Magnetic sensing employing non-equilibrium long-lived coherences in driven quantum spin ensembles

Ensembles of interacting quantum spins, driven by imperfect control pulses, often demonstrate non-equilibrium coherence that survives far beyond the "usual" decay time, i.e. beyond the Hahn echo decay time [1,2]. This effect occurs in various spin systems of different dimensionalities. The long-lived coherences arise due to accumulation of the control imperfections and inter-spin couplings; they are very robust, and can extend the coherence time by up to five orders of magnitude.

We present theoretical design and analysis of control protocols which employ the long-lived coherences for quantum-assisted sensing of ac magnetic fields. In these protocols the direction of the control pulses periodically changes, inducing the long-lived oscillations with the frequency which depends on the magnitude of the ac magnetic field; the baseline of the oscillations also depends on the ac field, enabling two modalities of magnetic sensing. Theory shows that this approach is a promising alternative to the sensing methods based on more traditional dynamical decoupling techniques. Experimental results utilizing ensembles of dipolar interacting nitrogen vacancy centers in diamond are presented.

[1] Y. Dong et al, Phys. Rev. Lett. 100, 247601 (2008).

[2] W. Hahn and V. V. Dobrovitski, New J. Phys. 23 073029 (2021).