  Dynamical polarization of nuclear spin ensembles is of central importance for
magnetic resonance studies, precision sensing and for applications in quantum
information theory. Here we propose a scheme to generate long-lived singlet
pairs in an unpolarized nuclear spin ensemble which is dipolar coupled to the
electron spins of a Nitrogen Vacancy center in diamond. The quantum mechanical
back-action induced by frequent spin-selective readout of the NV centers allows
the nuclear spins to pair up into maximally entangled singlet pairs.
Counterintuitively, the robustness of the pair formation to dephasing noise
improves with increasing size of the spin ensemble. We also show how the paired
nuclear spin state allows for enhanced sensing capabilities of NV centers in
diamond.
