Martin Weitz and colleagues at the University of Bonn in Germany have shown that photons that are cooled and confined inside an optical cavity can form a quantum system with two distinct energy states (Phys. Rev. Lett., doi: 10.1103/kynj-l87s). For small numbers of photons they find that the particles become evenly distributed between the two energy levels, while for larger populations almost all the photons accumulate in the ground state of the system.

Cooling things down

Weitz and his team have pioneered the study of photons that are cooled through repeated interactions with dye molecules within a small and reflective optical cavity. The energy lost by the photons through these interactions reduces their effective temperature to around 300 K. At the same time, the spacing between the cavity mirrors produces a single optical mode that prevents the photons from moving in the vertical direction.

The difference in energy between the two states was around 100 times smaller than the thermal energy of the photons.

In this regime, the photons form a two-dimensional gas that behaves in a similar way to a cloud of ultracold atoms. In one striking example, Weitz and his team showed in 2010 that photons can form the optical equivalent of a Bose–Einstein condensate, with all the photons collapsing into the lowest energy state of the system.

Two-state quantum system

While previous experiments have focused on photons with a continuous energy spectrum, in this work the researchers investigated the behavior of a two-state quantum system. By creating two small indents in one of the cavity mirrors, they produced two optical modes that forced the photons to occupy one of two possible energy states. The difference in energy between the two states was around 100 times smaller than the thermal energy of the photons, with the researchers showing that a pulsed laser could be used to switch the photons between the two states.

In experiments without this strong laser pumping, small numbers of cooled photons within the optical cavity become evenly spread between the two energy states. However, larger populations tend to accumulate in the lower energy level, with more than 90% of the measured light emission coming from photons in this ground state of the system.

Understanding this fundamental behavior could offer new strategies for generating quantum states for novel sensing applications or quantum key distribution. The tendency of large photon populations to occupy the same energy state could also inform the design of more powerful laser sources, since it could provide a mechanism for combining multiple sources of radiation without the need for precise phase locking. “Our findings suggest this could work, but there’s a long way to go until the technology is up and running,” says Weitz.