  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.
