Polarization of a Photon Bose-Einstein Condensate

Abstract

A Bose-Einstein condensate is a state of matter where bosonic particles macroscopically occupy the ground state of a system. It was predicted as early as 1924 by Satyendra Nath Bose \& Albert Einstein. Although this state of matter was deemed possible, it was only experimentally achieved relatively recently. In 1995 by Eric Cornell, Carl Wieman \& Wolfgang Ketterle. This Bose-Einstein condensate was made using massive bosonic particles. But all bosons should be able to under go Bose-Einstein condensation so why not the most common one of them all, the photon?

This question was mostly waved away with a quick reference to the fact that photons adhere to Planck's law. For Bose-Einstein condensation one should be able to tune the temperature and particle number independently. It was thus thought that photons could not be made to condense. This all changed when Martin Weitz showed that he could tune both the temperature and particle number of photons independently by confining them to a white-wall box filled with a medium.

In 2010 they became the first group to achieve a Bose-Einstein condensate of photons. They used an optical dye filled microcavity to make a two-dimensional photon gas Bose-Einstein condense. In 2015 we became the second group in the world to replicate these results.

One of the characteristics of a photon Bose-Einstein condensate that has not yet been researched is its polarization. As Bose-Einstein condensation is an example of spontaneous symmetry breaking. One could expect each condensate to pick a well defined but random polarization. However, slight anisotropy could also force each condensate to pick the same polarization. To answer these questions we build an experimental setup which can measure all four Stokes parameters at the same time.

We find that the polarization of every condensate was the same. The polarization of the condensate shows a clear distinction from the polarization of the thermal cloud. We find that the thermal cloud is unpolarized while the condensate is a superposition of multiple polarization states with a strong linear contribution.