Intramode correlations enhanced phase sensitivities in an SU(1,1) interferometer

TL;DR

This paper theoretically analyzes the quantum Fisher information bounds in an SU(1,1) interferometer, highlighting the role of intramode correlations and photon number fluctuations in enhancing phase sensitivity beyond traditional limits.

Contribution

It derives bounds on quantum Fisher information for SU(1,1) interferometers and links intramode correlations to quantum enhancement in phase sensitivity.

Findings

Large intramode correlations improve phase sensitivity.

Photon-subtracted squeezed vacuum states can surpass standard scaling.

Heisenberg limit should account for photon number fluctuations.

Abstract

We theoretically derive the lower and upper bounds of quantum Fisher information (QFI) of an SU(1,1) interferometer whatever the input state chosen. According to the QFI, the crucial resource for quantum enhancement is shown to be large intramode correlations indicated by the Mandel $Q$-parameter. For a photon-subtracted squeezed vacuum state with high super-Poissonian statistics in one input port and a coherent state in the other input port, the quantum Cram\'{e}r-Rao bound of the SU(1,1) interferometer can beat $1/\langle\hat{N}\rangle$ scaling in presence of large fluctuations in the number of photons, with a given fixed input mean number of photons. The definition of the Heisenberg limit (HL) should take into account the amount of fluctuations. The HL considering the number fluctuation effect may be the ultimate phase limit.