Optimal observables for quantum-enhanced sensing and applications in a Floquet time crystal sensor

• URL: http://arxiv.org/abs/2606.26248v1
• Date: Wed, 24 Jun 2026 18:00:14 GMT
• Title: Optimal observables for quantum-enhanced sensing and applications in a Floquet time crystal sensor
• Abstract summary: We focus on a simple protocol, namely the method of moments (MoM)<n>We first demonstrate that the symmetric logarithmic derivative (SLD) operator saturates the quantum Fisher information bound.<n>We show that the SLD is generally too complex and typically non-local, rendering its direct experimental realization unfeasible.
• Score: 41.99844472131922
• License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/
• Abstract: In this work, we discuss how to determine and implement feasible optimal observables for a metrology protocol that saturates the quantum Fisher information (QFI) bound. In particular, we focus our study on a simple protocol, namely the method of moments (MoM). We first demonstrate that the symmetric logarithmic derivative (SLD) operator, a Hermitian observable, once implemented in the MoM, saturates the QFI bound. However, the SLD is generally too complex and typically non-local, rendering its direct experimental realization unfeasible. To overcome this limitation, we explore its structure in a specific sensing model - a Floquet time crystal (FTC) acting as an ac field sensor - and show that the SLD can be approximated by substantially simpler observables, such as the bare spin magnetization or a parity observable, for different relevant initial state preparations. We further corroborate our theoretical predictions in a nuclear magnetic resonance system operating as an FTC sensor, employing experimentally motivated parameters to simulate its performance in a state-of-the-art implementation. In general, our results establish a practical route toward near-optimal metrology in FTC sensors, where the inaccessible SLD operator can be replaced by simpler observables while retaining quantum-enhanced sensitivity.

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