Magnetic resonance at the quantum limit and beyond
The detection and characterization of paramagnetic species by electron-spin resonance (ESR) spectroscopy has numerous applications in chemistry, biology, and materials science . 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 in which they are embedded. Using the tools offered by circuit Quantum Electrodynamics (QED), namely high quality factor superconducting micro-resonators and Josephson parametric amplifiers that operate at the quantum limit when cooled at 20mK , 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 . 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 , which opens the way to on-demand spin initialization via the Purcell effect. Finally we report recent results demonstrating that squeezed microwave signals can be used to enhance ESR sensitivity even further 
 A. Schweiger and G. Jeschke, Principles of Pulse Electron Magnetic Resonance (Oxford University Press, 2001)
 X. Zhou et al., Physical Review B 89, 214517 (2014).
 A. Bienfait et al., Nature Nanotechnology 11, 253 (2016)
 A. Bienfait et al., Nature 531, 74 (2016)
 A. Bienfait et al., in preparation (2016)