Bose-Einstein condensation of photons and grand-canonical condensate fluctuations

TL;DR

This paper reviews experiments demonstrating Bose-Einstein condensation of photons in a dye-filled microresonator, highlighting the unique conditions enabling photon thermalization, condensation, and grandcanonical fluctuations.

Contribution

It presents experimental evidence of photon BEC in a microcavity and discusses the grandcanonical ensemble behavior of the condensate, a novel observation in photon gases.

Findings

Photon thermalization via fluorescence in dye microcavities

Observation of Bose-Einstein condensation of photons

Evidence of grandcanonical ensemble fluctuations in the condensate

Abstract

We review recent experiments on the Bose-Einstein condensation of photons in a dye-filled optical microresonator. The most well-known example of a photon gas, photons in blackbody radiation, does not show Bose-Einstein condensation. Instead of massively populating the cavity ground mode, photons vanish in the cavity walls when they are cooled down. The situation is different in an ultrashort optical cavity imprinting a low-frequency cutoff on the photon energy spectrum that is well above the thermal energy. The latter allows for a thermalization process in which both temperature and photon number can be tuned independently of each other or, correspondingly, for a non-vanishing photon chemical potential. We here describe experiments demonstrating the fluorescence-induced thermalization and Bose-Einstein condensation of a two-dimensional photon gas in the dye microcavity. Moreover, recent measurements on the photon statistics of the condensate, showing Bose-Einstein condensation in the grandcanonical ensemble limit, will be reviewed.