Purification of an unpolarized spin ensemble into entangled singlet pairs

TL;DR

This paper proposes a scheme to generate long-lived entangled singlet pairs in unpolarized nuclear spin ensembles coupled to NV centers, enhancing quantum sensing and demonstrating robustness to noise.

Contribution

It introduces a novel method using quantum back-action from NV center readout to entangle nuclear spins into singlet pairs, improving robustness and sensing capabilities.

Findings

Long-lived singlet pairs can be generated in unpolarized nuclear ensembles.

The entanglement robustness increases with ensemble size.

Enhanced sensing capabilities are achieved through nuclear spin pairing.

Abstract

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.