Observation of squeezed light from one atom excited with two photons

Abstract

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 times¹. 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 light⁴, 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 photons⁵. Squeezed beams have been generated using macroscopic and mesoscopic media down to a few tens of atoms⁶, 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 nonlinearity¹⁰, 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 photons¹⁷, the squeezed light stems from the quantum coherence of photon pairs emitted from the system¹⁸. 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}.