Sea of photons made to act as one 'super-photon': First, the team placed two concave mirrors 1 micrometre apart, and filled the lens-shaped cavity between them with a red liquid dye. They then fired a green laser at the cavity. The dye absorbed photons from the laser and re-emitted them at lower-energy yellow wavelengths, which the mirrors focused at the centre of the cavity. While some photons were indeed absorbed by the mirrors, the large number present in the laser more than made up for this.
When the low-energy photons at the centre of the cavity reached a density of about a trillion photons per cubic centimetre, they began to act as a single photon, shifting in appearance from a blurry glow to a bright point (Nature, DOI: 10.1038/nature09567). "All the photons marched in lockstep," Weitz says.