The detection and characterization of paramagnetic species by electronâspin 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 microâresonators 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 stateâofâtheart, enabling the detection of 1700 Bismuth donor spins in silicon with a signalâtoânoise 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 . This constitutes the first observation of the Purcell effect for spins, and a first step towards circuit QED experiments with magnetically coupled individual spins.
 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, 253–257 (2016)
 A. Bienfait et al., Nature 531, 74–77 (2016)