Watching a BEC Condense

Observing the Formation of Long-Range Order during Bose-Einstein Condensation

Stephan Ritter, Anton \"{O}ttl, Tobias Donner, Thomas Bourdel, Michael K\"{o}hl, and Tilman Esslinger

PRL 98, 090402 (2007)

URL: http://link.aps.org/abstract/PRL/v98/e090402

Neat experiment!

The authors watched a Bose-Einstein condensate form. They took a cloud of bosonic atoms and shock cooled it (by lowering the temperature and getting rid of the hottest 30% of the population). This left a non-equilibrium gas of atoms whose equilibrium state was a Bose-Einstein condensate.

By measuring the formation of an interference pattern between two separated regions of the cloud, the authors were able to watch off-diagonal long-range order develop in real time. The visibility of the interference patter measures the phase coherence of the two regions.

The feature that differentiates a Bose-Einstein condensate from a cloud of cold atoms is off-diagonal long-range order. This means that the wave functions describing the density at regions far apart from one another maintains phase coherence --- kind of like entanglement, I suppose. It's known that this is the correct order parameter to study for BEC, superfluid, and superconducting phase transitions, but exactly how it goes from being zero to being not zero is an interesting question. The dynamics of a phase transition.

There are two stages of the condensation process.

• Kinematic: Collisions between particles bring the system into its equilibrium state --- i.e. the energy distribution is thermalized. The time scale of this phase of condensation is set by the collision time.

• Coherent: In this stage of condensation, small regions of coherent atoms merge into a single coherent state.

The authors find that the size of coherent regions grows at a speed about 1/5 the maximum speed of sound in the cloud. As far as I can tell, there is no theoretical estimate of what this speed should be --- just the intuitive idea that it shouldn't be faster than the speed of sound.

The plots in the paper really tell the story. The authors have done an excellent job of presenting their data.