The Planck distribution of phonons in a Bose-Einstein condensate

TL;DR

This paper reports the experimental observation of a Planck distribution of thermal phonons in a Bose-Einstein condensate, confirming the quantum nature of its excitations and revealing non-equilibrium effects during rapid cooling.

Contribution

First direct observation of the phonon Planck distribution in a 3D Bose-Einstein condensate, confirming theoretical predictions about quantized excitations.

Findings

Thermal phonons follow a Planck distribution in a BEC.

Density fluctuations increase with temperature due to non-conservation of phonons.

Rapid cooling leads to non-equilibrium phonon temperatures, affecting condensate properties.

Abstract

The Planck distribution of photons emitted by a black body led to the development of quantum theory. An analogous distribution of phonons should exist in a Bose-Einstein condensate. We observe this Planck distribution of thermal phonons in a 3D condensate. This observation provides an important confirmation of the basic nature of the condensate's quantized excitations. In contrast to the bunching effect, the density fluctuations are seen to increase with increasing temperature. This is due to the non-conservation of the number of phonons. In the case of rapid cooling, the phonon temperature is out of equilibrium with the surrounding thermal cloud. In this case, a Bose-Einstein condensate is not as cold as previously thought. These measurements are enabled by our in situ k-space technique.