High sensitivity quantum limited electron spin resonance spectroscopy

Electron spin resonance (ESR) is a well-established method to analyze paramagnetic species, reaction products and complex molecules in materials science, chemistry and molecular biology. Despite widespread use, the conventional ESR based on the inductive detection method has very limited sensitivity, in part due to the small single spin-photon coupling g. Following recent advances in circuit quantum electrodynamics, we have employed high quality factor superconducting resonators to reduce the mode volume around the spins and operate ESR in so-called Purcell regime [1,2] where a larger g not only leads to a larger signal but also provides a high repetition rate. In particular with a nanometric inductor, we show g/2π of 3 kHz, which together with Josephson parametric amplifier pushes the spin sensitivity to be 10 spins for unit signal to noise ratio per second of averaging.

[1] A. Bienfait et al., Nature Nano 11, 253 (2016)
[2] S. Probst et al., Appl Phys Lett, 111, 202604 (2017)