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.