Linear Optical Quantum Metrology with Single Photons: Exploiting Spontaneously Generated Entanglement to Beat the Shot-Noise Limit

TL;DR

This paper demonstrates that passive linear optical interferometers fed with single photons can generate entanglement that surpasses the shot-noise limit, enabling practical quantum metrology with existing technology.

Contribution

It shows that simple, passive linear optical devices with single-photon inputs can exploit spontaneously generated entanglement to achieve super-sensitive measurements beyond the shot-noise limit.

Findings

Passive linear optical interferometers generate large entanglement.

Single-photon inputs can beat the shot-noise limit.

Practical quantum metrology is feasible with current technology.

Abstract

Quantum number-path entanglement is a resource for super-sensitive quantum metrology and in particular provides for sub-shotnoise or even Heisenberg-limited sensitivity. However, such number-path entanglement has thought to have been resource intensive to create in the first place --- typically requiring either very strong nonlinearities, or nondeterministic preparation schemes with feed-forward, which are difficult to implement. Very recently, arising from the study of quantum random walks with multi-photon walkers, as well as the study of the computational complexity of passive linear optical interferometers fed with single-photon inputs, it has been shown that such passive linear optical devices generate a superexponentially large amount of number-path entanglement. A logical question to ask is whether this entanglement may be exploited for quantum metrology. We answer that question here in the affirmative by showing that a simple, passive, linear-optical interferometer --- fed with only uncorrelated, single-photon inputs, coupled with simple, single-mode, disjoint photodetection --- is capable of significantly beating the shotnoise limit. Our result implies a pathway forward to practical quantum metrology with readily available technology.