Single electron spin resonance by microwave photon counting

Electron spin resonance (ESR) with single spin addressability is an approach to characterize paramagnetic species and gives coherent manipulation of the spins. This goal has been reached in various systems or techniques, such as gate-defined quantum dots, spin-dependent photoluminescence and scanning-probe techniques. However, these approaches require specific systems or restrict themselves to small detection volume. Therefore, operational and universal single spin detection remains a challenge. Here, we demonstrate single electron spin resonance by spin fluorescence detection with a superconducting-qubit-based single microwave photon detector at cryogenic temperature. We couple individual paramagnetic erbium ions in a scheelite crystal to a high-quality factor superconducting resonator to enhance their radiative decay rate. By counting the spontaneously emitted photons from ions, we resolve their ESR spectrum down to single impurity level. In one second integration time of the detection, we reach a signal-to-noise ratio of 1.45. The observed photon anti-bunching in the emission proves that the fluorescence signal comes from single emitters. Our results pave the way for practical ESR spectroscopy of arbitrary paramagnetic species with single spin resolution and it may also find potential applications for quantum computing.