Probing Phase Fluctuations in a 2D Degenerate Bose Gas by Free Expansion

TL;DR

This study measures the density fluctuation spectrum of a 2D Bose gas during free expansion, revealing temperature-dependent phase fluctuations, oscillatory spectral features, and vortex dynamics, advancing understanding of 2D quantum gases.

Contribution

It introduces a method to probe phase fluctuations via power spectrum analysis during free expansion, demonstrating temperature dependence and vortex formation in 2D Bose gases.

Findings

Spectral peak positions are temperature-independent and scale during expansion.

Phase fluctuation strength decreases at lower temperatures, confirming thermal origin.

Observation of free vortices with ring-shaped density ripples after relaxation.

Abstract

We measure the power spectrum of the density distribution of a freely expanding 2D degenerate Bose gas, where irregular density modulations gradually develop due to the initial phase fluctuations in the sample. The spectrum has an oscillatory shape, where the peak positions are found to be independent of temperature and show scaling behavior in the course of expansion. The relative intensity of phase fluctuations is estimated from the normalized spectral peak strength and observed to decrease at lower temperatures, confirming the thermal nature of the phase fluctuations. We investigate the relaxation dynamics of nonequilibrium states using the power spectrum. Free vortices are observed with ring-shaped density ripples in a perturbed sample after a long relaxation time.