Fisher information of a squeezed-state interferometer with a finite photon-number resolution

TL;DR

This paper analyzes the Fisher information in squeezed-state interferometers with finite photon-number resolution, showing how detector limitations affect phase estimation precision and providing formulas to quantify information loss.

Contribution

It demonstrates that the Fisher information for each photon-number component saturates the quantum Fisher information and introduces an analytical formula to estimate information loss due to finite photon resolution.

Findings

Classical Fisher information saturates quantum Fisher information for each N-photon component.

Total Fisher information is additive over photon-number components.

Over 96% of the Heisenberg limit can be achieved with photon-number resolution larger than 5 times the mean photon number.

Abstract

Squeezed-state interferometry plays an important role in quantum-enhanced optical phase estimation, as it allows the estimation precision to be improved up to the Heisenberg limit by using ideal photon-number-resolving detectors at the output ports. Here we show that for each individual $N$-photon component of the phase-matched coherent $\otimes$ squeezed vacuum input state, the classical Fisher information always saturates the quantum Fisher information. Moreover, the total Fisher information is the sum of the contributions from each individual $N$-photon components, where the largest $N$ is limited by the finite number resolution of available photon counters. Based on this observation, we provide an approximate analytical formula that quantifies the amount of lost information due to the finite photon number resolution, e.g., given the mean photon number $\bar{n}$ in the input state, over $96$ percent of the Heisenberg limit can be achieved with the number resolution larger than $5\bar{n}$.