distance from the Earth. A major caveat concerning detectability is whether the nonaxisymmetries observed in existing simulations with \#\#IMG\#\# [http://ej.iop.org/images/0004-637X/807/2/132/apj515723ieqn4.gif] \${\textbackslash}lesssim 10{\textasciicircum}3\$ vortices extrapolate to realistic neutron stars with \#\#IMG\#\# [http://ej.iop.org/images/0004-637X/807/2/132/apj515723ieqn5.gif] \${\textbackslash}gtrsim 10{\textasciicircum}15\$ vortices. The arguments for and against extrapolation are discussed critically in the context of avalanche dynamics in self-organized critical systems, but the issue cannot be resolved without larger simulations and tighter observational limits on \#\#IMG\#\# [http://ej.iop.org/images/0004-637X/807/2/132/apj515723ieqn6.gif] \${\textbackslash}eta {\textbackslash}rm{\textbackslash}Delta {\textbackslash}phi {\textbackslash}rm{\textbackslash}Delta {\textbackslash}rmØmega \_{\textbackslash}mathrmmax\$ from future LIGO (non)detections and radio timing campaigns.},
number = {2},
urldate = {2016-01-04TZ},
journal = {The Astrophysical Journal},
author = {Melatos, A. and Douglass, J. A. and Simula, T. P.},
year = {2015},
pages = {132}
}

### 2014

• Precise wavefunction engineering with magnetic resonance,
L. M. Bennie, P. B. Wigley, S. S. Szigeti, M. Jasperse, J. J. Hope, L. D. Turner, and R. P. Anderson.
[arXiv] [Bibtex] [Abstract]

Controlling quantum fluids at their fundamental length scale will yield superlative quantum simulators, precision sensors, and spintronic devices. This scale is typically below the optical diffraction limit, precluding precise wavefunction engineering using optical potentials alone. We present a protocol to rapidly control the phase and density of a quantum fluid down to the healing length scale using strong time-dependent coupling between internal states of the fluid in a magnetic field gradient. We demonstrate this protocol by simulating the creation of a single stationary soliton and double soliton states in a Bose-Einstein condensate with control over the individual soliton positions and trajectories, using experimentally feasible parameters. Such states are yet to be realized experimentally, and are a path towards engineering soliton gases and exotic topological excitations.

@article{bennie_precise_2014,
title = {Precise wavefunction engineering with magnetic resonance},
url = {http://arxiv.org/abs/1412.6854},
abstract = {Controlling quantum fluids at their fundamental length scale will yield superlative quantum simulators, precision sensors, and spintronic devices. This scale is typically below the optical diffraction limit, precluding precise wavefunction engineering using optical potentials alone. We present a protocol to rapidly control the phase and density of a quantum fluid down to the healing length scale using strong time-dependent coupling between internal states of the fluid in a magnetic field gradient. We demonstrate this protocol by simulating the creation of a single stationary soliton and double soliton states in a Bose-Einstein condensate with control over the individual soliton positions and trajectories, using experimentally feasible parameters. Such states are yet to be realized experimentally, and are a path towards engineering soliton gases and exotic topological excitations.},
urldate = {2016-01-04TZ},
journal = {arXiv:1412.6854},
author = {Bennie, L. M. and Wigley, P. B. and Szigeti, S. S. and Jasperse, M. and Hope, J. J. and Turner, L. D. and Anderson, R. P.},
month = dec,
year = {2014},
eprint = {1412.6854},
keywords = {Condensed Matter - Quantum Gases, Quantum Physics}
}
• Emergence of Order from Turbulence in an Isolated Planar Superfluid,
T. Simula, M. J. Davis, and K. Helmerson.
Physical Review Letters 113, 165302 (2014).
[arXiv] [DOI] [Bibtex] [Abstract]

We study the relaxation dynamics of an isolated zero temperature quasi-two-dimensional superfluid Bose-Einstein condensate that is imprinted with a spatially random distribution of quantum vortices. Following a period of vortex annihilation the remaining vortices self-organize into two macroscopic coherent “Onsager vortex” clusters that are stable indefinitely—despite the absence of driving or external dissipation in the dynamics. We demonstrate that this occurs due to a novel physical mechanism—the evaporative heating of the vortices—that results in a negative-temperature phase transition in the vortex degrees of freedom. At the end of our simulations the system is trapped in a nonthermal state. Our computational results provide a pathway to observing Onsager vortex states in a superfluid Bose gas.

@article{simula_emergence_2014,
title = {Emergence of Order from Turbulence in an Isolated Planar Superfluid},
volume = {113},
doi = {10.1103/PhysRevLett.113.165302},
abstract = {We study the relaxation dynamics of an isolated zero temperature quasi-two-dimensional superfluid Bose-Einstein condensate that is imprinted with a spatially random distribution of quantum vortices. Following a period of vortex annihilation the remaining vortices self-organize into two macroscopic coherent “Onsager vortex” clusters that are stable indefinitely—despite the absence of driving or external dissipation in the dynamics. We demonstrate that this occurs due to a novel physical mechanism—the evaporative heating of the vortices—that results in a negative-temperature phase transition in the vortex degrees of freedom. At the end of our simulations the system is trapped in a nonthermal state. Our computational results provide a pathway to observing Onsager vortex states in a superfluid Bose gas.},
number = {16},
urldate = {2016-01-04TZ},
journal = {Physical Review Letters},
author = {Simula, Tapio and Davis, Matthew J. and Helmerson, Kristian},
month = oct,
year = {2014},
eprint = {1405.3399},
pages = {165302}
}
• Vortex Gyroscope Imaging of Planar Superfluids,
A.  T. Powis, S.  J. Sammut, and T.  P. Simula.
Physical Review Letters 113, 165303 (2014).
[arXiv] [DOI] [Bibtex] [Abstract]

We propose a robust imaging technique that makes it possible to distinguish vortices from antivortices in quasi-two-dimensional Bose-Einstein condensates from a single image of the density of the atoms. Tilting the planar condensate prior to standard absorption imaging excites a generalized gyroscopic mode of the condensate, revealing the sign and location of each vortex. This technique is anticipated to enable experimental measurement of the incompressible kinetic energy spectrum of the condensate and the observation of a negative-temperature phase transition of the vortex gas, driven by two-dimensional superfluid turbulence.

@article{powis_vortex_2014,
title = {Vortex Gyroscope Imaging of Planar Superfluids},
volume = {113},
doi = {10.1103/PhysRevLett.113.165303},
abstract = {We propose a robust imaging technique that makes it possible to distinguish vortices from antivortices in quasi-two-dimensional Bose-Einstein condensates from a single image of the density of the atoms. Tilting the planar condensate prior to standard absorption imaging excites a generalized gyroscopic mode of the condensate, revealing the sign and location of each vortex. This technique is anticipated to enable experimental measurement of the incompressible kinetic energy spectrum of the condensate and the observation of a negative-temperature phase transition of the vortex gas, driven by two-dimensional superfluid turbulence.},
number = {16},
urldate = {2016-01-04TZ},
journal = {Physical Review Letters},
author = {Powis, A. T. and Sammut, S. J. and Simula, T. P.},
month = oct,
year = {2014},
eprint = {1405.4352},
pages = {165303}
}
• Self-induced spatial dynamics to enhance spin squeezing via one-axis twisting in a two-component Bose–Einstein condensate,
S. A. Haine, J. Lau, R. P. Anderson, and M. T. Johnsson.
Physical Review A 90, 23613 (2014).
[arXiv] [DOI] [Bibtex] [Abstract]

We theoretically investigate a scheme to enhance relative number squeezing and spin squeezing in a two-component Bose-Einstein condensate (BEC) by utilizing the inherent mean-field dynamics of the condensate. Due to the asymmetry in the scattering lengths, the two components exhibit large density oscillations where they spatially separate and recombine. The effective nonlinearity responsible for the squeezing is increased by up to 3 orders of magnitude when the two components spatially separate. We perform a multimode simulation of the system using the truncated Wigner method and show that this method can be used to create significant squeezing in systems where the effective nonlinearity would ordinarily be too small to produce any significant squeezing in sensible time frames, and we show that strong spatial dynamics resulting from large particle numbers aren’t necessarily detrimental to generating squeezing. We develop a simplified semianalytic model that gives good agreement with our multimode simulation and will be useful for predicting squeezing in a range of different systems.

