Nonequilibrium thermometry via an ensemble of initially correlated qubits

TL;DR

This paper explores a quantum thermometry method using correlated qubits to measure bath temperature, highlighting the role of initial quantum correlations and transient dynamics in enhancing measurement sensitivity.

Contribution

It demonstrates that initial quantum correlations among qubits improve thermometric precision and identifies optimal initial states for high-accuracy temperature sensing.

Findings

Quantum Fisher Information peaks during early thermalization, indicating enhanced sensitivity.

Quantum correlations among qubits always improve the QFI compared to uncorrelated states.

Maximally entangled states approach the standard quantum limit at high bath temperatures.

Abstract

We investigate a nonequilibrium quantum thermometry protocol in which an ensemble of qubits, acting as temperature probes, is weakly coupled to a macroscopic thermal bath. The temperature of the bath, the parameter of interest, is encoded in the dissipator of a Markovian thermalization process. For some relevant initial states, we observe a peak in the Quantum Fisher Information (QFI) during the transient of the thermalization, indicating enhanced sensitivity in early-time dynamics. This effect becomes more pronounced at higher bath temperatures and is further enhanced when the initial reduced state of the qubits has a large ground-state population and/or it is highly coherent. We also focus on the role of initial quantum correlations in the thermometric performance, which emerge as a central feature of this work. We find strong numerical evidence that, given same single-qubit reduced states, the inclusion of quantum correlations among the qubits of the ensemble always yields an enhanced QFI. Moreover, even if none of the considered states outperform the (pure, separable) ground state, maximally entangled states display QFIs values remarkably close to the standard quantum limit when probing extremely hot thermal baths. Finally, although the Markovian dynamics does not permit superlinear scaling of the QFI with the number of probes, we identify the most effective initial states for designing high-precision quantum thermometers within this setting. We also provide concrete guidelines for experimental implementations.