Noisy metrology beyond the standard quantum limit

TL;DR

This paper demonstrates that in noisy quantum metrology, it is possible to achieve super-classical precision scaling by engineering the Hamiltonian and noise directions, overcoming previous limitations.

Contribution

It introduces a scenario where quantum advantage persists despite noise, by orienting the Hamiltonian and noise in perpendicular directions.

Findings

Super-classical scaling achieved with directional noise.

Optimal states identified for perpendicular Hamiltonian and noise.

Quantum advantage persists beyond previous noise restrictions.

Abstract

Parameter estimation is of fundamental importance in areas from atomic spectroscopy and atomic clocks to gravitational wave detection. Entangled probes provide a significant precision gain over classical strategies in the absence of noise. However, recent results seem to indicate that any small amount of realistic noise restricts the advantage of quantum strategies to an improvement by at most a multiplicative constant. Here, we identify a relevant scenario in which one can overcome this restriction and attain superclassical precision scaling even in the presence of uncorrelated noise. We show that precision can be significantly enhanced when the noise is concentrated along some spatial direction, while the Hamiltonian governing the evolution which depends on the parameter to be estimated can be engineered to point along a different direction. In the case of perpendicular orientation, we find superclassical scaling and identify a state which achieves the optimum.