Loss-resistant unambiguous phase measurement

TL;DR

This paper proposes a practical method to generate loss-resistant quantum states for phase measurement using parametric down-conversion, achieving near-optimal performance and unambiguous measurement beyond the standard quantum limit.

Contribution

It introduces a feasible approach to produce near-optimal loss-resistant states for phase measurement, enabling unambiguous quantum measurements with improved accuracy.

Findings

States generated are close to optimal in Fisher information.

Sequences of these states enable beating the standard quantum limit.

Optimal parameters differ from Fisher information maximization.

Abstract

Entangled multi-photon states have the potential to provide improved measurement accuracy, but are sensitive to photon loss. It is possible to calculate ideal loss-resistant states that maximize the Fisher information, but it is unclear how these could be experimentally generated. Here we propose a set of states that can be obtained by processing the output from parametric down-conversion. Although these states are not optimal, they provide performance very close to that of optimal states for a range of parameters. Moreover, we show how to use sequences of such states in order to obtain an unambiguous phase measurement that beats the standard quantum limit. We consider the optimization of parameters in order to minimize the final phase variance, and find that the optimum parameters are different from those that maximize the Fisher information.