Transmission estimation at the Cramér-Rao bound for squeezed states of light in the presence of loss and imperfect detection

Abstract

Enhancing the precision of a measurement requires maximizing the information that can be gained about the quantity of interest from probing a system. For optical based measurements, such an enhancement can be achieved through two approaches, increasing the number of photons used to interrogate the system and using quantum states of light to increase the amount of quantum Fisher information gained per photon. Here we consider the use of quantum states of light with a large number of photons, namely the bright single-mode and two-mode squeezed states, that take advantage of both of these approaches for the problem of transmission estimation. We show that, in the limit of large squeezing, these states approach the maximum possible quantum Fisher information per photon for transmission estimation that is achieved with the Fock state and the vacuum two-mode squeezed state. Since the bright states we consider can be generated at much higher powers than the quantum states that achieve the maximum quantum Fisher information per photon, they can achieve an much higher absolute precision as quantified by the quantum Cram\'er-Rao bound. We discuss the effects of losses external to the system on the precision of transmission estimation and identify simple measurements techniques that can saturate the quantum Cram\'er-Rao bound for the bright squeezed states even in the presence of such external losses.