Observation of squeezed light from one atom excited with two photons

The light field emitted by a single atom can exhibit non-classical effects such as 'squeezing', which is characterized by sub-shot noise amplitude or phase fluctuations. This phenomenon was predicted 30 years ago, but has been observed experimentally only for macroscopic and mesoscopic media down to a few tens of atoms. Ourjoumtsev et al. now report the observation of squeezed light generated from a single atom excited by laser light in a high-finesse optical resonator. In contrast to the emission of single photons, a more easily observed event, the squeezed light derives from the quantum coherence of photon pairs emitted from the system. This may offer new perspectives for photonic quantum logic with single emitters.