↑ Return to Quantum Matter

Quantum Matter Publications

2017

  • Dipolar fermions in a multilayer geometry,
    M. Callegari, M. M. Parish, and F. M. Marchetti.
    Physical Review B 95, 85124 (2017).
    [DOI] [Bibtex] [Abstract]

    We investigate the behavior of identical dipolar fermions with aligned dipole moments in two-dimensional multilayers at zero temperature. We consider density instabilities that are driven by the attractive part of the dipolar interaction and, for the case of bilayers, we elucidate the properties of the stripe phase recently predicted to exist in this interaction regime. When the number of layers is increased, we find that this “attractive” stripe phase exists for an increasingly larger range of dipole angles, and if the interlayer distance is sufficiently small, the stripe phase eventually spans the full range of angles, including the situation where the dipole moments are aligned perpendicular to the planes. However, in this regime, we expect the behavior to be strongly modified by the binding of dipoles between layers. In the limit of an infinite number of layers, we derive an analytic expression for the mean-field interlayer effects in the density-density response function and, using this result, we find that the stripe phase is replaced by a collapse of the dipolar system.

    @article{callegari_dipolar_2017,
    title = {Dipolar fermions in a multilayer geometry},
    volume = {95},
    url = {https://link.aps.org/doi/10.1103/PhysRevB.95.085124},
    doi = {10.1103/PhysRevB.95.085124},
    abstract = {We investigate the behavior of identical dipolar fermions with aligned dipole moments in two-dimensional multilayers at zero temperature. We consider density instabilities that are driven by the attractive part of the dipolar interaction and, for the case of bilayers, we elucidate the properties of the stripe phase recently predicted to exist in this interaction regime. When the number of layers is increased, we find that this “attractive” stripe phase exists for an increasingly larger range of dipole angles, and if the interlayer distance is sufficiently small, the stripe phase eventually spans the full range of angles, including the situation where the dipole moments are aligned perpendicular to the planes. However, in this regime, we expect the behavior to be strongly modified by the binding of dipoles between layers. In the limit of an infinite number of layers, we derive an analytic expression for the mean-field interlayer effects in the density-density response function and, using this result, we find that the stripe phase is replaced by a collapse of the dipolar system.},
    number = {8},
    urldate = {2017-11-17TZ},
    journal = {Physical Review B},
    author = {Callegari, M. and Parish, M. M. and Marchetti, F. M.},
    month = feb,
    year = {2017},
    pages = {085124}
    }
  • Universality of the unitary Fermi gas: a few-body perspective,
    J. Levinsen, P. Massignan, S. Endo, and M. M. Parish.
    Journal of Physics B: Atomic, Molecular and Optical Physics 50, 72001 (2017).
    [DOI] [Bibtex] [Abstract]

    We revisit the properties of the two-component Fermi gas with short-range interactions in three dimensions, in the limit where the s -wave scattering length diverges. Such a unitary Fermi gas possesses universal thermodynamic and dynamical observables that are independent of any interaction length scale. Focusing on trapped systems of N fermions, where \#\#IMG\#\# [http://ej.iop.org/images/0953-4075/50/7/072001/jpbaa5a1eieqn1.gif] \$N{\textbackslash}leqslant 10\$ , we investigate how well we can determine the zero-temperature behavior of the many-body system from published few-body data on the ground-state energy and the contact. For the unpolarized case, we find that the Bertsch parameters extracted from trapped few-body systems all lie within 15\% of the established value. Furthermore, the few-body values for the contact are well within the range of values determined in the literature for the many-body system. In the limit of large spin polarization, we obtain a similar accuracy for the polaron energy, and we estimate the polaron’s effective mass from the dependence of its energy on N . We also compute an upper bound for the squared wave-function overlap between the unitary Fermi system and the non-interacting ground state, both for the trapped and uniform cases. This allows us to prove that the trapped unpolarized ground state at unitarity has zero overlap with its non-interacting counterpart in the many-body limit \#\#IMG\#\# [http://ej.iop.org/images/0953-4075/50/7/072001/jpbaa5a1eieqn2.gif] \$N{\textbackslash}to ınfty \$ .

