# Observation of Atom Number Fluctuations in a Bose-Einstein Condensate

## TL;DR

This paper reports the first experimental observation of atom number fluctuations in a Bose-Einstein condensate, revealing a sharp increase near the critical temperature, achieved through advanced stabilization and correlation techniques.

## Contribution

It introduces a stabilization method enabling precise measurement of atom number fluctuations in interacting BECs at finite temperature.

## Key findings

- Fluctuations increase sharply near the critical temperature.
- Technical noise sources were effectively eliminated.
- Correlations from evaporative cooling enabled precise fluctuation measurement.

## Abstract

Fluctuations are a key property of both classical and quantum systems. While the fluctuations are well understood for many quantum systems at zero temperature, the case of an interacting quantum system at finite temperature still poses numerous challenges. Despite intense theoretical investigations of atom number fluctuations in Bose-Einstein condensates (BECs), their amplitude in experimentally relevant interacting systems is still poorly understood. Moreover, technical limitations have prevented their experimental observation to date. Here we report the first observation of these fluctuations. Our experiments are based on a stabilization technique, which allows for the preparation of ultracold thermal clouds at the shot noise level, thereby eliminating numerous technical noise sources. Furthermore, we make use of the correlations established by the evaporative cooling process to precisely determine the fluctuations and the sample temperature. This allows us to observe a telltale signature: the sudden increase in fluctuations of the condensate atom number close to the critical temperature.