Cold Atoms seminar

Rob Nyman, Imperial College London. Talk title:Bose-Einstein Condensates of Photons in a Dye-FilledMicrocavity. Bose-Einstein condensation (BEC) is a universalphenomenon which occurs when a system of identical bosons at thermalequilibrium occupy the ground state in enormous numbers. By optically pumping a1.5-micron long, dye-filled resonator, we can achieve both thermal equilibriumof photons and a well-defined ground state. Thus, the first room temperatureBEC was demonstrated [1]. We have become only the second laboratory to createthis quantum-fluid state of light. There are many recent published theoretical models ofphoton BEC, some using rate equations, other fully quantised matter-lightinteractions. Some of our steady-state observations, such as the variation ofcritical pump power with pump spot size, contradict the predictions of thesesimple models, giving a challenge to our theory collaborators [2]. We have observed the interference of the thermalizedlight and the visibility of fringes relates directly to the non-equilibriumcorrelation function. We have shown that the condensate has long rangecoherence in both space and time. We have also started experiments aimed atmeasuring the interactions between photons, which are currently poorlyunderstood. We expect that even very weak photon-photon interactions should bemeasurable this way [3].