Long-living coherences and magnetic sensing with strongly-coupled quantum spins driven by pulse trains

The systems comprised of a large number of strongly coupled quantum spins, driven by trains of 180° pulses, often demonstrate long-living quantum coherences, extending far beyond the "usual" coherence decay time (e.g. Hahn echo decay time) [1-3]. This effect is generic for various spin systems in different dimensions, from 1D to ∞-D (where every spin is coupled to all others), and the appearance of the long-living coherences is associated with the accumulated effect of the spin dynamics during the pulses (such as rotation angle deviating from 180°, or finite pulse width).
In this work, we present the protocols that employ the long-living coherences for quantum-assisted sensing of ac magnetic fields. In these protocols the pulse polarity is periodically changed, thus modulating the effect of pulses, so that the resulting long-living coherence oscillates in time with the frequency depending on the ac magnetic field. We also discuss application of such protocols beyong sensing of ac magnetic fields, to other quantum-assisted metrology tasks.
[1] M. A. Frey et al, PNAS 109, 5190 (2012).
[2] S. Choi et al, Nature 543, 221 (2017).
[3] J. Rovny et al, PRL 120, 180603 (2018).