@article{haine_self-induced_2014,
title = {Self-induced spatial dynamics to enhance spin squeezing via one-axis twisting in a two-component {Bose--Einstein} condensate},
volume = {90},
doi = {10.1103/PhysRevA.90.023613},
abstract = {We theoretically investigate a scheme to enhance relative number squeezing and spin squeezing in a two-component Bose-Einstein condensate (BEC) by utilizing the inherent mean-field dynamics of the condensate. Due to the asymmetry in the scattering lengths, the two components exhibit large density oscillations where they spatially separate and recombine. The effective nonlinearity responsible for the squeezing is increased by up to 3 orders of magnitude when the two components spatially separate. We perform a multimode simulation of the system using the truncated Wigner method and show that this method can be used to create significant squeezing in systems where the effective nonlinearity would ordinarily be too small to produce any significant squeezing in sensible time frames, and we show that strong spatial dynamics resulting from large particle numbers aren't necessarily detrimental to generating squeezing. We develop a simplified semianalytic model that gives good agreement with our multimode simulation and will be useful for predicting squeezing in a range of different systems.},
number = {2},
urldate = {2016-01-04TZ},
journal = {Physical Review A},
author = {Haine, S. A. and Lau, J. and Anderson, R. P. and Johnsson, M. T.},
month = aug,
year = {2014},
eprint = {1402.0307},
pages = {023613}
}
• Unifying interpretations of the Gouy phase anomaly for electron waves,
T. C. Petersen, D. M. Paganin, M. Weyland, T. P. Simula, S. A. Eastwood, and M. J. Morgan.
Physical Review A 89, 63801 (2014).
[DOI] [Bibtex] [Abstract]

We provide a unified description of the Gouy phase anomaly, highlighting the pivotal role of local fluctuations in transverse momentum and quantal phase changes associated with electron trajectories that touch caustics. Using arguments based on wave optics, at the interface between classical and quantum mechanics, we derive the magnitude of the quantized Gouy phase changes for scalar waves. Our analysis unifies disparate descriptions of the Gouy anomaly within a single theoretical framework. In particular, we find that the phase anomaly connects Maslov indices, quantal Berry phases, Heisenberg momentum fluctuations, and confinement-induced Lévy-Leblond phase shifts.

@article{petersen_unifying_2014,
title = {Unifying interpretations of the Gouy phase anomaly for electron waves},
volume = {89},
doi = {10.1103/PhysRevA.89.063801},
abstract = {We provide a unified description of the Gouy phase anomaly, highlighting the pivotal role of local fluctuations in transverse momentum and quantal phase changes associated with electron trajectories that touch caustics. Using arguments based on wave optics, at the interface between classical and quantum mechanics, we derive the magnitude of the quantized Gouy phase changes for scalar waves. Our analysis unifies disparate descriptions of the Gouy anomaly within a single theoretical framework. In particular, we find that the phase anomaly connects Maslov indices, quantal Berry phases, Heisenberg momentum fluctuations, and confinement-induced Lévy-Leblond phase shifts.},
number = {6},
urldate = {2016-01-04TZ},
journal = {Physical Review A},
author = {Petersen, T. C. and Paganin, D. M. and Weyland, M. and Simula, T. P. and Eastwood, S. A. and Morgan, M. J.},
month = jun,
year = {2014},
pages = {063801}
}
• Condensed-matter physics: History matters for a stirred superfluid,
M. J. Davis and K. Helmerson.
Nature 506, 166-167 (2014).
[DOI] [Bibtex] [Abstract]

The observation of path dependence in the response of a superfluid to stirring promises potential applications in precision rotation sensing, and provides a test bed for microscopic theories of ultracold atomic gases. See Letter p.200

@article{davis_condensed-matter_2014,
title = {Condensed-matter physics: History matters for a stirred superfluid},
volume = {506},
issn = {0028-0836},
shorttitle = {Condensed-matter physics},
url = {http://www.nature.com/nature/journal/v506/n7487/full/506166a.html},
doi = {10.1038/506166a},
abstract = {The observation of path dependence in the response of a superfluid to stirring promises potential applications in precision rotation sensing, and provides a test bed for microscopic theories of ultracold atomic gases. See Letter p.200},
number = {7487},
urldate = {2016-01-04TZ},
journal = {Nature},
author = {Davis, Matthew J. and Helmerson, Kristian},
month = feb,
year = {2014},
pages = {166--167}
}

### 2013

• Diffraction catastrophes threaded by quantized vortex skeletons caused by atom-optical aberrations induced in trapped Bose–Einstein condensates,
T. P. Simula, T. C. Petersen, and D. M. Paganin.
Physical Review A 88, 43626 (2013).
[arXiv] [DOI] [Bibtex] [Abstract]

We propose a nonlinear atom-optics experiment to create diffraction catastrophes threaded by quantized vortex skeletons in Bose-Einstein-condensed matter waves. We show how atom-optical aberrations induced in trapped Bose-Einstein condensates evolve into specific caustic structures due to imperfect focusing. Vortex skeletons, whose cross sections are staggered vortex lattices, are observed to nucleate inside the universal diffraction catastrophes. Our observations shed further light on the structure and dynamics of Bose novae and suggest applications of matter wave diffraction catastrophes, including the detection of the order parameter pairing symmetry in cold gas experiments.

@article{simula_diffraction_2013,
title = {Diffraction catastrophes threaded by quantized vortex skeletons caused by atom-optical aberrations induced in trapped {Bose--Einstein} condensates},
volume = {88},
doi = {10.1103/PhysRevA.88.043626},
abstract = {We propose a nonlinear atom-optics experiment to create diffraction catastrophes threaded by quantized vortex skeletons in Bose-Einstein-condensed matter waves. We show how atom-optical aberrations induced in trapped Bose-Einstein condensates evolve into specific caustic structures due to imperfect focusing. Vortex skeletons, whose cross sections are staggered vortex lattices, are observed to nucleate inside the universal diffraction catastrophes. Our observations shed further light on the structure and dynamics of Bose novae and suggest applications of matter wave diffraction catastrophes, including the detection of the order parameter pairing symmetry in cold gas experiments.},
number = {4},
urldate = {2016-01-04TZ},
journal = {Physical Review A},
author = {Simula, T. P. and Petersen, T. C. and Paganin, D. M.},
month = oct,
year = {2013},
eprint = {1305.6413},
pages = {043626}
}
• Measurement of the Gouy phase anomaly for electron waves,
T. C. Petersen, D. M. Paganin, M. Weyland, T. P. Simula, S. A. Eastwood, and M. J. Morgan.
Physical Review A 88, 43803 (2013).
[DOI] [Bibtex] [Abstract]

We measure the Gouy phase anomaly for matter waves using in-line holography to retrieve the full complex field of an astigmatic electron wave function. Sequential phase shifts of π/2 rad are observed for electron trajectories along the optic axis that pass through each line-focus caustic of subnanometer transverse width. Our observations demonstrate that anomalous phase shifts of matter waves in the vicinity of caustics can be robustly measured using phase retrieval, extending the current scope of singular electron optics.

@article{petersen_measurement_2013,
title = {Measurement of the Gouy phase anomaly for electron waves},
volume = {88},
doi = {10.1103/PhysRevA.88.043803},
abstract = {We measure the Gouy phase anomaly for matter waves using in-line holography to retrieve the full complex field of an astigmatic electron wave function. Sequential phase shifts of π/2 rad are observed for electron trajectories along the optic axis that pass through each line-focus caustic of subnanometer transverse width. Our observations demonstrate that anomalous phase shifts of matter waves in the vicinity of caustics can be robustly measured using phase retrieval, extending the current scope of singular electron optics.},
number = {4},
urldate = {2016-01-04TZ},
journal = {Physical Review A},
author = {Petersen, T. C. and Paganin, D. M. and Weyland, M. and Simula, T. P. and Eastwood, S. A. and Morgan, M. J.},
month = oct,
year = {2013},
pages = {043803}
}
• A scripted control system for autonomous hardware-timed experiments,
P. T. Starkey, C. J. Billington, S. P. Johnstone, M. Jasperse, K. Helmerson, L. D. Turner, and R. P. Anderson.
Review of Scientific Instruments 84, 85111 (2013).
[arXiv] [DOI] [Bibtex] [Abstract]

We present the labscript suite, an open-source experiment control system for automating shot-based experiments and their analysis. Experiments are composed as Python code, which is used to produce low-level hardware instructions. They are queued up and executed on the hardware in real time, synchronized by a pseudoclock. Experiment parameters are manipulated graphically, and analysis routines are run as new data are acquired. With this system, we can easily automate exploration of parameter spaces, including closed-loop optimization.

@article{starkey_scripted_2013,
title = {{A} scripted control system for autonomous hardware-timed experiments},
volume = {84},
issn = {0034-6748, 1089-7623},
url = {http://scitation.aip.org/content/aip/journal/rsi/84/8/10.1063/1.4817213},
doi = {10.1063/1.4817213},
abstract = {We present the labscript suite, an open-source experiment control system for automating shot-based experiments and their analysis. Experiments are composed as Python code, which is used to produce low-level hardware instructions. They are queued up and executed on the hardware in real time, synchronized by a pseudoclock. Experiment parameters are manipulated graphically, and analysis routines are run as new data are acquired. With this system, we can easily automate exploration of parameter spaces, including closed-loop optimization.},
number = {8},
urldate = {2014-04-25TZ},
journal = {Review of Scientific Instruments},
author = {Starkey, P. T. and Billington, C. J. and Johnstone, S. P. and Jasperse, M. and Helmerson, K. and Turner, L. D. and Anderson, R. P.},
month = aug,
year = {2013},
eprint = {1303.0080},
pages = {085111}
}
• Vibrations of a columnar vortex in a trapped Bose–Einstein condensate,
L. Koens, T. P. Simula, and A. M. Martin.
Physical Review A 87, 63614 (2013).
[arXiv] [DOI] [Bibtex] [Abstract]

We derive a governing equation for a Kelvin wave supported on a vortex line in a Bose-Einstein condensate, in a rotating cylindrically symmetric parabolic trap, where it is assumed that the shape of the vortex line is dominated by the properties of the condensate at the center of the trap. From this solution the Kelvin wave dispersion relation is determined. In the limit of an oblate trap and in the absence of longitudinal trapping our results are consistent with previous work. We show that the derived Kelvin wave dispersion in the general case is in quantitative agreement with numerical calculations of the Bogoliubov spectrum.

