The impact of microwave phase noise on diamond quantum sensing

TL;DR

This paper investigates how microwave phase noise affects the sensitivity of diamond NV center sensors, revealing its impact on measurement limits and proposing strategies for mitigation to enhance quantum sensing performance.

Contribution

It provides a detailed frequency domain model of microwave phase noise effects on NV sensors and demonstrates a gradiometry-based suppression method.

Findings

Microwave phase noise sets a pT s^{1/2} noise floor in NV magnetometry.

The noise impact varies with microwave frequency and pulse sequence parameters.

A gradiometry technique achieves over 10-fold suppression of phase noise effects.

Abstract

Precision optical measurements of the electron-spin precession of nitrogen-vacancy (NV) centers in diamond form the basis of numerous applications. The most sensitivity-demanding applications, such as femtotesla magnetometry, require the ability to measure changes in GHz spin transition frequencies at the sub-millihertz level, corresponding to a fractional resolution of better than 10^{-12}. Here we study the impact of microwave (MW) phase noise on the response of an NV sensor. Fluctuations of the phase of the MW waveform cause undesired rotations of the NV spin state. These fluctuations are imprinted in the optical readout signal and, left unmitigated, are indistinguishable from magnetic field noise. We show that the phase noise of several common commercial MW generators results in an effective pT s^{1/2}-range noise floor that varies with the MW carrier frequency and the detection frequency of the pulse sequence. The data are described by a frequency domain model incorporating the MW phase noise spectrum and the filter-function response of the sensing protocol. For controlled injection of white and random-walk phase noise, the observed NV magnetic noise floor is described by simple analytic expressions that accurately capture the scaling with pulse sequence length and the number of pi pulses. We outline several strategies to suppress the impact of MW phase noise and implement a version, based on gradiometry, that realizes a >10-fold suppression. Our study highlights an important challenge in the pursuit of sensitive diamond quantum sensors and is applicable to other qubit systems with a large transition frequency.