Can some lasers operate as Bose Einstein condensates?

Stephane Barland

Université Côte d'Azur - CNRS - stephane.barland@univ-cotedazur.fr

Bose Einstein condensates consist of a large number of bosons in a single quantum state. This is strongly reminiscent of laser light, where many photons occupy a single coherent state, but there is a key difference: In BECs, bosons at equilibrium thermalize and, if their density is large enough for their wavefunctions to significantly overlap, eventually condense to a single state. On the other hand, the coherent emission threshold in lasers is attained when optical gain compensates losses. Thus, the lasing transition is typically an out of equilibrium process, in contrast to BEC.

Here we experimentally address the question of BEC of light in a Vertical Cavity Surface Emitting Laser. We measure the dispersion relation of light in the resonator and observe spectra compatible with a room-temperature thermal equilibrium distribution. A phase transition takes place when the phase space density is of order unity, as expected for a BEC transition.