Abstract
Interferometric phase measurement is widely used to precisely determine quantities such as length, speed and material properties1,2,3. Without quantum correlations, the best phase sensitivity \({\boldsymbol{\Delta }}{\boldsymbol{\phi }}\) achievable using n photons is the shot-noise limit, \({\boldsymbol{\Delta }}{\boldsymbol{\phi }}=1\,/\sqrt{{n}}\). Quantum-enhanced metrology promises better sensitivity, but, despite theoretical proposals stretching back decades3,4, no measurement using photonic (that is, definite photon number) quantum states has truly surpassed the shot-noise limit. Instead, all such demonstrations, by discounting photon loss, detector inefficiency or other imperfections, have considered only a subset of the photons used. Here, we use an ultrahigh-efficiency photon source and detectors to perform unconditional entanglement-enhanced photonic interferometry. Sampling a birefringent phase shift, we demonstrate precision beyond the shot-noise limit without artificially correcting our results for loss and imperfections. Our results enable quantum-enhanced phase measurements at low photon flux and open the door to the next generation of optical quantum metrology advances.
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Acknowledgements
This work was supported by the Australian Research Council (grant DP140100648). The authors thank J. Ho for help with SNSPDs.
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G.J.P. conceived the idea and supervised the project. S.S. and M.M.W. constructed and carried out the experiment with help from H.M.C. L.K.S., V.B.V. and S.W.N. developed the high-efficiency SNSPDs. All authors discussed the results and contributed to writing the manuscript.
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Slussarenko, S., Weston, M.M., Chrzanowski, H.M. et al. Unconditional violation of the shot-noise limit in photonic quantum metrology. Nature Photon 11, 700–703 (2017). https://doi.org/10.1038/s41566-017-0011-5
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DOI: https://doi.org/10.1038/s41566-017-0011-5
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