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Cooling phonon modes of a Bose condensate with uniform few body losses
by I. Bouchoule, M. Schemmer, C. Henkel
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|As Contributors:||Isabelle Bouchoule|
|Arxiv Link:||https://arxiv.org/abs/1806.08759v2 (pdf)|
|Date submitted:||2018-07-04 02:00|
|Submitted by:||Bouchoule, Isabelle|
|Submitted to:||SciPost Physics|
We present a general analysis of the cooling produced by losses on condensates or quasi-condensates. We study how the occupations of the collective phonon modes evolve in time, assuming that the loss process is slow enough so that each mode adiabatically follows the decrease of the mean density. The theory is valid for any loss process whose rate is proportional to the $j$th power of the density, but otherwise spatially uniform. We cover both homogeneous gases and systems confined in a smooth potential. For a low-dimensional gas, we can take into account the modified equation of state due to the broadening of the cloud width along the tightly confined directions, which occurs for large interactions. We find that at large times, the temperature decreases proportionally to the energy scale $mc^2$, where $m$ is the mass of the particles and $c$ the sound velocity. We compute the asymptotic ratio of these two quantities for different limiting cases: a homogeneous gas in any dimension and a one-dimensional gas in a harmonic trap.
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Anonymous Report 1 on 2018-7-26 (Invited Report)
- Cite as: Anonymous, Report on arXiv:1806.08759v2, delivered 2018-07-26, doi: 10.21468/SciPost.Report.540
1- Significant extension and generalization of previous theoretical models, which enables the interpretation of recent experiments studying cooling through losses in cold atomic gases.
2- Comprehensive, timely and relevant.
3- Clear explanation of all steps of the derivation.
None (see below for minor questions)
The authors present a general formalism for the cooling of quantum gases through uniform particle loss. While often associated with heating it is shown that under certain conditions few-particle loss processes can also lead to cooling. The study is motivated by a series of recent experiments, e.g. in the groups of Schmiedmayer and Bouchoule. As such, it is timely and relevant.
The present work is particularly interesting because the existing formalism to describe such cooling is significantly extended to cover general N-body losses, as well as arbitrary dimensions and realistic trapping potentials. For example, the cooling of a 1D gas through 3-body losses has recently been studied in an experiment by some of the authors for the first time. The results show good agreement with the theoretical predictions outlined in the present study. It will be very interesting to check experimentally if the predictions also hold in 3D situations, where evaporation (i.e. cooling through selective removal of particles with above average energy) will compete with the mechanism studied here (uniform loss of particles).
The study is well written and clearly explains all assumptions and steps of the derivation, as well as some limits of the theory. In particular, it nicely describes the inherent competition between the cooling of the low energy modes and their heating through shot noise.
A few minor questions:
1- Footnote 1: It is not quite clear to me what exactly the remark on Ito / Stratonovich formalisms refers to here? Could the authors briefly elaborate to make this clearer for the general reader?
2- Is there a simple way to estimate for which experimental parameters one would expect the assumption of Poissonian, uncorrelated shot noise to hold for the loss process?
3- In realistic experiments there will often be different loss processes present at once (e.g. three-body loss from interatomic collisions due high atomic densities, and one-body loss from background gas collisions), each with their own asymptotic temperature. What would one expect to see in these cases?
4- Is there a deeper meaning / interpretation behind the asymptotic temperature? Particularly Fig. 1b seems to suggest that the cooling exhibits some universal properties?
5- It would be nice to discuss if the gases become more degenerate during the cooling process. Can new regimes be reached, e.g. for 1D Bose gases?