Quantum Joule-Thomson Effect in a Saturated Homogeneous Bose Gas

Abstract

We study the thermodynamics of Bose-Einstein condensation in a weakly interacting quasi-homogeneous atomic gas, prepared in an optical-box trap. We characterise the critical point for condensation and observe saturation of the thermal component in a partially condensed cloud, in agreement with Einstein's textbook picture of a purely statistical phase transition. Finally, we observe the quantum Joule-Thomson effect, namely isoenthalpic cooling of an (essentially) ideal gas. In our experiments this cooling occurs spontaneously, due to energy-independent collisions with the background gas in the vacuum chamber. We extract a Joule-Thomson coefficient µJT > 10 9 K/bar, about ten orders of magnitude larger than observed in classical gases. [3]. However, they are traditionally produced in harmonic traps, which makes them spatially inhomogeneous and thus different both from other correlated systems, that they often aim to emulate, and from textbook models. This difference can often be addressed using local density approximation (LDA), and is in some cases even beneficial [4][5][6][7][8][9][10]. However, spatial inhomogeneity is also often problematic. Integrating over the varying density can smear or even qualitatively change experimental signatures (see, e.g., [11][12][13][14][15]). Moreover, LDA breaks down close to phase transitions, where the correlation length diverges [16]. Recently, a Bose-Einstein condensate (BEC) of atoms in an essentially uniform potential was achieved [17], opening new possibilities for studies of both quantum-statistical and interaction effects in many-body systems.In this Letter, we explore the thermodynamics of a weakly interacting quasi-homogeneous Bose gas, prepared in an optical-box trap (see Fig. 1). We characterise the critical point for condensation and demonstrate two purely quantumstatistical phenomena that are, despite weak interactions, obscured in harmonically trapped gases. First, we observe saturation of the thermal component in a partially condensed gas, in agreement with Einstein's textbook picture of condensation as a purely statistical phase transition. Second, we observe and quantitatively explain cooling of partially condensed clouds through isoenthalpic rarefaction, driven by collisions with the background gas in the vacuum chamber. This phenomenon is the quantum version of the Joule-Thomson (JT) effect, exploited in thermal machines such as refrigerators and heat pumps. In the classical JT process, isoenthalpic cooling occurs only due to interactions, whereas a quantum Bose gas is expected to show it even in their absence. This phenomenon was predicted as early as 1937 [18], and was at the time proposed as a way to experimentally observe the quantum statistics obeyed by a gas, the effect being opposite for fermions (see also [19]). However, to our knowledge, JT cooling has never before been experimentally observed in the regime where quantum-statistical effects dominate over the interaction ones. Outside the realm of ultracold atomic gases, the quantum JT effect is ma...

Cooling of a Bose-Einstein Condensate by Spin Distillation

“…This competition can lead to an increase in condensate fraction, because BEC atoms have zero energy. Hence, melting of the BEC cools the thermal gas in m min s , which can then be saturated at a lower temperature, as already observed in [20] and [21].…”

Negative Differential Conductivity in an Interacting Quantum Gas

“…4). This is an extreme manifestation of the Joule Thompson effect in the quantum regime [24], where the interaction energy is converted into thermal energy. For larger filling, the atoms condense, and the width of the density distribution gets smaller.…”

Homogeneous Atomic Fermi Gases

“…This is most severe for supersolid states, such as the elusive FFLO state [21][22][23], where the emergent spatial period is well defined only in a homogeneous setting. A natural solution to these problems is the use of uniform potentials, which have recently proved to be advantageous for thermodynamic and coherence measurements with Bose gases [24][25][26][27].…”