Tracking the precession of single nuclear spins by weak measurements.

Abstract

Nuclear magnetic resonance (NMR) spectroscopy is a powerful technique for analysing the structure and function of molecules, and for performing three-dimensional imaging of their spin densities. At the heart of NMR spectrometers is the detection of electromagnetic radiation, in the form of a free induction decay signal¹, generated by nuclei precessing around an applied magnetic field. Whereas conventional NMR requires signals from 10¹² or more nuclei, recent advances in sensitive magnetometry^2,3 have dramatically lowered the required number of nuclei to a level where a few or even individual nuclear spins can be detected^4-6. It is unclear whether continuous detection of the free induction decay can still be applied at the single-spin level, or whether quantum back-action (the effect that a detector has on the measurement itself) modifies or suppresses the NMR response. Here we report the tracking of single nuclear spin precession using periodic weak measurements^7-9. Our experimental system consists of nuclear spins in diamond that are weakly interacting with the electronic spin of a nearby nitrogen vacancy centre, acting as an optically readable meter qubit. We observe and minimize two important effects of quantum back-action: measurement-induced decoherence¹⁰ and frequency synchronization with the sampling clock^11,12. We use periodic weak measurements to demonstrate sensitive, high-resolution NMR spectroscopy of multiple nuclear spins with a priori unknown frequencies. Our method may provide a useful route to single-molecule NMR^13,14 at atomic resolution.