@article{koens_vibrations_2013,
title = {Vibrations of a columnar vortex in a trapped {Bose--Einstein} condensate},
volume = {87},
doi = {10.1103/PhysRevA.87.063614},
abstract = {We derive a governing equation for a Kelvin wave supported on a vortex line in a Bose-Einstein condensate, in a rotating cylindrically symmetric parabolic trap, where it is assumed that the shape of the vortex line is dominated by the properties of the condensate at the center of the trap. From this solution the Kelvin wave dispersion relation is determined. In the limit of an oblate trap and in the absence of longitudinal trapping our results are consistent with previous work. We show that the derived Kelvin wave dispersion in the general case is in quantitative agreement with numerical calculations of the Bogoliubov spectrum.},
number = {6},
urldate = {2016-01-04TZ},
journal = {Physical Review A},
author = {Koens, Lyndon and Simula, Tapio P. and Martin, Andrew M.},
month = jun,
year = {2013},
eprint = {1208.1313},
pages = {063614}
}
• PHI: A powerful new program for the analysis of anisotropic monomeric and exchange-coupled polynuclear d- and f-block complexes,
N. F. Chilton, R. P. Anderson, L. D. Turner, A. Soncini, and K. S. Murray.
Journal of Computational Chemistry 34, 1164-1175 (2013).
[DOI] [Bibtex] [Abstract]

A new program, PHI, with the ability to calculate the magnetic properties of large spin systems and complex orbitally degenerate systems, such as clusters of d-block and f-block ions, is presented. The program can intuitively fit experimental data from multiple sources, such as magnetic and spectroscopic data, simultaneously. PHI is extensively parallelized and can operate under the symmetric multiprocessing, single process multiple data, or GPU paradigms using a threaded, MPI or GPU model, respectively. For a given problem PHI is been shown to be almost 12 times faster than the well-known program MAGPACK, limited only by available hardware. © 2013 Wiley Periodicals, Inc.

@article{chilton_phi:_2013,
title = {{PHI:} {A} powerful new program for the analysis of anisotropic monomeric and exchange-coupled polynuclear d- and f-block complexes},
volume = {34},
issn = {1096-987X},
shorttitle = {{PHI}},
url = {http://onlinelibrary.wiley.com/doi/10.1002/jcc.23234/abstract},
doi = {10.1002/jcc.23234},
abstract = {A new program, PHI, with the ability to calculate the magnetic properties of large spin systems and complex orbitally degenerate systems, such as clusters of d-block and f-block ions, is presented. The program can intuitively fit experimental data from multiple sources, such as magnetic and spectroscopic data, simultaneously. PHI is extensively parallelized and can operate under the symmetric multiprocessing, single process multiple data, or GPU paradigms using a threaded, MPI or GPU model, respectively. For a given problem PHI is been shown to be almost 12 times faster than the well-known program MAGPACK, limited only by available hardware. © 2013 Wiley Periodicals, Inc.},
number = {13},
urldate = {2014-04-25TZ},
journal = {Journal of Computational Chemistry},
author = {Chilton, Nicholas F. and Anderson, Russell P. and Turner, Lincoln D. and Soncini, Alessandro and Murray, Keith S.},
month = may,
year = {2013},
pages = {1164--1175}
}
• A versatile high resolution objective for imaging quantum gases,
L. M. Bennie, P. T. Starkey, M. Jasperse, C. J. Billington, R. P. Anderson, and L. D. Turner.
Optics Express 21, 9011-9016 (2013).
[arXiv] [DOI] [Bibtex] [Abstract]

We present a high resolution objective lens made entirely from catalog singlets that has a numerical aperture of 0.36. It corrects for aberrations introduced by a glass window and has a long working distance of 35 mm, making it suitable for imaging objects within a vacuum system. This offers simple high resolution imaging for many in the quantum gas community. The objective achieves a resolution of 1.3 μm at the design wavelength of 780 nm, and a diffraction-limited field of view of 360 μm when imaging through a 5 mm thick window. Images of a resolution target and a pinhole show quantitative agreement with the simulated lens performance. The objective is suitable for diffraction-limited monochromatic imaging on the D2 line of all the alkalis by changing only the aperture diameter, retaining numerical apertures above 0.32. The design corrects for window thicknesses of up to 15 mm if the singlet spacings are modified.

@article{bennie_versatile_2013,
title = {{A} versatile high resolution objective for imaging quantum gases},
volume = {21},
url = {http://www.opticsexpress.org/abstract.cfm?URI=oe-21-7-9011},
doi = {10.1364/OE.21.009011},
abstract = {We present a high resolution objective lens made entirely from catalog singlets that has a numerical aperture of 0.36. It corrects for aberrations introduced by a glass window and has a long working distance of 35 mm, making it suitable for imaging objects within a vacuum system. This offers simple high resolution imaging for many in the quantum gas community. The objective achieves a resolution of 1.3 μm at the design wavelength of 780 nm, and a diffraction-limited field of view of 360 μm when imaging through a 5 mm thick window. Images of a resolution target and a pinhole show quantitative agreement with the simulated lens performance. The objective is suitable for diffraction-limited monochromatic imaging on the D2 line of all the alkalis by changing only the aperture diameter, retaining numerical apertures above 0.32. The design corrects for window thicknesses of up to 15 mm if the singlet spacings are modified.},
number = {7},
urldate = {2014-04-25TZ},
journal = {Optics Express},
author = {Bennie, L. M. and Starkey, P. T. and Jasperse, M. and Billington, C. J. and Anderson, R. P. and Turner, L. D.},
month = apr,
year = {2013},
eprint = {1302.4166},
pages = {9011--9016}
}
• Collective dynamics of vortices in trapped Bose–Einstein condensates,
T. Simula.
Physical Review A 87, 23630 (2013).
[arXiv] [DOI] [Bibtex] [Abstract]

We have calculated collective-mode spectra for three-dimensional, rotating Bose-Einstein condensates in oblate harmonic traps using the microscopic Bogoliubov–de Gennes field theory. For condensates with Nv vortices, Nv Kelvin-Tkachenko-mode branches are obtained. The features of these modes are compared with those predicted by a classical point-vortex model. We have created movies to visualize the motion of the vortices corresponding to the Kelvin-Tkachenko waves.

@article{simula_collective_2013,
title = {Collective dynamics of vortices in trapped {Bose--Einstein} condensates},
volume = {87},
doi = {10.1103/PhysRevA.87.023630},
abstract = {We have calculated collective-mode spectra for three-dimensional, rotating Bose-Einstein condensates in oblate harmonic traps using the microscopic Bogoliubov–de Gennes field theory. For condensates with Nv vortices, Nv Kelvin-Tkachenko-mode branches are obtained. The features of these modes are compared with those predicted by a classical point-vortex model. We have created movies to visualize the motion of the vortices corresponding to the Kelvin-Tkachenko waves.},
number = {2},
urldate = {2016-01-04TZ},
journal = {Physical Review A},
author = {Simula, Tapio},
month = feb,
year = {2013},
eprint = {1303.0947},
pages = {023630}
}
• Wideband laser locking to an atomic reference with modulation transfer spectroscopy,
V. Negnevitsky and L. D. Turner.
Optics Express 21, 3103 (2013).
[arXiv] [DOI] [Bibtex] [Abstract]

We demonstrate that conventional modulated spectroscopy apparatus, used for laser frequency stabilization in many atomic physics laboratories, can be enhanced to provide a wideband lock delivering deep suppression of frequency noise across the acoustic range. Using an acousto-optic modulator driven with an agile oscillator, we show that wideband frequency modulation of the pump laser in modulation transfer spectroscopy produces the unique single lock-point spectrum previously demonstrated with electro-optic phase modulation. We achieve a laser lock with 100 kHz feedback bandwidth, limited by our laser control electronics. This bandwidth is sufficient to reduce frequency noise by 30 dB across the acoustic range and narrows the imputed linewidth by a factor of five.

