Quantum Phase Estimation with Entangled Photons produced by Parametric Down Conversion

TL;DR

This paper investigates how entangled photon pairs from parametric down-conversion can be used for high-precision quantum measurements, demonstrating that entanglement isn't essential for ultimate sensitivity and analyzing the effects of losses.

Contribution

It shows that high-precision quantum measurements can be achieved with entangled photons without relying on entanglement, and characterizes the impact of photon losses on measurement precision.

Findings

Heisenberg-limited scaling possible with photon counting in lossless conditions

Precision degrades to shot-noise limit as losses approach 100%

Bayesian simulation confirms hypersensitivity predictions

Abstract

We explore the advantages offered by twin light beams produced in parametric down-conversion for precision measurement. The symmetry of these bipartite quantum states, even under losses, suggests that monitoring correlations between the divergent beams permits a high-precision inference of any symmetry-breaking effect, e.g. fiber birefringence. We show that the quantity of entanglement is not the key feature for such an instrument. In a lossless setting, scaling of precision at the ultimate `Heisenberg' limit is possible with photon counting alone. Even as photon losses approach 100% the precision is shot-noise limited, and we identify the crossover point between quantum and classical precision as a function of detected flux. The predicted hypersensitivity is demonstrated with a Bayesian simulation.