Thermalization, condensate growth, and defect formation in an out-of-equilibrium Bose gas

Brown, Dylan; McPhail, Alexander; White, D. H.; Baillie, D.; Ruddell, S. K.; Hoogerland, M. D.

doi:10.1103/physreva.98.013606

“…In Brown et al [11], we showed that an entire Bose condensate formed in a non-zero momentum state, as opposed to some components being in a metastable state as in the experiments with solitons and vortices. We used a Bragg pulse to excite 50% of a population of rubidium-87 atoms in the ground state of an approximately harmonic trap into the |2 hk momentum state.…”

Section: Introductionsupporting

confidence: 50%

A Bose–Einstein condensate is a Bose condensate in the laboratory ground state

McPhail

¹

,

Hoogerland

²

2021

Proc. R. Soc. A.

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Bose–Einstein condensates of weakly interacting, ultra-cold atoms have become a workhorse for exploring quantum effects on atomic motion, but does this condensate need to be in the ground state of the system? Researchers often perform transformations so that their Hamiltonians are easier to analyse. However, changing Hamiltonians can require an energy shift. We show that transforming into a rotating or oscillating frame of reference of a Bose condensate does not then satisfy Einstein’s requirement that a condensate exists in the zero kinetic energy state. We show that Bose condensation can occur above the ground state and at room temperature, referring to recent literature.

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“…For many T = 0 non-equilibrium phenomena it is sufficient to treat this initial condition as a multimode coherent state that is sampled by seeding the initial mean-field condensate wavefunction with on average half an atom of vacuum noise per mode [48]. Zero temperature TW with this initial condition has successfully modelled BEC dynamics in regimes where nonclassical particle correlations become important [49][50][51][52][53][54][55][56][57][58]. For finite-temperature studies, the relevant c-field theory is the stochastic projected Gross-Pitaevskii equation (SPGPE), which describes interactions between degenerate modes of the quantum field with a static thermal reservoir [59].…”

Section: Classical Field Methodologymentioning

confidence: 99%

Superflow decay in a toroidal Bose gas: The effect of quantum and thermal fluctuations

Mehdi

¹

,

Bradley

²

,

Hope

³

et al. 2021

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We theoretically investigate the stochastic decay of persistent currents in a toroidal ultracold atomic superfluid caused by a perturbing barrier. Specifically, we perform detailed three-dimensional simulations to model the experiment of Kumar et al. in [Phys. Rev. A 95 021602 (2017)], which observed a strong temperature dependence in the timescale of superflow decay in an ultracold Bose gas. Our ab initio numerical approach exploits a classical-field framework that includes thermal fluctuations due to interactions between the superfluid and a thermal cloud, as well as the intrinsic quantum fluctuations of the Bose gas. In the low-temperature regime our simulations provide a quantitative description of the experimental decay timescales, improving on previous numerical and analytical approaches. At higher temperatures, our simulations give decay timescales that range over the same orders of magnitude observed in the experiment, however, there are some quantitative discrepancies that are not captured by any of the mechanisms we explore. Our results suggest a need for further experimental and theoretical studies into superflow stability.

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“…For many T = 0 non-equilibrium phenomena it is sufficient to treat this initial condition as a multimode coherent state that is sampled by seeding the initial mean-field condensate wavefunction with on average half an atom of vacuum noise per mode [47]. Zero temperature TW with this initial condition has successfully modelled BEC dynamics in regimes where nonclassical particle correlations become important [48][49][50][51][52][53][54][55][56][57]. For finite-temperature studies, the relevant c-field theory is the stochastic projected Gross-Pitaevskii equation (SPGPE), which describes interactions between degenerate modes of the quantum field with a static thermal reservoir [58].…”

Section: Classical Field Methodologymentioning

confidence: 99%

Superflow decay in a toroidal Bose gas: The effect of quantum and thermal fluctuations

Mehdi,

Bradley,

Hope

et al. 2021

Preprint

0

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We theoretically investigate the stochastic decay of persistent currents in a toroidal ultracold atomic superfluid caused by a perturbing barrier. Specifically, we perform detailed threedimensional simulations to model the experiment of Kumar et al. in [Phys. Rev. A 95 021602 (2017)], which observed a strong temperature dependence in the timescale of superflow decay in an ultracold Bose gas. Our ab initio numerical approach exploits a classical-field framework that includes thermal fluctuations due to interactions between the superfluid and a thermal cloud, as well as the intrinsic quantum fluctuations of the Bose gas. In the low-temperature regime our simulations provide a quantitative description of the experimental decay timescales. At higher temperatures, our simulations give decay timescales that range over the same orders of magnitude observed in the experiment, however there are some quantitative discrepancies. In particular, we find a much larger perturbing barrier strength is required to simulate a particular decay timescale (between ∼0.15µ and ∼0.5µ), as compared to the experiment. We rule out imprecise estimation of simulation parameters, systematic errors in experimental barrier calibration, and shot-to-shot atom number fluctuations as causes of the discrepancy. However our model does not account for technical noise on the trapping lasers, which may have enhanced the superflow decay in the experiment. For the intermediate temperatures studied in the experiment, we also observe some discrepancy in the sensitivity of the decay timescale to small changes in the barrier height, which may be due to the breakdown of our model's validity in this regime.

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Thermalization, condensate growth, and defect formation in an out-of-equilibrium Bose gas

Cited by 11 publications

References 33 publications

A Bose–Einstein condensate is a Bose condensate in the laboratory ground state

A Bose–Einstein condensate is a Bose condensate in the laboratory ground state

Superflow decay in a toroidal Bose gas: The effect of quantum and thermal fluctuations

Superflow decay in a toroidal Bose gas: The effect of quantum and thermal fluctuations

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