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Quantum Matter Publications

2016

  • 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}
    }