Observation of squeezed light from one atom excited with two photons

Squeezing light from a single atom 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. Single quantum emitters such as atoms are well known as non-classical light sources with reduced noise in the intensity, capable of producing photons one by one at given times 1 . However, the light field emitted by a single atom can exhibit much richer dynamics. A prominent example 2 , 3 is the predicted ability of a single atom to produce quadrature-squeezed light 4 , which has fluctuations of amplitude or phase that are below the shot-noise level. However, such squeezing is much more difficult to observe than the emission of single photons 5 . Squeezed beams have been generated using macroscopic and mesoscopic media down to a few tens of atoms 6 , but despite experimental efforts 7 , 8 , 9 , single-atom squeezing has so far escaped observation. Here we generate squeezed light with a single atom in a high-finesse optical resonator. The strong coupling of the atom to the cavity field induces a genuine quantum mechanical nonlinearity 10 , which is several orders of magnitude larger than in typical macroscopic media 11 , 12 , 13 . This produces observable quadrature squeezing 14 , 15 , 16 , with an excitation beam containing on average only two photons per system lifetime. In sharp contrast to the emission of single photons 17 , the squeezed light stems from the quantum coherence of photon pairs emitted from the system 18 . The ability of a single atom to induce strong coherent interactions between propagating photons opens up new perspectives for photonic quantum logic with single emitters 19 , 20 , 21 , 22 , 23 , 24 .