Broadband electron spin resonance spectroscopy with a superconducting resonator, Part 1 : Theory

Electron spin resonance (ESR) spectroscopy is a useful tool for characterizing spin defects relevant to quantum technologies. Typical ESR systems are based on a resonant exchange interaction between the spins and a high-Q resonant cavity or resonator. While sensitive, the small magnetic-dipole coupling strength limits the ESR detection bandwidth in the dispersive regime and alternative coupling strategies must be used. One such strategy is to rely instead on a longitudinal interaction where the spin magnetization directly influences the resonator frequency, irrespective of the spin transition frequency.

In this talk, we show how we can realize such broadband spectroscopy of spin systems using high-kinetic-inductance superconducting resonators. We demonstrate that the longitudinal interaction is dominated by spins close to the resonator surface and discuss how the coupling strength and detection sensitivity can be optimized. This analysis paves the way towards a broadband ESR spectrometer suitable for novel quantum materials, as well as numerous other applications in chemistry, biology and material sciences.