Joint estimation of real squeezing and displacement

We study the problem of joint estimation of real squeezing and amplitude of the radiation field, deriving the measurement that maximizes the probability density of detecting the true value of the unknown parameters. More generally, we provide a solution for the problem of estimating the unknown unitary action of a nonunimodular group in the maximum likelihood approach. Remarkably, in this case the optimal measurements do not coincide with the so-called square-root measurements. In the case of squeezing and displacement we analyse in detail the sensitivity of estimation for coherent states and displaced squeezed states, deriving the asymptotic relation between the uncertainties in the joint estimation and the corresponding uncertainties in the optimal separate measurements of squeezing and displacement. A two-mode set-up is also analysed, showing how entanglement between optical modes can be used to approximate perfect estimation. PACS numbers: 03.67.−a, 03.65.Ta, 42.50.−p (Some figures in this article are in colour only in the electronic version)