Ensemble based quantum metrology

TL;DR

This paper explores how ensemble-based quantum metrology can surpass classical limits by leveraging initial system polarization and specific quantum strategies, especially under dephasing conditions.

Contribution

It identifies the importance of initial polarization as a resource and proposes a quantum measurement strategy that can outperform the standard quantum limit under realistic decoherence.

Findings

Performance depends on decoherence type

Quantum strategy can beat the standard quantum limit

Initial polarization is a crucial resource

Abstract

The field of quantum metrology promises measurement devices that are fundamentally superior to conventional technologies. Specifically, when quantum entanglement is harnessed the precision achieved is supposed to scale more favourably with the resources employed, such as system size and the time required. Here we consider measurement of magnetic field strength using an ensemble of spins, and we identify a third essential resource: the initial system polarisation, i.e. the low entropy of the original state. We find that performance depends crucially on the form of decoherence present; for a plausible dephasing model, we describe a quantum strategy which can indeed beat the standard quantum limit.