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Quadrature squeezed photons from a two-level system


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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Figure 1: Homodyne intensity-correlation detection.
Figure 2: Phase-dependent quadrature variance of resonance fluorescence.
Figure 3: Excitation-power dependence of quadrature squeezing.


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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.

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Authors and Affiliations



C.H.H.S. and M.A. devised the experiment, C.H.H.S., J.H., A.E.J., C.M. and C.L.G. performed the experiments, C.H.H.S., J.H. and C.L.G. developed the models and analysed the data, all authors contributed to the discussion of the results and the manuscript preparation. C.H.H.S. and C.M. processed the quantum dot device.

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Correspondence to Mete Atatüre.

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The authors declare no competing financial interests.

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Supplementary Information

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. (PDF 108 kb)

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Schulte, C., Hansom, J., Jones, A. et al. Quadrature squeezed photons from a two-level system. Nature 525, 222–225 (2015).

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