Magnetic sensing employing long-lived coherences in quantum spin ensembles*

Ensembles of interacting quantum spins, driven by imperfect control pulses, often demonstrate 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; 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.