Resonance fluorescence arises from the interaction of an optical field with a two-level system, and has played a fundamental role in the development of quantum optics and its applications. Despite its conceptual simplicity, it entails a wide range of intriguing phenomena, such as the Mollow-triplet emission spectrum1, photon antibunching2 and coherent photon emission3. One fundamental aspect of resonance fluorescence—squeezing in the form of reduced quantum fluctuations in the single photon stream from an atom in free space—was predicted more than 30 years ago4. However, the requirement to operate in the weak excitation regime, together with the combination of modest oscillator strength of atoms and low collection efficiencies, has continued to necessitate stringent experimental conditions for the observation of squeezing with atoms. Attempts to circumvent these issues had to sacrifice antibunching, owing to either stimulated forward scattering from atomic ensembles5,6 or multi-photon transitions inside optical cavities7,8. Here, we use an artificial atom with a large optical dipole enabling 100-fold improvement of the photon detection rate over the natural atom counterpart9 and reach the necessary conditions for the observation of quadrature squeezing in single resonance-fluorescence photons. By implementing phase-dependent homodyne intensity-correlation detection9,10,11, we demonstrate that the electric field quadrature variance of resonance fluorescence is three per cent below the fundamental limit set by vacuum fluctuations, while the photon statistics remain antibunched. The presence of squeezing and antibunching simultaneously is a fully non-classical outcome of the wave–particle duality of photons.
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We acknowledge financial support from the University of Cambridge, the European Research Council ERC Consolidator Grant Agreement No. 617985 and the EU-FP7 Marie Curie Initial Training Network S3NANO. C.M. acknowledges Clare College Cambridge for financial support through a Junior Research Fellowship. We thank E. Clarke, M. Hugues and the EPSRC National Centre for III-V Technologies for the wafer and C. Baune, R. Moghadas Nia, W. Vogel, G. Rempe, H. J. Carmichael and A. Ourjoumtsev for discussions.
This file contains Supplementary Text and Data comprising: 1 Theory of homodyne intensity autocorrelation measurement; 2 Wigner Functions; 3 Theoretical power dependence for Figure 3; and additional references.
About this article
Nature Communications (2016)