Proposal for enhanced resolution in nanoscale NMR: quantum sensing with pulses of finite duration

TL;DR

This paper reveals that the finite duration of microwave pulses in NV center quantum sensing can be used as a control parameter to enhance NMR resolution by manipulating quantum-state crossings, leading to more precise spectroscopy.

Contribution

It introduces the concept that pulse duration, previously considered an error source, can be exploited to control quantum-state crossings for improved NMR resolution.

Findings

Identification of quantum-state crossings activated by pulse-width adjustments.

Control over the position and strength of coherence dips.

Potential for higher resolution spectroscopy using pulse-width tuning.

Abstract

The nitrogen vacancy (NV) color center in diamond is an enormously important platform for the development of quantum sensors, including for single spin and single molecule NMR. Detection of weak single-spin signals is greatly enhanced by repeated sequences of microwave pulses; in these dynamical decoupling (DD) techniques, the key control parameters swept in the experiment are the time intervals, $\tau$, between pulses. Here we show that, in fact, the pulse duration offers a powerful additional control parameter. While previously, a non-negligible pulse-width has been considered simply a source of experimental error, here we elucidate the underlying quantum dynamics: we identify a landscape of quantum-state crossings which are usually closed (inactive) but may be controllably activated (opened) by adjusting the pulse-width from zero. We identify these crossings with recently observed but unexpected dips (so called spurious dips) seen in the quantum coherence of the NV spin. With this new understanding, both the position and strength of these sharp features may be accurately controlled; they co-exist with the usual broader coherence dips of short-duration microwave pulses, but their sharpness allows for higher resolution spectroscopy with quantum diamond sensors, or their analogues.