Dynamical freezing for magnetometry in an interacting spin ensemble

Understanding and controlling non-equilibrium dynamics in quantum many-body systems is a fundamental challenge in modern physics^1-5, with profound implications for advancing quantum technologies. Typically, periodically driven systems in the absence of conservation laws thermalize to a featureless 'infinite-temperature' state, erasing all memory of their initial conditions^6-8. However, this pattern can break down through mechanisms such as integrability⁹, many-body localization^2,3,10,11, quantum many-body scars⁴ and Hilbert space fragmentation^12,13. Here we report the experimental observation of dynamical freezing, a distinct mechanism of thermalization breakdown in driven systems^14-19, and demonstrate its application in quantum sensing using an ensemble of approximately 10⁴ interacting nitrogen-vacancy (NV) spins in diamond. By precisely controlling the driving frequency and detuning, we observe emergent long-lived spin magnetization and coherent oscillatory micromotions, persisting over timescales exceeding the interaction-limited coherence time (T₂) by more than an order of magnitude. By using these unconventional dynamics, we develop a dynamical-freezing-enhanced a.c. magnetometry that extends optimal sensing times far beyond T₂, outperforming conventional dynamical decoupling magnetometry with a 2.7-fold sensitivity enhancement. Our results not only provide clear experimental observation of dynamical freezing-a peculiar mechanism defying thermalization through emergent conservation laws-but also establish a robust control method generally applicable to diverse physical platforms, with broad implications in quantum metrology and beyond.