    @article{levinsen_universality_2017,
    title = {Universality of the unitary {Fermi} gas: a few-body perspective},
    volume = {50},
    issn = {0953-4075},
    shorttitle = {Universality of the unitary {Fermi} gas},
    url = {http://stacks.iop.org/0953-4075/50/i=7/a=072001},
    doi = {10.1088/1361-6455/aa5a1e},
    abstract = {We revisit the properties of the two-component Fermi gas with short-range interactions in three dimensions, in the limit where the s -wave scattering length diverges. Such a unitary Fermi gas possesses universal thermodynamic and dynamical observables that are independent of any interaction length scale. Focusing on trapped systems of N fermions, where \#\#IMG\#\# [http://ej.iop.org/images/0953-4075/50/7/072001/jpbaa5a1eieqn1.gif] \$N{\textbackslash}leqslant 10\$ , we investigate how well we can determine the zero-temperature behavior of the many-body system from published few-body data on the ground-state energy and the contact. For the unpolarized case, we find that the Bertsch parameters extracted from trapped few-body systems all lie within 15\% of the established value. Furthermore, the few-body values for the contact are well within the range of values determined in the literature for the many-body system. In the limit of large spin polarization, we obtain a similar accuracy for the polaron energy, and we estimate the polaron’s effective mass from the dependence of its energy on N . We also compute an upper bound for the squared wave-function overlap between the unitary Fermi system and the non-interacting ground state, both for the trapped and uniform cases. This allows us to prove that the trapped unpolarized ground state at unitarity has zero overlap with its non-interacting counterpart in the many-body limit \#\#IMG\#\# [http://ej.iop.org/images/0953-4075/50/7/072001/jpbaa5a1eieqn2.gif] \$N{\textbackslash}to ınfty \$ .},
    number = {7},
    urldate = {2017-11-17TZ},
    journal = {Journal of Physics B: Atomic, Molecular and Optical Physics},
    author = {Levinsen, Jesper and Massignan, Pietro and Endo, Shimpei and Parish, Meera M.},
    year = {2017},
    pages = {072001}
    }

2016

  • Lifetime of Feshbach dimers in a Fermi-Fermi mixture of \${\textasciicircum}\6\{\textbackslash}text\{{Li}\}\$ and \${\textasciicircum}\40\{\textbackslash}text\{{{K}}\}\$,
    M. Jag, M. Cetina, R. S. Lous, R. Grimm, J. Levinsen, and D. S. Petrov.
    Physical Review A 94, 62706 (2016).
    [DOI] [Bibtex] [Abstract]

    We present a joint experimental and theoretical investigation of the lifetime of weakly bound dimers formed near narrow interspecies Feshbach resonances in mass-imbalanced Fermi-Fermi systems, considering the specific example of a mixture of 6Li and 40K atoms. Our work addresses the central question of the increase in the stability of the dimers resulting from Pauli suppression of collisional losses, which is a well-known effect in mass-balanced fermionic systems near broad resonances. We present measurements of the spontaneous dissociation of dimers in dilute samples, and of the collisional losses in dense samples arising from both dimer-dimer processes and from atom-dimer processes. We find that all loss processes are suppressed close to the Feshbach resonance. Our general theoretical approach for fermionic mixtures near narrow Feshbach resonances provides predictions for the suppression of collisional decay as a function of the detuning from resonance, and we find excellent agreement with the experimental benchmarks provided by our 6Li−40K system. We finally present model calculations for other Feshbach-resonant Fermi-Fermi systems, which are of interest for experiments in the near future.

    @article{jag_lifetime_2016,
    title = {Lifetime of {Feshbach} dimers in a Fermi-{Fermi} mixture of \${\textasciicircum}\6\{\textbackslash}text\{{Li}\}\$ and \${\textasciicircum}\40\{\textbackslash}text\{{{K}}\}\$},
    volume = {94},
    url = {https://link.aps.org/doi/10.1103/PhysRevA.94.062706},
    doi = {10.1103/PhysRevA.94.062706},
    abstract = {We present a joint experimental and theoretical investigation of the lifetime of weakly bound dimers formed near narrow interspecies Feshbach resonances in mass-imbalanced Fermi-Fermi systems, considering the specific example of a mixture of 6Li and 40K atoms. Our work addresses the central question of the increase in the stability of the dimers resulting from Pauli suppression of collisional losses, which is a well-known effect in mass-balanced fermionic systems near broad resonances. We present measurements of the spontaneous dissociation of dimers in dilute samples, and of the collisional losses in dense samples arising from both dimer-dimer processes and from atom-dimer processes. We find that all loss processes are suppressed close to the Feshbach resonance. Our general theoretical approach for fermionic mixtures near narrow Feshbach resonances provides predictions for the suppression of collisional decay as a function of the detuning from resonance, and we find excellent agreement with the experimental benchmarks provided by our 6Li−40K system. We finally present model calculations for other Feshbach-resonant Fermi-Fermi systems, which are of interest for experiments in the near future.},
    number = {6},
    urldate = {2017-11-17TZ},
    journal = {Physical Review A},
    author = {Jag, M. and Cetina, M. and Lous, R. S. and Grimm, R. and Levinsen, J. and Petrov, D. S.},
    month = dec,
    year = {2016},
    pages = {062706}
    }
  • Quantum dynamics of impurities coupled to a Fermi sea,
    M. M. Parish and J. Levinsen.
    Physical Review B 94, 184303 (2016).
    [DOI] [Bibtex] [Abstract]

