The Role of Electrolytes in the Relaxation of Near-Surface Spin Defects in Diamond

TL;DR

This study reveals that diamagnetic electrolytes can increase the relaxation time of near-surface NV centers in diamond by reducing interfacial noise, suggesting new avenues for quantum sensing applications in biological and chemical systems.

Contribution

It uncovers a novel effect of diamagnetic ions on NV center relaxation and proposes a mechanism involving interfacial band bending and electric double layer formation.

Findings

Diamagnetic electrolytes increase NV center $T_1$ times.

Electric double layer formation stabilizes interfacial charges.

Reduction of magnetic and electric noise observed with electrolytes.

Abstract

Quantum sensing with spin defects in diamond, such as the nitrogen-vacancy (NV) center, enables the detection of various chemical species on the nanoscale. Molecules or ions with unpaired electronic spins are typically probed by their influence on the NV center's spin relaxation. Whereas it is well-known that paramagnetic ions reduce the NV center's relaxation time ($T_1$), here we report on the opposite effect for diamagnetic ions. We demonstrate that millimolar concentrations of aqueous diamagnetic electrolyte solutions increase the $T_1$ time of near-surface NV center ensembles compared to pure water. To elucidate the underlying mechanism of this surprising effect, single and double quantum NV experiments are performed, which indicate a reduction of magnetic and electric noise in the presence of diamagnetic electrolytes. In combination with ab initio simulations, we propose that a change in the interfacial band bending due to the formation of an electric double layer leads to a stabilization of fluctuating charges at the interface of an oxidized diamond. This work not only helps to understand noise sources in quantum systems but could also broaden the application space of quantum sensors towards electrolyte sensing in cell biology, neuroscience and electrochemistry.