Intrinsic metrological resolution as a distance measure and nonclassical light

Abstract

We elaborate on a Hilbert-Schmidt distance measure assessing the intrinsic metrological accuracy in the detection of signals imprinted on quantum probe states by signal-dependent transformations. For small signals this leads to a probe-transformation measure Λ fully symmetric on the probe ρ and the generator G of the transformation Λ(ρ,G)=Λ(G,ρ). Although Λ can be regarded as a generalization of variance, we show that no uncertainty relation holds for the product of measures corresponding to complementary generators. We show that all states with resolution larger than coherent states are nonclassical. We apply this formalism to feasible probes and transformations.