  Electron spin resonance (ESR) spectroscopy is the method of choice for
characterizing paramagnetic impurities, with applications ranging from
chemistry to quantum computing, but it gives access only to ensemble-averaged
quantities due to its limited signal-to-noise ratio. Single-electron-spin
sensitivity has however been reached using spin-dependent photoluminescence,
transport measurements, and scanning-probe techniques. These methods are
system-specific or sensitive only in a small detection volume, so that
practical single spin detection remains an open challenge. Here, we demonstrate
single electron magnetic resonance by spin fluorescence detection, using a
microwave photon counter at cryogenic temperatures. We detect individual
paramagnetic erbium ions in a scheelite crystal coupled to a high-quality
factor planar superconducting resonator to enhance their radiative decay rate,
with a signal-to-noise ratio of 1.9 in one second integration time. The
fluorescence signal shows anti-bunching, proving that it comes from individual
emitters. Coherence times up to 3 ms are measured, limited by the spin
radiative lifetime. The method has the potential to apply to arbitrary
paramagnetic species with long enough non-radiative relaxation time, and allows
single-spin detection in a volume as large as the resonator magnetic mode
volume ( 10 um^3 in the present experiment), orders of magnitude larger than
other single-spin detection techniques. As such, it may find applications in
magnetic resonance and quantum computing.
