High sensitivity Electron Spin Resonance spectroscopy for quantum technologies

In a conventional electron spin resonance (ESR) spectrometer based on the inductive detection method, the paramagnetic spins precess in an external magnetic field radiating weak microwave signals into a resonant cavity. Despite its widespread use, ESR spectroscopy has limited sensitivity and large amounts of spins ~10^12 are necessary to accumulate sufficient signal. Exploiting recent progress in circuit-quantum electrodynamics, we have combined high quality factor superconducting micro-resonators and noise-less Josephson Parametric Amplifiers to perform ESR spectroscopy at millikelvin temperatures, reaching a new regime where the sensitivity is limited by the quantum vacuum fluctuations of the microwave field. In this talk, I will discuss this new ESR regime and its prospects for quantum technologies.