# Theoretical Physics

**25 June 2015**

**Time:**n/a

**Location:**

Theoretical Physics Seminar: Rob Nyman (Imperial)

Bose-Einstein Condensates of Photons in a Dye-Filled Microcavity

Rob Nyman, Imperial College London

Bose-Einstein condensation (BEC) is a universal phenomenon which occurs when a system of identical bosons at thermal equilibrium occupy the ground state in enormous numbers. By optically pumping a 1.5-micron long, dye-filled resonator, we can achieve both thermal equilibrium of photons and a well-defined ground state. Thus, the first room temperature BEC was demonstrated [1]. We have become only the second laboratory to create this quantum-fluid state of light.

There are many recent published theoretical models of photon BEC, some using rate equations, other fully quantised matter-light interactions. Some of our steady-state observations, such as the variation of critical pump power with pump spot size, contradict the predictions of these simple models, giving a challenge to our theory collaborators [2].

We have observed the interference of the thermalised light and the visibility of fringes relates directly to the non-equilibrium correlation function. We have shown that the condensate has long range coherence in both space and time. We have also started experiments aimed at measuring the interactions between photons, which are currently poorly understood. We expect that even very weak photon-photon interactions should be measurable this way [3].

[1] J. Klaers et al, Nature, vol. 468, p545 (2010)

[2] J. Marelic and R.A. Nyman, Phys. Rev. A, vol 91, 033813 (2015)

[3] R.A. Nyman and M.H. Szymanska, Phys. Rev. A, vol 89, 033844 (2014)

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