Observation of first and second sound in a BKT superfluid

Christodoulou, Panayiotis; Gałka, Maciej; Dogra, Nishant; Lopes, Raphaël; Schmitt, Julian; Hadzibabic, Zoran

doi:10.1038/s41586-021-03537-9

Cited by 56 publications

(103 citation statements)

References 49 publications

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“… 16 ) and second sound. Second sound has recently been observed by Sidorenkov et al 20 in a unitary Fermi gas and by Ville et al 21 and Christodoulou et al 22 in a two-dimensional bosonic SF. Second sound was also possibly present in an experiment by Meppelink et al 23 as pointed out in ref.…”

Section: Introductionmentioning

confidence: 77%

Second sound in the crossover from the Bose-Einstein condensate to the Bardeen-Cooper-Schrieffer superfluid

et al. 2021

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Second sound is an entropy wave which propagates in the superfluid component of a quantum liquid. Because it is an entropy wave, it probes the thermodynamic properties of the quantum liquid. Here, we study second sound propagation for a large range of interaction strengths within the crossover between a Bose-Einstein condensate (BEC) and the Bardeen-Cooper-Schrieffer (BCS) superfluid, extending previous work at unitarity. In particular, we investigate the strongly-interacting regime where currently theoretical predictions only exist in terms of an interpolation in the crossover. Working with a quantum gas of ultracold fermionic 6Li atoms with tunable interactions, we show that the second sound speed varies only slightly in the crossover regime. By varying the excitation procedure, we gain deeper insight on sound propagation. We compare our measurement results with classical-field simulations, which help with the interpretation of our experiments.

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Section: Introductionmentioning

confidence: 77%

Second sound in the crossover from the Bose-Einstein condensate to the Bardeen-Cooper-Schrieffer superfluid

et al. 2021

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“…Ultracold atomic quantum systems in dimensions lower than three bear interesting physics [1,2]. In 2D an interacting Bose gas can undergo a crossover from a Bose-Einstein condensate to a Berezinskii-Kosterlitz-Thouless phase [3][4][5], where vortex and anti-vortex pairs are produced and can move through the gas. In one-dimensional systems large phase fluctuations are detected [6], which induce an algebraic decay of the correlation function, in contrast to an exponential decay in higher dimensions.…”

Section: Introductionmentioning

confidence: 99%

Photon BEC with thermo-optic interaction at dimensional crossover

Stein

Pelster²

2022

New J. Phys.

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Since the advent of experiments with photon Bose-Einstein condensates in dye-filled microcavities in 2010, many investigations have focused upon the emerging effective photon-photon interaction. Despite its smallness, it can be identified to stem from two physically distinct mechanisms. On the one hand, a Kerr nonlinearity of the dye medium yields a photon-photon contact interaction. On the other hand, a heating of the dye medium leads to an additional thermo-optic interaction, which is both delayed and non-local. The latter turns out to represent the leading contribution to the effective interaction for the current 2D experiments. Here we analyse theoretically how the effective photon-photon interaction increases when the system dimension is reduced from 2D to 1D. To this end, we consider an anisotropic harmonic trapping potential and determine via a variational approach how the properties of the photon Bose-Einstein condensate in general, and both aforementioned interaction mechanisms in particular, change with increasing anisotropy. We find that the thermo-optic interaction strength increases at first linearly with the trap aspect ratio and lateron saturates at a certain value of the trap aspect ratio. Furthermore, in the strong 1D limit the roles of both interactions get reversed as the thermo-optic interaction remains saturated and the contact Kerr interaction becomes the leading interaction mechanism. Finally, we discuss how the predicted effects can be measured experimentally.

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“…In ultracold atoms, the crossover between saturation-driven BEC and interaction-driven BKT superfluidity has been investigated in 2D harmonically trapped Bose gases by tuning the interactions exploiting a Feshbach resonance [16], while studies of homogeneous gases in box potentials have focused on the interacting regime [17][18][19]. In uniform gases of exciton-polaritons [20], on the other hand, the observation of BEC is hampered by reservoir-induced interactions and non-equilibrium effects.…”

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confidence: 99%

“…At the largest investigated box sizes, we have (8). For interacting 2D gases characterized by an interaction strength g and realized in ultracold atoms [6,7,[15][16][17]19], the BKT phase transition to a superfluid usually occurs before BEC; for example, for homogeneous gases, D c,BKT = log(380/g) = 6.5 for g = 0.6 [19]. Quite distinctly, for photon gases in dye-microcavities, self-interactions with g ≤ 10 −6 [23] imply a significantly larger D c,BKT ≥ 20 and, accordingly, both phases are expected to be well separated.…”

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confidence: 99%

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Compressibility and the Equation of State of an Optical Quantum Gas in a Box

Busley,

Miranda,

Redmann

et al. 2021

Preprint

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The compressibility of a medium, quantifying its response to mechanical perturbations, is a fundamental property determined by the equation of state. For gases of material particles, studies of the mechanical response are well established, in fields from classical thermodynamics to cold atomic quantum gases. Here we demonstrate a measurement of the compressibility of a two-dimensional quantum gas of light in a box potential and obtain the equation of state for the optical medium. The experiment is carried out in a nanostructured dye-filled optical microcavity. We observe signatures of Bose-Einstein condensation at high phase-space densities in the finite-size system. Strikingly, upon entering the quantum degenerate regime, the measured density response to an external force sharply increases, hinting at the peculiar prediction of an infinite compressibility of the deeply degenerate Bose gas.Quantum gases of atoms, exciton-polaritons, and photons provide a test bed for many-body physics under both in-and out-of-equilibrium settings [1][2][3]. Experimental control over dimensionality, potential energy landscapes, or the coupling to reservoirs offer wide possibilities to explore different phases of matter. For cold atomic gases, thermodynamic susceptibilities and transport properties have been extracted from density measurements [4][5][6][7][8][9] and have proven to be direct manifestations of the equation of state (EOS). In general, the EOS of a material, e.g., its pressure-volume relation, describes both the thermodynamic state of a system under a given set of physical conditions as well as its response to perturbations, as mechanical compression.Experimental investigations of the EOS in quantum gases constitute a tool for the characterisation of phases and the identification of phase transitions, enabling important tests of physical models in a wide range of systems, from the ideal gas to superfluids and the interior of stars.Quantum gases of light have so far been experimentally realized in low-dimensional settings, mostly twodimensional (2D) systems [3]. Thermalized photon gases with non-vanishing chemical potential µ, as well as Bose-Einstein condensation (BEC) have been demonstrated in dye-filled optical microcavities at harmonic confinement [10][11][12], including measurements of density-insensitive thermodynamic quantities [13]. In contrast, the isothermal compressibility κ T = n −2 (∂n/∂µ) T at temperature T depends on the (local) particle density n in the gas; for a systematic study, it thus is desirable to avoid spatially inhomogeneous density distributions inherent to harmonically trapped gases, and instead prepare uniform samples, where applying a spatially uniform force directly allows one to compress the gas and probe κ T .

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Observation of first and second sound in a BKT superfluid

Cited by 56 publications

References 49 publications

Second sound in the crossover from the Bose-Einstein condensate to the Bardeen-Cooper-Schrieffer superfluid

Second sound in the crossover from the Bose-Einstein condensate to the Bardeen-Cooper-Schrieffer superfluid

Photon BEC with thermo-optic interaction at dimensional crossover

Compressibility and the Equation of State of an Optical Quantum Gas in a Box

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