Hyperpolarisation of external nuclear spins using nitrogen-vacancy centre ensembles

TL;DR

This paper demonstrates the potential of using dense NV ensembles in diamond to hyperpolarise external nuclear spins, showing promising transfer rates and discussing challenges and improvements needed for practical NMR sensitivity enhancement.

Contribution

It provides the first evidence of polarisation transfer from a dense NV ensemble to external nuclear spins over a micrometre scale, characterising transfer rates and limitations.

Findings

Maximum transfer rate of ~7500 spins/sec/NV for solid hydrogen targets

Reduced efficiency in liquid targets due to molecular diffusion

Feasibility of NMR enhancement with material improvements

Abstract

The nitrogen-vacancy (NV) centre in diamond has emerged as a candidate to non-invasively hyperpolarise nuclear spins in molecular systems to improve the sensitivity of nuclear magnetic resonance (NMR) experiments. Several promising proof of principle experiments have demonstrated small-scale polarisation transfer from single NVs to hydrogen spins outside the diamond. However, the scaling up of these results to the use of a dense NV ensemble, which is a necessary prerequisite for achieving realistic NMR sensitivity enhancement, has not yet been demonstrated. In this work, we present evidence for a polarising interaction between a shallow NV ensemble and external nuclear targets over a micrometre scale, and characterise the challenges in achieving useful polarisation enhancement. In the most favourable example of the interaction with hydrogen in a solid state target, a maximum polarisation transfer rate of $\approx 7500$ spins per second per NV is measured, averaged over an area containing order $10^6$ NVs. Reduced levels of polarisation efficiency are found for liquid state targets, where molecular diffusion limits the transfer. Through analysis via a theoretical model, we find that our results suggest implementation of this technique for NMR sensitivity enhancement is feasible following realistic diamond material improvements.