| 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 Bose-Einstein condensate. This observation provides an important confirmation of the basic nature of quantized excitations in the condensate. 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. When the quench time drops below the measured thermal equilibration time, the phonon temperature is out of equilibrium with the surrounding thermal cloud. Thus, for rapid cooling rates, a Bose-Einstein condensate is not as cold as previously thought. These measurements are enabled by our in situ k-space technique. This technique also allows for a quantitative observation of bunching above the critical temperature. |
