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

Interferometric phase measurement is widely used to precisely determine quantities such as length, speed and material properties 1–3 . Without quantum correlations, the best phase sensitivity Δ φ achievable using n photons is the shot-noise limit, Δ φ =1\,/√n . Quantum-enhanced metrology promises better sensitivity, but, despite theoretical proposals stretching back decades 3,4 , 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. Unconditional entanglement-enhanced photonic interferometry is implemented by using a state-of-the-art photon source and detectors. Sampling a birefringent phase shift, precision beyond the shot-noise limit is demonstrated without data correction.