Direct observation of quantum phonon fluctuations in a one dimensional Bose gas

TL;DR

This paper presents the first direct measurement of quantum phonon fluctuations in a one-dimensional Bose gas, revealing quantum effects at microscopic scales through in situ imaging and statistical analysis.

Contribution

It provides the first direct observation of collective quantum fluctuations in a continuous 1D Bose gas using high-resolution in situ imaging.

Findings

Quantum phonons constitute up to 20% of fluctuations in the cloud center.

Deviations from classical predictions indicate dominant quantum fluctuations at short length scales.

Quantum effects emerge as the thermodynamic limit breaks down.

Abstract

We report the first direct observation of collective quantum fluctuations in a continuous field. Shot-to-shot atom number fluctuations in small sub-volumes of a weakly interacting ultracold atomic 1D cloud are studied using \textit{in situ} absorption imaging and statistical analysis of the density profiles. In the cloud centers, well in the \textit{quantum quasicondensate} regime, the ratio of chemical potential to thermal energy is $\mu/ k_B T\simeq4$, and, owing to high resolution, up to 20% of the microscopically observed fluctuations are quantum phonons. Within a non-local analysis at variable observation length, we observe a clear deviation from a classical field prediction, which reveals the emergence of dominant quantum fluctuations at short length scales, as the thermodynamic limit breaks down.