Feasible phase detection with ideal sensitivity

Abstract. We present a first feasible scheme for detecting the phase of a single mode of radiation with ideal RMS sensitivity Nn−1 versus the average number of photons Nn. It involves pairs of alternate independent homodyne measurements of two conjugated quadratures on a weakly squeezed state at the input. Nonunit quantum efficiency of photodetectors degrades phase sensitivity leading to a power law Nn−γ./, with γ increasing versus . In phase-sensing interferometers minute variations of environmental parameters are detected through changes in the phase shift of a light beam relative to a local oscillator (LO). The back-action effect of radiation pressure on the measured parameter poses the problem of optimizing phase sensitivity for a given average number of photons Nn [1, 2]. In a shot-noise limited homodyne interferometer that uses coherent states, the root-mean-square (RMS) phase error is proportional to Nn−1=2. Sensitivity can be improved up to Nn−1 using squeezed states [1–3]. Such a power law, however, only holds in a small neighbourhood of a fixed working point that should be pursued by a feedback, whereas any sizeable phase shift would greatly degrade sensitivity. Moreover, nonideal quantum efficiency < 1 of photodetectors leads again to shot noise for Nn> =[8.1 − /]