We report measurements of electron spin echo envelope modulation (ESEEM) performed at millikelvin temperatures in a custom-built high-sensitivity spectrometer based on superconducting micro-resonators. The high quality factor and small mode volume (down to 0.2pL) of the resonator allow to probe a small number of spins, down to 5⋅102. We measure 2-pulse ESEEM on two systems: erbium ions coupled to 183W nuclei in a natural-abundance CaWO4 crystal, and bismuth donors coupled to residual 29Si nuclei in a silicon substrate that was isotopically enriched in the 28Si isotope. We also measure 3- and 5-pulse ESEEM for the bismuth donors in silicon. Quantitative agreement is obtained for both the hyperfine coupling strength of proximal nuclei, and the nuclear spin concentration.
@article{arxiv.2001.04854,
title = {Hyperfine spectroscopy in a quantum-limited spectrometer},
author = {S. Probst and G. L. Zhang and M. Rančić and V. Ranjan and M. Le Dantec and Z. Zhong and B. Albanese and A. Doll and R. B. Liu and J. J. L. Morton and T. Chanelière and P. Goldner and D. Vion and D. Esteve and P. Bertet},
journal= {arXiv preprint arXiv:2001.04854},
year = {2021}
}