Unbiased estimation of an optical loss at the ultimate quantum limit with twin-beams

Loss measurements are at the base of spectroscopy and imaging, thus perme- ating all the branches of science, from chemistry and biology to physics and material science. However, quantum mechanics laws set the ultimate limit to the sensitivity, constrained by the probe mean energy. This can be the main source of uncertainty, for example when dealing with delicate system such as biological samples or photosensitive chemicals. It turns out that ordinary (clas- sical) probe beams, namely with Poissonian photon number distribution, are fundamentally inadequate to measure small losses with the highest sensitivity. Conversely, we demonstrate that a quantum-correlated pair of beams, known as twin-beam state, allows reaching the ultimate sensitivity for all energy regimes (even less than one photon per mode) with the simplest measurement strategy. One beam of the pair addresses the sample, while the second one is used as a reference to compensate both for classical drifts and for uctuation at the most fundamental quantum level. This scheme is also absolute and accurate, since it self-compensates for unavoidable instability of the sources and detectors, which could otherwise lead to strongly biased results. Moreover, we report the best sensitivity per photon ever achieved in loss estimation experiments.