Letter | Published:

Unconditional violation of the shot-noise limit in photonic quantum metrology

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.

Author information

Affiliations

1. Centre for Quantum Dynamics, Griffith University, Brisbane, Queensland, 4111, Australia

• Sergei Slussarenko
• , Morgan M. Weston
• , Helen M. Chrzanowski
•  & Geoff J. Pryde
2. Clarendon Laboratory, University of Oxford, Parks Road, Oxford, OX1 3PU, UK

• Helen M. Chrzanowski
3. National Institute of Standards and Technology, 325 Broadway, Boulder, CO, 80305, USA

• Lynden K. Shalm
• , Varun B. Verma
•  & Sae Woo Nam

Contributions

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.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Geoff J. Pryde.