In their experiments, the resesearchers used ultrathin crystals consisting of a single layer of atoms. These sheet was sandwiched between two layers of mirror-like materials. The whole structure acts like a cage for light and is called a "microcavity".
This setup was cooled to a few degrees above absolute zero. The researchers stimulated the crystal in the middle by short pulses of laser light (not shown). A sudden increase in the light emissions from the sample (red) indicated that a Bose-Einstein Condensate out of exciton-polaritons had been formed.