A new method for detecting the magnetic resonance of electronic spins at low temperature is demonstrated. It consists in measuring the signal emitted by the spins with a superconducting qubit that acts as a single-microwave-photon detector, resulting in an enhanced sensitivity. We implement this new type of electron-spin resonance spectroscopy using a hybrid quantum circuit in which a transmon qubit is coupled to a spin ensemble consisting of NV centers in diamond. With this setup we measure the NV center absorption spectrum at 30mK at an excitation level of \thicksim15\,\mu_{B} out of an ensemble of 10^{11} spins.
@article{arxiv.1205.5659,
title = {Electron spin resonance detected by a superconducting qubit},
author = {Y. Kubo and I. Diniz and C. Grezes and T. Umeda and J. Isoya and H. Sumiya and T. Yamamoto and H. Abe and S. Onoda and T. Ohshima and V. Jacques and A. Dréau and J. -F. Roch and A. Auffeves and D. Vion and D. Esteve and P. Bertet},
journal= {arXiv preprint arXiv:1205.5659},
year = {2015}
}