Quantum-limited metrology of macroscopic spin ensembles

Nature Physics (2026)Cite this article Quantum effects are usually observed and utilized in microscopic systems, where qubits can be manipulated and measured with precise control. However, larger qubit ensembles should, in principle, enhance performance in sensing and metrology applications. There is an inherent tension between the sensitivity afforded by large-scale experiments and the ability to use quantum protocols, since quantum phenomena are usually rapidly swamped by classical noise as the system size is scaled up. Here we show that spin quantum fluctuations are present in macroscopic spin qubit ensembles that might be expected to behave classically. Quantum-limited detection sensitivity enables us to perform magnetic resonance spectroscopy of quantum spin fluctuations without any external excitation. We demonstrate non-equilibrium spin-state preparation and single-shot measurements of subsequent ultraslow thermalization dynamics. Quantum-limited metrology of millimole-scale ensemble dynamics brings the tools of quantum sensing into the macroscopic regime. This enables truly non-invasive magnetic resonance spectroscopy and precision searches for fundamental physics.This is a preview of subscription content, access via your institution Access Nature and 54 other Nature Portfolio journals Get Nature+, our best-value online-access subscription $32.99 / 30 days cancel any timeSubscribe to this journal Receive 12 print issues and online access $259.00 per yearonly $21.58 per issueBuy this articleUSD 39.95Prices may be subject to local taxes which are calculated during checkoutSource data are provided with this paper.Degen, C. L., Reinhard, F. & Cappellaro, P. Quantum sensing. Rev. Mod. Phys. 89, 035002 (2017).Article ADS MathSciNet Google Scholar Bothwell, T. et al. Resolving the gravitational redshift across a millimetre-scale atomic sample. Nature 602, 420–424 (2022).Article ADS Google Scholar LIGO Scientific Collaboration and Virgo Collaboration et al. Observation of gravitational waves from a binary black hole merger. Phys. Rev. Lett. 116, 061102 (2016).Article ADS MathSciNet Google Scholar Aasi, J. et al. Enhanced sensitivity of the LIGO gravitational wave detector by using squeezed states of light. Nat. Photon. 7, 613–619 (2013).Article ADS Google Scholar Tse, M. et al. Quantum-enhanced advanced LIGO detectors in the era of gravitational-wave astronomy. Phys. Rev. Lett. 123, 231107 (2019).Article ADS Google Scholar Virgo Collaboration et al. Increasing the astrophysical reach of the Advanced Virgo Detector via the application of squeezed vacuum states of light. Phys. Rev. Lett. 123, 231108 (2019).Article ADS Google Scholar Robinson, J. M. et al. Direct comparison of two spin-squeezed optical clock ensembles at the 10−17 level. Nat. Phys. 20, 208–213 (2024).Article Google Scholar Aprile, E. et al. Observation of two-neutrino double electron capture in 124Xe with XENON1T. Nature 568, 532–535 (2019).Article ADS Google Scholar Abe, K. et al. Constraint on the matter–antimatter symmetry-violating phase in neutrino oscillations. Nature 580, 339–344 (2020).Article Google Scholar Ning, X. et al. Limits on the luminance of dark matter from xenon recoil data. Nature 618, 47–50 (2023).Article ADS Google Scholar IceCube Collaboration. Observation of high-energy neutrinos from the Galactic plane. Science 380, 1338–1343 (2023).Article ADS Google Scholar Budker, D. & Romalis, M. Optical magnetometry. Nat. Phys. 3, 227–234 (2007).Article Google Scholar Budker, D., Graham, P. W., Ledbetter, M., Rajendran, S. & Sushkov, A. O. Proposal for a cosmic axion spin precession experiment (CASPEr). Phys. Rev. X 4, 021030 (2014).

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