@article{negnevitsky_wideband_2013,
title = {Wideband laser locking to an atomic reference with modulation transfer spectroscopy},
volume = {21},
issn = {1094-4087},
url = {https://www.osapublishing.org/oe/abstract.cfm?uri=oe-21-3-3103},
doi = {10.1364/OE.21.003103},
abstract = {We demonstrate that conventional modulated spectroscopy apparatus, used for laser frequency stabilization in many atomic physics laboratories, can be enhanced to provide a wideband lock delivering deep suppression of frequency noise across the acoustic range. Using an acousto-optic modulator driven with an agile oscillator, we show that wideband frequency modulation of the pump laser in modulation transfer spectroscopy produces the unique single lock-point spectrum previously demonstrated with electro-optic phase modulation. We achieve a laser lock with 100 kHz feedback bandwidth, limited by our laser control electronics. This bandwidth is sufficient to reduce frequency noise by 30 dB across the acoustic range and narrows the imputed linewidth by a factor of five.},
number = {3},
urldate = {2016-01-04TZ},
journal = {Optics Express},
author = {Negnevitsky, V. and Turner, L. D.},
month = feb,
year = {2013},
eprint = {1204.5240},
pages = {3103}
}
• Precision atomic gravimeter based on Bragg diffraction,
P. A. Altin, M. T. Johnsson, V. Negnevitsky, G. R. Dennis, R. P. Anderson, J. E. Debs, S. S. Szigeti, K. S. Hardman, S. Bennetts, G. D. McDonald, L. D. Turner, J. D. Close, and N. P. Robins.
New Journal of Physics 15, 23009 (2013).
[arXiv] [DOI] [Bibtex] [Abstract]

We present a precision gravimeter based on coherent Bragg diffraction of freely falling cold atoms. Traditionally, atomic gravimeters have used stimulated Raman transitions to separate clouds in momentum space by driving transitions between two internal atomic states. Bragg interferometers utilize only a single internal state, and can therefore be less susceptible to environmental perturbations. Here we show that atoms extracted from a magneto-optical trap using an accelerating optical lattice are a suitable source for a Bragg atom interferometer, allowing efficient beamsplitting and subsequent separation of momentum states for detection. Despite the inherently multi-state nature of atom diffraction, we are able to build a Mach–Zehnder interferometer using Bragg scattering which achieves a sensitivity to the gravitational acceleration of Δg/g = 2.7 × 10−9 with an integration time of 1000 s. The device can also be converted to a gravity gradiometer by a simple modification of the light pulse sequence.

@article{altin_precision_2013,
title = {Precision atomic gravimeter based on {Bragg} diffraction},
volume = {15},
issn = {1367-2630},
url = {http://iopscience.iop.org/1367-2630/15/2/023009},
doi = {10.1088/1367-2630/15/2/023009},
abstract = {We present a precision gravimeter based on coherent Bragg diffraction of freely falling cold atoms. Traditionally, atomic gravimeters have used stimulated Raman transitions to separate clouds in momentum space by driving transitions between two internal atomic states. Bragg interferometers utilize only a single internal state, and can therefore be less susceptible to environmental perturbations. Here we show that atoms extracted from a magneto-optical trap using an accelerating optical lattice are a suitable source for a Bragg atom interferometer, allowing efficient beamsplitting and subsequent separation of momentum states for detection. Despite the inherently multi-state nature of atom diffraction, we are able to build a Mach–Zehnder interferometer using Bragg scattering which achieves a sensitivity to the gravitational acceleration of Δg/g = 2.7 × 10−9 with an integration time of 1000 s. The device can also be converted to a gravity gradiometer by a simple modification of the light pulse sequence.},
number = {2},
urldate = {2014-04-25TZ},
journal = {New Journal of Physics},
author = {Altin, P. A. and Johnsson, M. T. and Negnevitsky, V. and Dennis, G. R. and Anderson, R. P. and Debs, J. E. and Szigeti, S. S. and Hardman, K. S. and Bennetts, S. and McDonald, G. D. and Turner, L. D. and Close, J. D. and Robins, N. P.},
month = feb,
year = {2013},
eprint = {1207.1595},
pages = {023009}
}
• Electron Vortex Production and Control Using Aberration Induced Diffraction Catastrophes,
T. C. Petersen, M. Weyland, D. M. Paganin, T. P. Simula, S. A. Eastwood, and M. J. Morgan.
Physical Review Letters 110, 33901 (2013).
[DOI] [Bibtex] [Abstract]

An aberration corrected electron microscope is used to create electron diffraction catastrophes, containing arrays of intensity zeros threading vortex cores. Vortices are ascribed to these arrays using catastrophe theory, scalar diffraction integrals, and experimentally retrieved phase maps. From measured wave function phases, obtained using focal-series phase retrieval, the orbital angular momentum density is mapped for highly astigmatic electron probes. We observe vortex rings and topological reconnections of nodal lines by tracking the vortex cores using the retrieved phases.

@article{petersen_electron_2013,
title = {Electron Vortex Production and Control Using Aberration Induced Diffraction Catastrophes},
volume = {110},
doi = {10.1103/PhysRevLett.110.033901},
abstract = {An aberration corrected electron microscope is used to create electron diffraction catastrophes, containing arrays of intensity zeros threading vortex cores. Vortices are ascribed to these arrays using catastrophe theory, scalar diffraction integrals, and experimentally retrieved phase maps. From measured wave function phases, obtained using focal-series phase retrieval, the orbital angular momentum density is mapped for highly astigmatic electron probes. We observe vortex rings and topological reconnections of nodal lines by tracking the vortex cores using the retrieved phases.},
number = {3},
urldate = {2016-01-04TZ},
journal = {Physical Review Letters},
author = {Petersen, T. C. and Weyland, M. and Paganin, D. M. and Simula, T. P. and Eastwood, S. A. and Morgan, M. J.},
month = jan,
year = {2013},
pages = {033901}
}
• Zero-energy states in rotating trapped Bose–Einstein condensates,
T. Simula.
Journal of Physics: Condensed Matter 25, 285602 (2013).
[arXiv] [DOI] [Bibtex] [Abstract]

We have calculated low-lying quasiparticle excitation spectra of rotating three-dimensional Bose–Einstein condensates. We find, as opposed to the prediction of hydrodynamic continuum theories, a minimum in the Tkachenko mode spectrum at intermediate rotation frequencies of the harmonic trap. Such a minimum can harbour a Tkachenko quasiparticle with zero excitation energy. We discuss the experimental signatures of such a zero mode.

@article{simula_zero-energy_2013,
title = {Zero-energy states in rotating trapped {Bose}–{Einstein} condensates},
volume = {25},
issn = {0953-8984},
url = {http://stacks.iop.org/0953-8984/25/i=28/a=285602},
doi = {10.1088/0953-8984/25/28/285602},
abstract = {We have calculated low-lying quasiparticle excitation spectra of rotating three-dimensional Bose–Einstein condensates. We find, as opposed to the prediction of hydrodynamic continuum theories, a minimum in the Tkachenko mode spectrum at intermediate rotation frequencies of the harmonic trap. Such a minimum can harbour a Tkachenko quasiparticle with zero excitation energy. We discuss the experimental signatures of such a zero mode.},
number = {28},
urldate = {2016-01-04TZ},
journal = {Journal of Physics: Condensed Matter},
author = {Simula, Tapio},
year = {2013},
eprint = {1207.5892},
pages = {285602}
}

### 2012

• Partial-transfer absorption imaging: A versatile technique for optimal imaging of ultracold gases,
A. Ramanathan, S. R. Muniz, K. C. Wright, R. P. Anderson, W. D. Phillips, K. Helmerson, and G. K. Campbell.
Review of Scientific Instruments 83, 83119 (2012).
[arXiv] [DOI] [Bibtex] [Abstract]

Partial-transfer absorption imaging is a tool that enables optimal imaging of atomic clouds for a wide range of optical depths. In contrast to standard absorption imaging, the technique can be minimally destructive and can be used to obtain multiple successive images of the same sample. The technique involves transferring a small fraction of the sample from an initial internal atomic state to an auxiliary state and subsequently imaging that fraction absorptively on a cycling transition. The atoms remaining in the initial state are essentially unaffected. We demonstrate the technique, discuss its applicability, and compare its performance as a minimally destructive technique to that of phase-contrast imaging.

@article{ramanathan_partial-transfer_2012,
title = {Partial-transfer absorption imaging: {A} versatile technique for optimal imaging of ultracold gases},
volume = {83},
issn = {0034-6748, 1089-7623},
shorttitle = {Partial-transfer absorption imaging},
url = {http://scitation.aip.org/content/aip/journal/rsi/83/8/10.1063/1.4747163},
doi = {10.1063/1.4747163},
abstract = {Partial-transfer absorption imaging is a tool that enables optimal imaging of atomic clouds for a wide range of optical depths. In contrast to standard absorption imaging, the technique can be minimally destructive and can be used to obtain multiple successive images of the same sample. The technique involves transferring a small fraction of the sample from an initial internal atomic state to an auxiliary state and subsequently imaging that fraction absorptively on a cycling transition. The atoms remaining in the initial state are essentially unaffected. We demonstrate the technique, discuss its applicability, and compare its performance as a minimally destructive technique to that of phase-contrast imaging.},
number = {8},
urldate = {2014-04-25TZ},
journal = {Review of Scientific Instruments},
author = {Ramanathan, Anand and Muniz, Sérgio R. and Wright, Kevin C. and Anderson, Russell P. and Phillips, William D. and Helmerson, Kristian and Campbell, Gretchen K.},
month = aug,
year = {2012},
eprint = {1206.7048},
pages = {083119}
}
• Synthetic charge-flux quantum liquids,
T. P. Simula.
Physical Review B 85, 144521 (2012).
[arXiv] [DOI] [Bibtex] [Abstract]

We apply rotating optical flux lattices to spinor Bose-Einstein condensates. Distinct quantum states emerge for fractional ratios of vortex charge density to optical flux density. We exhibit the calculated charge-flux states and discuss their topological structure and experimental signatures.

