Exploring the thermodynamics of a two-dimensional Bose gas

TL;DR

This study investigates the thermodynamic properties of a two-dimensional Bose gas using in situ measurements, revealing low entropy levels and methods to analyze energy contributions, with implications for cooling quantum fluids.

Contribution

The paper provides the first detailed equations of state for a 2D Bose gas and demonstrates how to measure entropy and energy contributions using experimental techniques.

Findings

Entropy per particle as low as 0.06 k_B in the degenerate regime

Method to disentangle kinetic, potential, and interaction energies

Evidence of reduced density fluctuations in non-coherent clouds

Abstract

Using \emph{in situ} measurements on a quasi two-dimensional, harmonically trapped $^{87}$Rb gas, we infer various equations of state for the equivalent homogeneous fluid. From the dependence of the total atom number and the central density of our clouds with the chemical potential and temperature, we obtain the equations of state for the pressure and the phase-space density. Then using the approximate scale invariance of this two-dimensional system, we determine the entropy per particle. We measure values as low as $0.06\,\kB$ in the strongly degenerate regime, which shows that a 2D Bose gas can constitute an efficient coolant for other quantum fluids. We also explain how to disentangle the various contributions (kinetic, potential, interaction) to the energy of the trapped gas using a time-of-flight method, from which we infer the reduction of density fluctuations in a non fully coherent cloud.