    We consider the dynamics of an impurity atom immersed in an ideal Fermi gas at zero temperature. We focus on the coherent quantum evolution of the impurity following a quench to strong impurity-fermion interactions, where the interactions are assumed to be short range like in cold-atom experiments. To approximately model the many-body time evolution, we use a truncated basis method, where at most two particle-hole excitations of the Fermi sea are included. When the system is initially noninteracting, we show that our method exactly captures the short-time dynamics following the quench, and we find that the overlap between initial and final states displays a universal nonanalytic dependence on time in this limit. We further demonstrate how our method can be used to compute the impurity spectral function, as well as describe many-body phenomena involving coupled impurity spin states, such as Rabi oscillations in a medium or highly engineered quantum quenches.

    @article{parish_quantum_2016,
    title = {Quantum dynamics of impurities coupled to a {Fermi} sea},
    volume = {94},
    url = {https://link.aps.org/doi/10.1103/PhysRevB.94.184303},
    doi = {10.1103/PhysRevB.94.184303},
    abstract = {We consider the dynamics of an impurity atom immersed in an ideal Fermi gas at zero temperature. We focus on the coherent quantum evolution of the impurity following a quench to strong impurity-fermion interactions, where the interactions are assumed to be short range like in cold-atom experiments. To approximately model the many-body time evolution, we use a truncated basis method, where at most two particle-hole excitations of the Fermi sea are included. When the system is initially noninteracting, we show that our method exactly captures the short-time dynamics following the quench, and we find that the overlap between initial and final states displays a universal nonanalytic dependence on time in this limit. We further demonstrate how our method can be used to compute the impurity spectral function, as well as describe many-body phenomena involving coupled impurity spin states, such as Rabi oscillations in a medium or highly engineered quantum quenches.},
    number = {18},
    urldate = {2017-11-17TZ},
    journal = {Physical Review B},
    author = {Parish, Meera M. and Levinsen, Jesper},
    month = nov,
    year = {2016},
    pages = {184303}
    }
  • 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},
    copyright = {Copyright © 2016, American Association for the Advancement of Science},
    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}
    }
  • 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},
    url = {http://link.aps.org/doi/10.1103/PhysRevLett.117.055302},
    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}
    }
  • 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},
    url = {http://link.aps.org/doi/10.1103/PhysRevLett.115.265302},
    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}
    }
  • 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},
    url = {http://link.aps.org/doi/10.1103/PhysRevLett.115.247202},
    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}
    }
  • 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},
    url = {http://link.aps.org/doi/10.1103/PhysRevLett.115.160401},
    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},
    url = {http://link.aps.org/doi/10.1103/PhysRevLett.115.125302},
    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}
    }
  • Strong-coupling ansatz for the one-dimensional Fermi gas in a harmonic potential,
    J. Levinsen, P. Massignan, G. M. Bruun, and M. M. Parish.
    Science Advances 1, e1500197 (2015).
    [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},
    url = {http://advances.sciencemag.org/content/1/6/e1500197},
    doi = {10.1126/sciadv.1500197},
    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},
    journal = {Science Advances},
    author = {Levinsen, Jesper and Massignan, Pietro and Bruun, Georg M. and Parish, Meera M.},
    month = jul,
    year = {2015},
    pages = {e1500197}
    }
  • 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},
    url = {http://link.aps.org/doi/10.1103/PhysRevA.91.013606},
    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}
    }