@article{simula_synthetic_2012,
title = {Synthetic charge-flux quantum liquids},
volume = {85},
doi = {10.1103/PhysRevB.85.144521},
abstract = {We apply rotating optical flux lattices to spinor Bose-Einstein condensates. Distinct quantum states emerge for fractional ratios of vortex charge density to optical flux density. We exhibit the calculated charge-flux states and discuss their topological structure and experimental signatures.},
number = {14},
urldate = {2016-01-04TZ},
journal = {Physical Review B},
author = {Simula, Tapio P.},
month = apr,
year = {2012},
eprint = {1201.4636},
pages = {144521}
}
• Coherence simplices,
T. P. Simula and D. M. Paganin.
New Journal of Physics 14, 113015 (2012).
[arXiv] [DOI] [Bibtex] [Abstract]

Coherence simplices are generic topological correlation-function defects supported by a hierarchy of coherence functions. We classify coherence simplices based on their topology and discuss their structure and dynamics, together with their relevance to several physical systems.

@article{simula_coherence_2012,
title = {Coherence simplices},
volume = {14},
issn = {1367-2630},
url = {http://stacks.iop.org/1367-2630/14/i=11/a=113015},
doi = {10.1088/1367-2630/14/11/113015},
abstract = {Coherence simplices are generic topological correlation-function defects supported by a hierarchy of coherence functions. We classify coherence simplices based on their topology and discuss their structure and dynamics, together with their relevance to several physical systems.},
number = {11},
urldate = {2016-01-04TZ},
journal = {New Journal of Physics},
author = {Simula, Tapio P. and Paganin, David M.},
year = {2012},
eprint = {1111.5114},
pages = {113015}
}

### 2011

• Coherence vortices in one spatial dimension,
T. P. Simula and D. M. Paganin.
Physical Review A 84, 52104 (2011).
[arXiv] [DOI] [Bibtex] [Abstract]

Coherence vortices are screw-type topological defects in the phase of Glauber’s two-point degree of quantum coherence, associated with pairs of spatial points at which an ensemble-averaged stochastic quantum field is uncorrelated. Coherence vortices may be present in systems whose dimensionality is too low to support spatial vortices. We exhibit lattices of such quantum-coherence phase defects for a one-dimensional model quantum system. We discuss the physical meaning of coherence vortices and propose how they may be realized experimentally.

@article{simula_coherence_2011,
title = {Coherence vortices in one spatial dimension},
volume = {84},
doi = {10.1103/PhysRevA.84.052104},
abstract = {Coherence vortices are screw-type topological defects in the phase of Glauber's two-point degree of quantum coherence, associated with pairs of spatial points at which an ensemble-averaged stochastic quantum field is uncorrelated. Coherence vortices may be present in systems whose dimensionality is too low to support spatial vortices. We exhibit lattices of such quantum-coherence phase defects for a one-dimensional model quantum system. We discuss the physical meaning of coherence vortices and propose how they may be realized experimentally.},
number = {5},
urldate = {2016-01-04TZ},
journal = {Physical Review A},
author = {Simula, Tapio P. and Paganin, David M.},
month = nov,
year = {2011},
eprint = {1111.5096},
pages = {052104}
}
• Cold-atom gravimetry with a Bose–Einstein condensate,
J. E. Debs, P. A. Altin, T. H. Barter, D. Döring, G. R. Dennis, G. McDonald, R. P. Anderson, J. D. Close, and N. P. Robins.
Physical Review A 84, 33610 (2011).
[arXiv] [DOI] [Bibtex] [Abstract]

We present a cold-atom gravimeter operating with a sample of Bose-condensed 87Rb atoms. Using a Mach-Zehnder configuration with the two arms separated by a two-photon Bragg transition, we observe interference fringes with a visibility of (83±6)\% at T=3 ms. We exploit large momentum transfer (LMT) beam splitting to increase the enclosed space-time area of the interferometer using higher-order Bragg transitions and Bloch oscillations. We also compare fringes from condensed and thermal sources and observe a reduced visibility of (58±4)\% for the thermal source. We suspect the loss in visibility is caused partly by wave-front aberrations, to which the thermal source is more susceptible due to its larger transverse momentum spread. Finally, we discuss briefly the potential advantages of using a coherent atomic source for LMT, and we present a simple mean-field model to demonstrate that with currently available experimental parameters, interaction-induced dephasing will not limit the sensitivity of inertial measurements using freely falling, coherent atomic sources.

@article{debs_cold-atom_2011,
title = {Cold-atom gravimetry with a {Bose--Einstein} condensate},
volume = {84},
doi = {10.1103/PhysRevA.84.033610},
abstract = {We present a cold-atom gravimeter operating with a sample of Bose-condensed 87Rb atoms. Using a Mach-Zehnder configuration with the two arms separated by a two-photon Bragg transition, we observe interference fringes with a visibility of (83±6)\% at T=3 ms. We exploit large momentum transfer (LMT) beam splitting to increase the enclosed space-time area of the interferometer using higher-order Bragg transitions and Bloch oscillations. We also compare fringes from condensed and thermal sources and observe a reduced visibility of (58±4)\% for the thermal source. We suspect the loss in visibility is caused partly by wave-front aberrations, to which the thermal source is more susceptible due to its larger transverse momentum spread. Finally, we discuss briefly the potential advantages of using a coherent atomic source for LMT, and we present a simple mean-field model to demonstrate that with currently available experimental parameters, interaction-induced dephasing will not limit the sensitivity of inertial measurements using freely falling, coherent atomic sources.},
number = {3},
urldate = {2012-02-06TZ},
journal = {Physical Review A},
author = {Debs, J. E. and Altin, P. A. and Barter, T. H. and Döring, D. and Dennis, G. R. and McDonald, G. and Anderson, R. P. and Close, J. D. and Robins, N. P.},
month = sep,
year = {2011},
eprint = {1011.5804},
pages = {033610}
}
• Long-lived periodic revivals of coherence in an interacting Bose–Einstein condensate,
M. Egorov, R. P. Anderson, V. Ivannikov, B. Opanchuk, P. Drummond, B. V. Hall, and A. I. Sidorov.
Physical Review A 84, 21605 (2011).
[arXiv] [DOI] [Bibtex] [Abstract]

We observe the coherence of an interacting two-component Bose-Einstein condensate (BEC) surviving for seconds in a trapped Ramsey interferometer. Mean-field-driven collective oscillations of two components lead to periodic dephasing and rephasing of condensate wave functions with a slow decay of the interference fringe visibility. We apply spin echo synchronous with the self-rephasing of the condensate to reduce the influence of state-dependent atom losses, significantly enhancing the visibility up to 0.75 at the evolution time of 1.5 s. Mean-field theory consistently predicts higher visibility than experimentally observed values. We quantify the effects of classical and quantum noise and infer a coherence time of 2.8 s for a trapped condensate of 5.5×104 interacting atoms.

@article{egorov_long-lived_2011,
title = {Long-lived periodic revivals of coherence in an interacting {Bose--Einstein} condensate},
volume = {84},
doi = {10.1103/PhysRevA.84.021605},
abstract = {We observe the coherence of an interacting two-component Bose-Einstein condensate (BEC) surviving for seconds in a trapped Ramsey interferometer. Mean-field-driven collective oscillations of two components lead to periodic dephasing and rephasing of condensate wave functions with a slow decay of the interference fringe visibility. We apply spin echo synchronous with the self-rephasing of the condensate to reduce the influence of state-dependent atom losses, significantly enhancing the visibility up to 0.75 at the evolution time of 1.5 s. Mean-field theory consistently predicts higher visibility than experimentally observed values. We quantify the effects of classical and quantum noise and infer a coherence time of 2.8 s for a trapped condensate of 5.5×104 interacting atoms.},
number = {2},
urldate = {2012-02-06TZ},
journal = {Physical Review A},
author = {Egorov, M. and Anderson, R. P. and Ivannikov, V. and Opanchuk, B. and Drummond, P. and Hall, B. V. and Sidorov, A. I.},
month = aug,
year = {2011},
eprint = {1012.3813},
pages = {021605}
}
• Crow instability in trapped Bose–Einstein condensates,
T. P. Simula.
Physical Review A 84, 21603 (2011).
[arXiv] [DOI] [Bibtex] [Abstract]

We show theoretically that elongated vortex-antivortex dipoles can be created controllably in trapped Bose-Einstein condensates, using known experimental techniques. Vortex dipoles of sufficient length are unstable and cascade into slow vortex rings which ultimately decay via sound emission. This instability of antiparallel vortex line elements, which self-generates Kelvin waves on vortex loops in trapped atomic gases, may play a role in bridging the Kelvin-wave and Kolmogorov-Richardson cascades of quantum turbulence.

