  Quantum emitters respond to resonant illumination by radiating
electromagnetic fields. A component of these fields is phase-coherent with the
driving tone, while another one is incoherent, consisting of spontaneously
emitted photons and forming the fluorescence signal. Atoms and molecules are
routinely detected by their fluorescence at optical frequencies, with important
applications in quantum technology and microscopy. Spins, on the other hand,
are usually detected by {their coherent response} at radio- or microwave
frequencies, either in continuous-wave or pulsed magnetic resonance. Indeed,
fluorescence detection of spins is hampered {by their low spontaneous emission
rate} and by the lack of single-photon detectors in this frequency range. Here,
using superconducting quantum devices, we demonstrate the detection of a small
ensemble of donor spins in silicon by their fluorescence at microwave frequency
and millikelvin temperatures. We enhance the spin radiative decay rate by
coupling them to a high-quality-factor and small-mode-volume superconducting
resonator, and we connect the device output to a newly-developed microwave
single-photon counter based on a superconducting qubit. We discuss the
potential of fluorescence detection as a novel method for magnetic resonance
spectroscopy of small numbers of spins.
