An ultimate goal of electron paramagnetic resonance (EPR) spectroscopy is to analyze molecular dynamics in place where it occurs, such as in a living cell. The nanodiamond (ND) hosting nitrogen-vacancy (NV) centers will be a promising EPR sensor to achieve this goal. However, ND-based EPR spectroscopy remains elusive, due to the challenge of controlling NV centers without well-defined orientations inside a flexible ND. Here, we show a generalized zero-field EPR technique with spectra robust to the sensor's orientation. The key is applying an amplitude modulation on the control field, which generates a series of equidistant Floquet states with energy splitting being the orientation-independent modulation frequency. We acquire the zero-field EPR spectrum of vanadyl ions in aqueous glycerol solution with embedded single NDs, paving the way towards \emph{in vivo} EPR.
@article{arxiv.2307.13349,
title = {In situ electron paramagnetic resonance spectroscopy using single nanodiamond sensors},
author = {Zhuoyang Qin and Zhecheng Wang and Fei Kong and Jia Su and Zhehua Huang and Pengju Zhao and Sanyou Chen and Qi Zhang and Fazhan Shi and Jiangfeng Du},
journal= {arXiv preprint arXiv:2307.13349},
year = {2023}
}