Achieving sub-shot-noise sensing at finite temperatures

TL;DR

This paper explores quantum spin chains for magnetic field sensing at finite temperatures, demonstrating how quantum phase crossovers can enhance sensitivity beyond traditional limits, with practical measurement strategies.

Contribution

It introduces an iterative feedforward protocol leveraging quantum phase crossovers to achieve super-extensive sensitivity scaling in spin chain sensors.

Findings

Magnetic sensitivity scales extensively with the number of spins in the XX chain.

The feedforward protocol enables super-extensive scaling of sensitivity.

Proposed observables can saturate quantum metrological bounds in realistic experiments.

Abstract

We investigate sensing of magnetic fields using quantum spin chains at finite temperature and exploit quantum phase crossovers to improve metrological bounds on the estimation of the chain parameters. In particular, we analyze the $ XX $ spin chain and show that the magnetic sensitivity of this system is dictated by its adiabatic magnetic susceptibility, which scales extensively (linearly) in the number of spins $ N $. Next, we introduce an iterative feedforward protocol that actively exploits features of quantum phase crossovers to enable super-extensive scaling of the magnetic sensitivity. Moreover, we provide experimentally realistic observables to saturate the quantum metrological bounds. Finally, we also address magnetic sensing in the Heisenberg $ XY $ spin chain.