3 years ago

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

Sae Woo Nam, Varun B. Verma, Geoff J. Pryde, Helen M. Chrzanowski, Morgan M. Weston, Lynden K. Shalm, Sergei Slussarenko
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 $Δ ϕ$ achievable using n photons is the shot-noise limit, $Δϕ = 1 ∕ n$. 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.

Publisher URL: https://www.nature.com/articles/s41566-017-0011-5

DOI: 10.1038/s41566-017-0011-5

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