@article{simula_crow_2011,
title = {Crow instability in trapped {Bose--Einstein} condensates},
volume = {84},
doi = {10.1103/PhysRevA.84.021603},
abstract = {We show theoretically that elongated vortex-antivortex dipoles can be created controllably in trapped Bose-Einstein condensates, using known experimental techniques. Vortex dipoles of sufficient length are unstable and cascade into slow vortex rings which ultimately decay via sound emission. This instability of antiparallel vortex line elements, which self-generates Kelvin waves on vortex loops in trapped atomic gases, may play a role in bridging the Kelvin-wave and Kolmogorov-Richardson cascades of quantum turbulence.},
number = {2},
urldate = {2016-01-04TZ},
journal = {Physical Review A},
author = {Simula, Tapio P.},
month = aug,
year = {2011},
eprint = {1107.3379},
pages = {021603}
}
• Optically trapped atom interferometry using the clock transition of large $^{87}$Rb Bose–Einstein condensates,
P. A. Altin, G. McDonald, D. Döring, J. E. Debs, T. H. Barter, J. D. Close, N. P. Robins, S. A. Haine, T. M. Hanna, and R. P. Anderson.
New Journal of Physics 13, 65020 (2011).
[arXiv] [DOI] [Bibtex]
@article{altin_optically_2011,
title = {Optically trapped atom interferometry using the clock transition of large {$^{87}$Rb} {Bose}–{Einstein} condensates},
volume = {13},
issn = {1367-2630},
url = {http://iopscience.iop.org/1367-2630/13/6/065020},
doi = {10.1088/1367-2630/13/6/065020},
number = {6},
urldate = {2012-02-06TZ},
journal = {New Journal of Physics},
author = {Altin, P A and McDonald, G and Döring, D and Debs, J E and Barter, T H and Close, J D and Robins, N P and Haine, S A and Hanna, T M and Anderson, R P},
month = jun,
year = {2011},
eprint = {1011.4713},
pages = {065020}
}
• Relative intensity squeezing by four-wave mixing with loss: an analytic model and experimental diagnostic,
M. Jasperse, L. D. Turner, and R. E. Scholten.
Optics Express 19, 3765 (2011).
[arXiv] [DOI] [Bibtex] [Abstract]

Four-wave mixing near resonance in an atomic vapor can produce relative intensity squeezed light suitable for precision measurements beyond the shot-noise limit. We develop an analytic distributed gain/loss model to describe the competition of mixing and absorption through the non-linear medium. Using a novel matrix calculus, we present closed-form expressions for the degree of relative intensity squeezing produced by this system. We use these theoretical results to analyze experimentally measured squeezing from a 85Rb vapor and demonstrate the analytic model’s utility as an experimental diagnostic.

@article{jasperse_relative_2011,
title = {Relative intensity squeezing by four-wave mixing with loss: an analytic model and experimental diagnostic},
volume = {19},
issn = {1094-4087},
shorttitle = {Relative intensity squeezing by four-wave mixing with loss},
url = {https://www.osapublishing.org/oe/abstract.cfm?uri=oe-19-4-3765},
doi = {10.1364/OE.19.003765},
abstract = {Four-wave mixing near resonance in an atomic vapor can produce relative intensity squeezed light suitable for precision measurements beyond the shot-noise limit. We develop an analytic distributed gain/loss model to describe the competition of mixing and absorption through the non-linear medium. Using a novel matrix calculus, we present closed-form expressions for the degree of relative intensity squeezing produced by this system. We use these theoretical results to analyze experimentally measured squeezing from a 85Rb vapor and demonstrate the analytic model’s utility as an experimental diagnostic.},
number = {4},
urldate = {2016-01-04TZ},
journal = {Optics Express},
author = {Jasperse, M. and Turner, L. D. and Scholten, R. E.},
month = feb,
year = {2011},
eprint = {1012.3482},
pages = {3765}
}
• Rotating Atoms with Light
K. Helmerson and W. D. Phillips.
In Twisted Photons, J. P. Torres and L. Torner, editors. Wiley-VCH Verlag GmbH & Co. KGaA, 2011, pp. 213-235.
[DOI] [Bibtex] [Abstract]

This chapter explains the techniques developed for manipulating and observing the rotational states of atoms using lasers beams that carry orbital angular momentum (OAM). It describes applications of these techniques for generating and studying persistent currents in a superfluid atomic gas confined in a ring-shaped container. In the case of linear momentum, the mechanical effects of light range from comet tails to laser cooling of atoms. The creation of Bose-Einstein condensates (BECs) in dilute atomic vapors is one of the major triumphs of the quest to control atoms. The chapter describes experiments generating vortex states of higher angular momentum and vortex states in spinor BECs. It also describes a matter wave amplification experiment on a vortex state. Although vortices have been generated and observed in atomic quantum degenerate gases, the related phenomena of persistent or supercurrents have not been clearly observed in atomic BECs.

@incollection{helmerson_rotating_2011,
title = {Rotating Atoms with Light},
isbn = {978-3-527-63536-8},
url = {http://onlinelibrary.wiley.com/doi/10.1002/9783527635368.ch12/summary},
abstract = {This chapter explains the techniques developed for manipulating and observing the rotational states of atoms using lasers beams that carry orbital angular momentum (OAM). It describes applications of these techniques for generating and studying persistent currents in a superfluid atomic gas confined in a ring-shaped container. In the case of linear momentum, the mechanical effects of light range from comet tails to laser cooling of atoms. The creation of Bose-Einstein condensates (BECs) in dilute atomic vapors is one of the major triumphs of the quest to control atoms. The chapter describes experiments generating vortex states of higher angular momentum and vortex states in spinor BECs. It also describes a matter wave amplification experiment on a vortex state. Although vortices have been generated and observed in atomic quantum degenerate gases, the related phenomena of persistent or supercurrents have not been clearly observed in atomic BECs.},
urldate = {2016-01-04TZ},
booktitle = {Twisted Photons},
publisher = {Wiley-VCH Verlag GmbH \& Co. KGaA},
author = {Helmerson, Kristian and Phillips, William D.},
editor = {Torres, Juan P. and Torner, Lluis},
year = {2011},
doi = {10.1002/9783527635368.ch12},
pages = {213--235}
}

### 2010

• Rotating Dipolar Spin-1 Bose–Einstein Condensates,
T. P. Simula, J. A. M. Huhtamäki, M. Takahashi, T. Mizushima, and K. Machida.
Journal of the Physical Society of Japan 80, 13001 (2010).
[arXiv] [DOI] [Bibtex] [Abstract]

We have computed phase diagrams for rotating spin-1 Bose–Einstein condensates with long-range magnetic dipole–dipole interactions. Spin textures including vortex sheets, staggered half-quantum- and skyrmion vortex lattices and higher order topological defects have been found. These systems exhibit both superfluidity and magnetic crystalline ordering and they could be realized experimentally by imparting angular momentum in the spinor condensate.

@article{simula_rotating_2010,
title = {Rotating Dipolar Spin-1 {Bose}–{Einstein} Condensates},
volume = {80},
issn = {0031-9015},
url = {http://journals.jps.jp/doi/abs/10.1143/JPSJ.80.013001},
doi = {10.1143/JPSJ.80.013001},
abstract = {We have computed phase diagrams for rotating spin-1 Bose–Einstein condensates with long-range magnetic dipole–dipole interactions. Spin textures including vortex sheets, staggered half-quantum- and skyrmion vortex lattices and higher order topological defects have been found. These systems exhibit both superfluidity and magnetic crystalline ordering and they could be realized experimentally by imparting angular momentum in the spinor condensate.},
number = {1},
urldate = {2016-01-04TZ},
journal = {Journal of the Physical Society of Japan},
author = {Simula, Tapio P. and Huhtamäki, Jukka A. M. and Takahashi, Masahiro and Mizushima, Takeshi and Machida, Kazushige},
month = dec,
year = {2010},
eprint = {1007.3551},
pages = {013001}
}
• Kelvin-Tkachenko waves of few-vortex arrays in trapped Bose–Einstein condensates,
T. P. Simula and K. Machida.
Physical Review A 82, 63627 (2010).
[arXiv] [DOI] [Bibtex] [Abstract]

We have calculated the low-lying elementary excitations of three-dimensional few-vortex arrays in trapped Bose-Einstein condensates. The number of different Kelvin-Tkachenko vortex wave branches found matches the number of vortices in the condensate. The lowest odd-parity modes exhibit superfluid gyroscopic vortex motion. Experimentally, these modes could be excited and observed individually or in connection with the formation and decay of quantum turbulence.

