THE WEAKLY-INTERACTING PHOTON GAS IN 2D: BEC, SUPERFLUIDITY, AND QUANTIZED VORTICES

摘要

The conditions under which a Bose-Einstein condensate (BEC) of photons can form is discussed. In analogy with the recently observed atomic BECs, a photonic BEC can occur when photons weakly interact with each other while inside a harmonic trap. Photon-photon collisions can happen when photons interact with each other via atoms excited off resonance by a few Doppler widths. At the macroscopic level, this leads to the atoms forming a Kerr nonlinear optical medium. The photons, along with the atomic medium, are placed inside an optical resonator which forms a harmonic trapping potential in two transverse dimensions for the photons. For this paraxial geometry, the effective mass of the photons for their transverse, two-dimensional dynamics is hω/c~2, where ω is the frequency of the light. The scattering length of S-wave photon-photon collisions is of the order of a few nanometers. This is in agreement with an earlier experiment, in which head-on photon-photon collisions mediated by rubidium atoms in a vapor cell was been observed. Multiple photon-photon collisions inside the resonator lead to the formation of a "photon fluid," which is closely connected with the formation of the photonic BEC. The signature of BEC would be the formation of a central spike in the transverse momentum distribution of the photons, on top of a broad pedestal (the depletion) of uncondensed photons. Further implications of this photonic BEC state are the superfluidity of the photon fluid, when produced by laser light in a pure quantum state, and the appearance of quantized vortices in this superfluid.