Microwave-Free Nuclear Magnetic Resonance at Molecular Scales

TL;DR

This paper introduces a microwave-free, less invasive method for nanoscale NMR using NV centers, enabling simpler and scalable quantum sensing of proton spins at molecular scales with sensitivity comparable to existing techniques.

Contribution

The authors develop and demonstrate a microwave-free protocol for NV-based nanoscale NMR, eliminating the need for complex microwave pulse sequences and simplifying the measurement process.

Findings

Achieved NMR spectroscopy of proton spins within a 10 nm cube.

Demonstrated that the sensitivity matches microwave-based NV NMR methods.

Provided both theoretical and experimental validation of the approach.

Abstract

The implementation of nuclear magnetic resonance (NMR) at the nanoscale is a major challenge, as conventional systems require relatively large ensembles of spins and limit resolution to mesoscopic scales. New approaches based on quantum spin probes, such as the nitrogen-vacancy (NV) centre in diamond, have recently achieved nano-NMR under ambient conditions. However, the measurement protocols require application of complex microwave pulse sequences of high precision and relatively high power, placing limitations on the design and scalability of these techniques. Here we demonstrate a microwave-free method for nanoscale NMR using the NV centre, which is a far less invasive, and vastly simpler measurement protocol. By utilising a carefully tuned magnetic cross-relaxation interaction between a subsurface NV spin and an external, organic environment of proton spins, we demonstrate NMR spectroscopy of $^1$H within a $\approx(10~{\rm nm})^3$ sensing volume. We also theoretically and experimentally show that the sensitivity of our approach matches that of existing microwave control-based techniques using the NV centre. Removing the requirement for coherent manipulation of either the NV or the environmental spin quantum states represents a significant step towards the development of robust, non-invasive nanoscale NMR probes.