@article{simula_kelvin-tkachenko_2010,
title = {{Kelvin}-Tkachenko waves of few-vortex arrays in trapped {Bose--Einstein} condensates},
volume = {82},
doi = {10.1103/PhysRevA.82.063627},
abstract = {We have calculated the low-lying elementary excitations of three-dimensional few-vortex arrays in trapped Bose-Einstein condensates. The number of different Kelvin-Tkachenko vortex wave branches found matches the number of vortices in the condensate. The lowest odd-parity modes exhibit superfluid gyroscopic vortex motion. Experimentally, these modes could be excited and observed individually or in connection with the formation and decay of quantum turbulence.},
number = {6},
urldate = {2016-01-04TZ},
journal = {Physical Review A},
author = {Simula, T. P. and Machida, K.},
month = dec,
year = {2010},
eprint = {1007.3548},
pages = {063627}
}
• Texture Control in a Pseudospin Bose–Einstein Condensate,
G. Ruben, M. J. Morgan, and D. M. Paganin.
Physical Review Letters 105, 220402 (2010).
[arXiv] [DOI] [Bibtex] [Abstract]

We describe a wave function engineering approach to the formation of textures in nonrotated multicomponent Bose-Einstein condensates. With numerical simulations of a viable two-component condensate experiment, we demonstrate the formation of a ballistically expanding regular lattice texture, composed of half-quantum vortices and spin-2 textures. The formation is described by a linear interference process in which the geometry and phase of three initially separated wave packets provide deterministic control over the resulting lattice texture.

@article{ruben_texture_2010,
title = {Texture Control in a Pseudospin {Bose--Einstein} Condensate},
volume = {105},
doi = {10.1103/PhysRevLett.105.220402},
abstract = {We describe a wave function engineering approach to the formation of textures in nonrotated multicomponent Bose-Einstein condensates. With numerical simulations of a viable two-component condensate experiment, we demonstrate the formation of a ballistically expanding regular lattice texture, composed of half-quantum vortices and spin-2 textures. The formation is described by a linear interference process in which the geometry and phase of three initially separated wave packets provide deterministic control over the resulting lattice texture.},
number = {22},
urldate = {2016-01-04TZ},
journal = {Physical Review Letters},
author = {Ruben, Gary and Morgan, Michael J. and Paganin, David M.},
month = nov,
year = {2010},
eprint = {1005.4760},
pages = {220402}
}
• Multi-Gaussian modes of diffusion in a quenched random medium,
T. Simula and M. Stenlund.
Physical Review E 82, 41125 (2010).
[arXiv] [DOI] [Bibtex] [Abstract]

We have studied a model of a random walk in a quenched random environment. In addition to featuring anomalous diffusion and localization, for special regimes of disorder parameters the particle density decomposes into multi-Gaussian structure while its cumulative distribution is normal. We explain the observed fine structure of the density and point out its significance to experiments.

@article{simula_multi-gaussian_2010,
title = {Multi-Gaussian modes of diffusion in a quenched random medium},
volume = {82},
doi = {10.1103/PhysRevE.82.041125},
abstract = {We have studied a model of a random walk in a quenched random environment. In addition to featuring anomalous diffusion and localization, for special regimes of disorder parameters the particle density decomposes into multi-Gaussian structure while its cumulative distribution is normal. We explain the observed fine structure of the density and point out its significance to experiments.},
number = {4},
urldate = {2016-01-04TZ},
journal = {Physical Review E},
author = {Simula, Tapio and Stenlund, Mikko},
month = oct,
year = {2010},
eprint = {1011.1992},
pages = {041125}
}
• Vortex and texture defects in radiation and matter wavefields,
G. Ruben, PhD Thesis.
[DOI] [Bibtex] [Abstract]

In this thesis the features that arise from the interference of simple coherent waves are described, namely the topological defects of phase known as phase vortices permeating such wavefields. Phase vortices are associated with the rotation of energy and mass and arise naturally in regions containing coherent light and/or matter. The simplest possible system exhibiting these defects is that of three linearly interfering plane waves, described using the classical theory of monochromatic complex scalar wavefields. We explore the related case of three outgoing waves produced by point sources, such as a laser-illuminated screen that has been punctured by three pinholes. We develop a description of the resulting vortex structure in the optical far-field, and relate this to the source arrangement and parameters. This allows an estimate of the number and positions of the vortices, and leads to connections to particular results from information theory and number theory. Numerical simulations of the near-field defect structure are also presented. Findings from this study are then applied to the time evolution of a nonlinear Bose-Einstein condensate of atoms, which shares the property of coherence that enables its description by a related model of interfering Gaussian wavepackets. Numerical modelling of the nonlinear system is performed for both trapped and untrapped Bose-Einstein condensates (BECs) in two and three spatial dimensions, for BEC pieces initially positioned at the three corners of an equilateral triangle. We demonstrate the production of a distorted honeycomb vortex-antivortex lattice, due to wavepacket interference. The wavepacket expansion is halted by the presence of a magnetic trap, causing the lattice to melt and providing a population of mobile vortices and antivortices. The addition of nonlinearity in this system over the linear case leads to the formation of vortex clusters, whose dynamics we investigate. A model of a Laguerre-Gauss (LG) laser beam, which carries a phase vortex along its axis, is studied following its interaction with a ground-glass plate. We present preliminary results showing that the beam acquires interference-generated vortices whose number density evolves as a function of propagation distance and beam topological charge, due to interaction with the roughened glass. Beyond the nonlinear coherent wavefields applied to the description of Bose-condensed gases, a pseudo-spinor system described as a nonlinear coupled system of two complex scalar fields is studied. Such systems admit the existence of texture defects similar to those that exist in ferromagnetic systems. We numerically model a pseudo-spinor system that we arrange to generate carefully aligned honeycomb vortex-antivortex lattices in the coupled components. We find that this corresponds to a texture defect lattice and identify its constituent texture defects as merons and a type of planar defect. Following the analogy with the single component BEC, we also study texture defects and clusters in the trapped pseudo-spinor system.

@phdthesis{ruben_vortex_2010,
type = {{PhD} {Thesis}},
title = {Vortex and texture defects in radiation and matter wavefields},
url = {http://hdl.handle.net/1959.1/243106},
abstract = {In this thesis the features that arise from the interference of simple coherent waves are described, namely the topological defects of phase known as phase vortices permeating such wavefields. Phase vortices are associated with the rotation of energy and mass and arise naturally in regions containing coherent light and/or matter. The simplest possible system exhibiting these defects is that of three linearly interfering plane waves, described using the classical theory of monochromatic complex scalar wavefields. We explore the related case of three outgoing waves produced by point sources, such as a laser-illuminated screen that has been punctured by three pinholes. We develop a description of the resulting vortex structure in the optical far-field, and relate this to the source arrangement and parameters. This allows an estimate of the number and positions of the vortices, and leads to connections to particular results from information theory and number theory. Numerical simulations of the near-field defect structure are also presented. Findings from this study are then applied to the time evolution of a nonlinear Bose-Einstein condensate of atoms, which shares the property of coherence that enables its description by a related model of interfering Gaussian wavepackets. Numerical modelling of the nonlinear system is performed for both trapped and untrapped Bose-Einstein condensates (BECs) in two and three spatial dimensions, for BEC pieces initially positioned at the three corners of an equilateral triangle. We demonstrate the production of a distorted honeycomb vortex-antivortex lattice, due to wavepacket interference. The wavepacket expansion is halted by the presence of a magnetic trap, causing the lattice to melt and providing a population of mobile vortices and antivortices. The addition of nonlinearity in this system over the linear case leads to the formation of vortex clusters, whose dynamics we investigate. A model of a Laguerre-Gauss (LG) laser beam, which carries a phase vortex along its axis, is studied following its interaction with a ground-glass plate. We present preliminary results showing that the beam acquires interference-generated vortices whose number density evolves as a function of propagation distance and beam topological charge, due to interaction with the roughened glass. Beyond the nonlinear coherent wavefields applied to the description of Bose-condensed gases, a pseudo-spinor system described as a nonlinear coupled system of two complex scalar fields is studied. Such systems admit the existence of texture defects similar to those that exist in ferromagnetic systems. We numerically model a pseudo-spinor system that we arrange to generate carefully aligned honeycomb vortex-antivortex lattices in the coupled components. We find that this corresponds to a texture defect lattice and identify its constituent texture defects as merons and a type of planar defect. Following the analogy with the single component BEC, we also study texture defects and clusters in the trapped pseudo-spinor system.},
school = {Monash University},
author = {Ruben, G},
month = jun,
year = {2010},
doi = {1959.1/243106}
}
• Spin textures in condensates with large dipole moments,
J. A. M. Huhtamäki, M. Takahashi, T. P. Simula, T. Mizushima, and K. Machida.
Physical Review A 81, 63623 (2010).
[arXiv] [DOI] [Bibtex] [Abstract]

We have solved numerically the ground states of a Bose-Einstein condensate in the presence of dipolar interparticle forces using a semiclassical approach. Our motivation is to model, in particular, the spontaneous spin textures emerging in quantum gases with large dipole moments, such as Cr52 or Dy condensates, or ultracold gases consisting of polar molecules. For a pancake-shaped harmonic (optical) potential, we present the ground-state phase diagram spanned by the strength of the nonlinear coupling and dipolar interactions. In an elongated harmonic potential, we observe a helical spin texture. The textures calculated according to the semiclassical model in the absence of external polarizing fields are predominantly analogous to previously reported results for a ferromagnetic F=1 spinor Bose-Einstein condensate, suggesting that the spin textures arising from the dipolar forces are largely independent of the value of the quantum number F or the origin of the dipolar interactions.

