Hyperpolarisation of nuclear spins: polarisation blockade

TL;DR

This paper studies how nuclear spin hyperpolarisation via NV centers is affected by a phenomenon called polarisation blockade, revealing that certain spins can disrupt resonance conditions and proposing solutions to improve hyperpolarisation protocols.

Contribution

It provides a detailed analysis of polarisation blockade effects in NV center-based hyperpolarisation and offers a closed-form expression for the displacement caused by blocking spins.

Findings

Polarisation resonances can be displaced by blocking spins.

The displacement occurs without weakening the resonance.

Solutions are proposed to mitigate the effects of blocking spins.

Abstract

Efficient hyperpolarisation of nuclear spins via optically active defect centers, such as the nitrogen vacancy (NV) center in diamond, has great potential for enhancing NMR based quantum information processing and nanoscale magnetic resonance imaging. Recently, pulse-based protocols have been shown to efficiently transfer optically induced polarisation of the electron defect spin to surrounding nuclear spins -- at particular resonant pulse intervals. In this work, we investigate the performance of these protocols, both analytically and experimentally, with the electronic spin of a single NV defect. We find that whenever polarisation resonances of nuclear spins are near-degenerate with a `blocking' spin, which is single spin with stronger off-diagonal coupling to the electronic central spin, they are displaced out of the central resonant region -- without, in general, significant weakening of the resonance. We analyse the underlying physical mechanism and obtain a closed form expression for the displacement. We propose that spin blocking represents a common but overlooked effect in hyperpolarisation of nuclear spins and suggest solutions for improved protocol performance in the presence of (naturally occurring) blocking nuclear spins.