ESR at the quantum limit using high-Q superconducting resonators

The detection and characterization of paramagnetic species by electron spin resonance (ESR) spectroscopy has numerous applications in chemistry, biology, and materials science [1]. Most ESR spectrometers rely on the inductive detection of the small microwave signals emitted by the spins during their Larmor precession into a microwave resonator. Using the tools offered by circuit Quantum Electrodynamics (cQED), namely high quality factor superconducting micro-resonators and Josephson parametric amplifiers that operate at the quantum limit when cooled at 20mK [2], we report an increase of the sensitivity of inductively detected ESR by 4 orders of magnitude over the state-of-the-art, enabling the detection of 1700 bismuth donor spins in silicon with a signal-to-noise ratio of 1 in a single echo [3]. We also demonstrate that the energy relaxation time of the spins is limited by spontaneous emission of microwave photons into the measurement line via the resonator [4]. This constitutes the first observation of the Purcell effect for spins [5], an important step towards the coherent magnetic coupling of individual spins to microwave photons. Finally, we show that the presence of this high-sensitivity on-chip ESR spectrometer imparts a large strain-induced quadrupole interaction on the bismuth donors, of order 100 MHz [6]. Through finite-element strain simulations and effective mass theory calculations of the donor electron wavefunction, we are able to reconstruct key features of our experiments, including the electron spin resonance spectra.

1. A. Schweiger and G. Jeschke, Principles of Pulse Electron Magnetic Resonance (Oxford University Press, 2001).
2. X. Zhou et al., Physical Review B 89, 214517 (2014).
3. A. Bienfait, et al., Nature Nanotechnology 11, 253 (2015).
4. A. Bienfait, et al., Nature 531, 74 (2016).
5. E. M. Purcell, Physical Review 69, 681 (1946).
6. J. J. Pla, arXiv:1608.07346 (2016).