@article{huhtamaki_spin_2010,
title = {Spin textures in condensates with large dipole moments},
volume = {81},
doi = {10.1103/PhysRevA.81.063623},
abstract = {We have solved numerically the ground states of a Bose-Einstein condensate in the presence of dipolar interparticle forces using a semiclassical approach. Our motivation is to model, in particular, the spontaneous spin textures emerging in quantum gases with large dipole moments, such as Cr52 or Dy condensates, or ultracold gases consisting of polar molecules. For a pancake-shaped harmonic (optical) potential, we present the ground-state phase diagram spanned by the strength of the nonlinear coupling and dipolar interactions. In an elongated harmonic potential, we observe a helical spin texture. The textures calculated according to the semiclassical model in the absence of external polarizing fields are predominantly analogous to previously reported results for a ferromagnetic F=1 spinor Bose-Einstein condensate, suggesting that the spin textures arising from the dipolar forces are largely independent of the value of the quantum number F or the origin of the dipolar interactions.},
number = {6},
urldate = {2016-01-04TZ},
journal = {Physical Review A},
author = {Huhtamäki, J. A. M. and Takahashi, M. and Simula, T. P. and Mizushima, T. and Machida, K.},
month = jun,
year = {2010},
eprint = {1007.2027},
pages = {063623}
}
• Finite-temperature phase transitions in quasi-two-dimensional spin-1 Bose gases,
V. Pietilä, T. P. Simula, and M. Möttönen.
Physical Review A 81, 33616 (2010).
[arXiv] [DOI] [Bibtex] [Abstract]

Recently, the Berezinskii-Kosterlitz-Thouless transition was found to be mediated by half-quantum vortices (HQVs) in two-dimensional (2D) antiferromagnetic Bose gases [S. Mukerjee, C. Xu, and J. E. Moore, Phys. Rev. Lett. 97, 120406 (2006)]. We study the thermal activation of HQVs in the experimentally relevant trapped quasi-2D system and find a crossover temperature at which free HQVs proliferate at the center of the trap. Above the crossover temperature, we observe transitions corresponding to the onset of a coherent condensate and a quasicondensate and discuss the absence of a fragmented condensate.

@article{pietila_finite-temperature_2010,
title = {Finite-temperature phase transitions in quasi-two-dimensional spin-1 {Bose} gases},
volume = {81},
doi = {10.1103/PhysRevA.81.033616},
abstract = {Recently, the Berezinskii-Kosterlitz-Thouless transition was found to be mediated by half-quantum vortices (HQVs) in two-dimensional (2D) antiferromagnetic Bose gases [S. Mukerjee, C. Xu, and J. E. Moore, Phys. Rev. Lett. 97, 120406 (2006)]. We study the thermal activation of HQVs in the experimentally relevant trapped quasi-2D system and find a crossover temperature at which free HQVs proliferate at the center of the trap. Above the crossover temperature, we observe transitions corresponding to the onset of a coherent condensate and a quasicondensate and discuss the absence of a fragmented condensate.},
number = {3},
urldate = {2016-01-04TZ},
journal = {Physical Review A},
author = {Pietilä, Ville and Simula, Tapio P. and Möttönen, Mikko},
month = mar,
year = {2010},
eprint = {0909.1898},
pages = {033616}
}
• A slow atom source using a collimated effusive oven and a single-layer variable pitch coil Zeeman slower,
S. C. Bell, M. Junker, M. Jasperse, L. D. Turner, Y. -J. Lin, I. B. Spielman, and R. E. Scholten.
Review of Scientific Instruments 81, 13105 (2010).
[DOI] [Bibtex] [Abstract]

We describe a simple slow atom source for loading a rubidiummagneto-optical trap. The source includes an effusive oven with a long heated collimation tube. Almost all components are standard vacuum parts. The heating elements and thermocouples are external to the vacuum, protecting them from the hostile hot alkali environment and allowing repair without breaking vacuum. The thermal source is followed by a Zeeman slower with a single-layer coil of variable winding pitch. The single-layer design is simple to construct and has low inductance which allows for rapid switching of the magnetic field. The coil pitch was determined by fitting the analytic form of the magnetic field for a variable winding pitch to the desired magnetic field profile required to slow atoms. The measured magnetic field for the constructed coil is in excellent agreement with the desired field. The source produces atoms at 35 m/s with a flux up to 2 × 10 10 cm − 2 s − 1 at 200 ° C .

@article{bell_slow_2010,
title = {{A} slow atom source using a collimated effusive oven and a single-layer variable pitch coil Zeeman slower},
volume = {81},
issn = {0034-6748, 1089-7623},
url = {http://scitation.aip.org/content/aip/journal/rsi/81/1/10.1063/1.3276712},
doi = {10.1063/1.3276712},
abstract = {We describe a simple slow atom source for loading a rubidiummagneto-optical trap. The source includes an effusive oven with a long heated collimation tube. Almost all components are standard vacuum parts. The heating elements and thermocouples are external to the vacuum, protecting them from the hostile hot alkali environment and allowing repair without breaking vacuum. The thermal source is followed by a Zeeman slower with a single-layer coil of variable winding pitch. The single-layer design is simple to construct and has low inductance which allows for rapid switching of the magnetic field. The coil pitch was determined by fitting the analytic form of the magnetic field for a variable winding pitch to the desired magnetic field profile required to slow atoms. The measured magnetic field for the constructed coil is in excellent agreement with the desired field. The source produces atoms at 35 m/s with a flux up to 2 × 10 10 cm − 2 s − 1 at 200 ° C .},
number = {1},
urldate = {2016-01-04TZ},
journal = {Review of Scientific Instruments},
author = {Bell, S. C. and Junker, M. and Jasperse, M. and Turner, L. D. and Lin, Y.-J. and Spielman, I. B. and Scholten, R. E.},
month = jan,
year = {2010},
keywords = {Atom trapping, Atomic and molecular beams, Coils, Zeeman effect, magnetic fields},
pages = {013105}
}

### 2009

• Mode stability of external cavity diode lasers,
S. D. Saliba, M. Junker, L. D. Turner, and R. E. Scholten.
Applied Optics 48, 6692 (2009).
[DOI] [Bibtex] [Abstract]

Mode stability is an important performance characteristic of external cavity diode lasers (ECDLs). It has been well established that the continuous mode-hop-free tuning range of a grating-feedback ECDL can be optimized by rotating the grating about a specific pivot location. We show that similar results can be obtained for other more convenient pivot locations by choosing instead the cavity length and grating location. The relative importance of the temperature stability of the diode and of the external cavity is also evaluated. We show that mechanically simple ECDL designs, using mostly standard components, can readily achieve a 35 GHz mode-hop-free tuning range at 780 nm.

@article{saliba_mode_2009,
title = {Mode stability of external cavity diode lasers},
volume = {48},
issn = {0003-6935, 1539-4522},
url = {https://www.osapublishing.org/abstract.cfm?URI=ao-48-35-6692},
doi = {10.1364/AO.48.006692},
abstract = {Mode stability is an important performance characteristic of external cavity diode lasers (ECDLs). It has been well established that the continuous mode-hop-free tuning range of a grating-feedback ECDL can be optimized by rotating the grating about a specific pivot location. We show that similar results can be obtained for other more convenient pivot locations by choosing instead the cavity length and grating location. The relative importance of the temperature stability of the diode and of the external cavity is also evaluated. We show that mechanically simple ECDL designs, using mostly standard components, can readily achieve a 35 GHz mode-hop-free tuning range at 780 nm.},
number = {35},
urldate = {2016-01-04TZ},
journal = {Applied Optics},
author = {Saliba, Sebastian D. and Junker, Mark and Turner, Lincoln D. and Scholten, Robert E.},
month = dec,
year = {2009},
pages = {6692}
}

### 2008

• Vortex-lattice formation and melting in a nonrotating Bose–Einstein condensate,
G. Ruben, D. M. Paganin, and M. J. Morgan.
Physical Review A 78, 13631 (2008).
[arXiv] [DOI] [Bibtex] [Abstract]

Numerical simulations of the interference of a three-way segmented nonrotating Bose-Einstein condensate reveal the production of a honeycomb vortex lattice containing significant numbers of vortices and antivortices. If confined within a trap, the lattice subsequently melts, exhibiting a rich assortment of vortex-antivortex interactions. In contrast with nonlinear vortex production mechanisms previously described for Bose-Einstein condensates, the process here is shown to be primarily one of linear superposition, with initial vortex locations approximately described by a linear theory of wave packet interference.

@article{ruben_vortex-lattice_2008,
title = {Vortex-lattice formation and melting in a nonrotating {Bose--Einstein} condensate},
volume = {78},
}