Phases and phase transitions of Bose condensed light

Bose-Einstein condensation of light [1] is characterized by two classical complex fields corresponding to two polarizations of light as well as by the distribution of dye molecules inducing light thermalization through dipolar transition coupled to the thermal bath of molecular vibrations. We emphasize a crucial role of removing the full degeneracy of the dipolar transition in forming algebraic order of the condensate in 2D. The resulting symmetries of the condensate can be characterized by groups O(2)XZ₂, O(2) and Z₂ order emerging before O(2). [If the transition is triple degenerate, the symmetry becomes O(4) which excludes the algebraic condensation at any finite temperature]. The main result of this work [2] addresses orientational disorder introduced by local dipolar anisotropy. It can destroy algebraic order in one-photon density matrix while preserving it in the two-photon one. This produces a condensate of photon pairs without any attraction between photons. We call such pairing geometrical.

[1] J. Klaers, F. Vewinger and M. Weitz, Nature Phys. 6 , 512 (2010); J. Klaers, J. Schmitt, F. Vewinger and M. Weitz, Nature 468, 545 (2010).
[2] V. Fleurov, A.B. Kuklov, arXiv:1808.09480.