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arXiv:2512.22739

A method for robust spin relaxometry in the presence of imperfect state preparation

Ella P. Walsh, Sepehr Ahmadi, Alexander J. Healey, David A. Simpson, Liam T. Hall

quant-phSubmitted Dec 28, 2025• Announced Dec 30, 2025• Revised Jan 6, 2026 (v2)
v1 · Dec 28, 2025v2 · Jan 6, 2026
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Abstract

Spin relaxometry based on quantum spin systems has developed as a valuable tool in medical and condensed matter systems, offering the advantage of operating without the need for external DC or RF fields. Spin relaxometry with nitrogen-vacancy (NV) centers has been applied to paramagnetic sensing using both single crystal diamond and nanodiamond materials. However, these methods often suffer from artifacts and systematic uncertainties, particularly due to imperfect spin state preparation, leading to artificially fast T$_1$ relaxation times. Current analysis techniques fail to adequately account for these issues, limiting the precision of parameter estimation. In this work, we introduce a minimal fitting procedure that enables more robust parameter estimation in the presence of imperfect spin polarization. Our model improves upon existing approaches by offering more accurate fits and provides a framework for efficiently parallelizing single-spin dynamics studies.

DOI
10.1002/qute.70291
Comments
13 pages, 3 figures in manuscript, 7 supplementary figures in appendix

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Spin relaxometry based on quantum spin systems has developed as a valuable tool in medical and condensed matter systems, offering the advantage of operating without the need for external DC or RF fields. Spin relaxometry with nitrogen-vacancy (NV) centers has been applied to paramagnetic sensing using both single crystal diamond and nanodiamond materials. However, these methods often suffer from artifacts and systematic uncertainties, particularly due to imperfect spin state preparation, leading to artificially fast T1_11​ relaxation times. Current analysis techniques fail to adequately account for these issues, limiting the precision of parameter estimation. In this work, we introduce a minimal fitting procedure that enables more robust parameter estimation in the presence of imperfect spin polarization. Our model improves upon existing approaches by offering more accurate fits and provides a framework for efficiently parallelizing single-spin dynamics studies.
Quantum relaxometry is one of the most successful applications of nitrogen-vacancy (NV) centers in diamond and, more broadly, solid-state spin qubits, enabling ultrasensitive detection of magnetic noise and paramagnetic species via measurements of the spin-lattice relaxation time T1T_1T1​. Conventional pulsed protocols, however, probe T1T_1T1​ efficiently only over a limited temporal range, which restricts the scope and throughput of the technique. Here we introduce a continuous-wave quantum relaxometry protocol that operates in the frequency domain. By measuring the frequency response of the optically detected magnetic resonance signal under low-frequency microwave amplitude modulation, we extract T1T_1T1​ from the characteristic response time of the spin system. The method enables efficient T1T_1T1​ measurements spanning more than three orders of magnitude -- directly demonstrated from 60 μμμs to 200 ms in our experiments -- across a broad temperature range and under substantial ensemble inhomogeneity. We further show that this protocol enables quantitative relaxometry-based sensing in nanodiamonds, achieving a substantial speed-up over the pulsed methods and offering a practical approach to optimizing nanodiamond size for enhanced sensitivity.
Nitrogen-vacancy centers in diamond have been long used for spin-based optical sensing and considered viable candidates for implementing spin-based quantum information protocols. Their unique advantages include their nanoscale size and the optical readout of the electron spin state. These features make them particularly fit for the use in integrated nanophotonic and quantum optical devices. We will discuss how the optical Purcell effect in plasmonic systems affects the NV spin readout signal and present the demonstration of this readout through optical plasmons in an integrated nanophotonic interface. We will show that spin relaxometry is a powerful tool that allows to probe the magnonic density of states and its electrical tuning with sub-um spatial resolution. Furthermore, we will discuss how spin signal readout sensitivity can be enhanced with the use of plasmonic nanostructures and Bayesian measurement methods.
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For quantum sensing, it is vital to develop an efficient technique for determining the quantum state of the sensor. We optimize the weighting of the photoluminescence intensity for readout of the spin state of the nitrogen-vacancy (NV) center in diamond. We find that adopting a physical model that considers the optical transitions and relaxations of the NV center allows for an efficient readout. Our method improves the signal-to-noise ratio of the readout by 5.4% in a short time of 3 s, while the existing methods typically require 1 min of integration time. We also show that our technique enhances the readout of the nuclear spin memory. The demonstrated way is helpful for a wide range of measurements, from a few minutes to several days.