Probing the local rapidity distribution of a 1D Bose gas

TL;DR

This paper introduces a method to measure the local rapidity distribution in a 1D Bose gas by analyzing the asymptotic momenta after expansion, revealing non-thermal features in out-of-equilibrium states.

Contribution

The authors develop a novel local probing technique for rapidity distributions in 1D Bose gases using expansion dynamics and Generalized Hydrodynamics theory.

Findings

Experimental validation of the local rapidity distribution in equilibrium.

Observation of a doubly-peaked local rapidity distribution in non-thermal states.

Finite expansion time effects can be accurately modeled with Generalized Hydrodynamics.

Abstract

One-dimensional Bose gases with contact repulsive interactions are characterized by the presence of infinite-lifetime quasiparticles whose momenta are called the `rapidities'. Here we develop a probe of the local rapidity distribution, based on the fact that rapidities are the asymptotic momenta of the particles after a long one-dimensional expansion. This is done by performing an expansion of a selected slice of the gas. We first apply this idea to a cloud in the quasi-condensate regime at equilibrium in a trap. We obtain an experimental picture of the position-dependent rapidity distribution which is in fair agreement with the theory prediction. The asymptotic regime is barely reached, but we show that finite expansion time can be taken into account using the Generalized Hydrodynamics theory. We then apply this local probe to an out-of-equilibrium situation where the local rapidity distribution is expected to be doubly peaked -- a hallmark of a non-thermal state -- even though the global rapidity distribution would possess no such distinctive feature. We observe the doubly-peaked local rapidity distribution.