Detecting spins with a microwave photon counter*

Operational single photon counters at microwave frequencies have been developed recently. Here we plug such a single microwave photon counter [R. Lescanne et al., PRX 10, 021038, 2020] to the output of a superconducting micro-resonator, itself coupled to an ensemble of bismuth donor electron spins in silicon at millikelvin temperatures in the Purcell regime [A. Bienfait et al., Nature, vol. 531, pp. 74 – 77, 2016]. We report the direct observation of the microwave photons emitted by the spins, during their energy relaxation following a pi pulse. Moreover by changing the area of the exciting pulse we were able to measured Rabi oscillations in the variation of the total number of detected photons. Finally we report the direct observation of the photons emitted from the spin ensemble in a Hahn echo experiment.