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 example2,3 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 efforts7,8,9, 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 media11,12,13. This produces observable quadrature squeezing14,15,16, 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 emitters19,20,21,22,23,24.
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Support from the Deutsche Forschungsgemeinschaft (Research Unit 635), the European Union (IST project AQUTE, ITN network CCQED) and the Bavarian PhD programme of excellence (QCCC) is acknowledged.
The authors declare no competing financial interests.
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Ourjoumtsev, A., Kubanek, A., Koch, M. et al. Observation of squeezed light from one atom excited with two photons. Nature 474, 623–626 (2011). https://doi.org/10.1038/nature10170
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