Spin fluorescence detection by a microwave photon counter : a sensitive new method for EPR spectroscopy*

The technique of choice for characterizing unpaired electron spins in a sample is Electron Paramagnetic Resonance (EPR) spectroscopy. In EPR, a dc magnetic field tunes the spins to resonance with a microwave cavity in which the sample is inserted. The spins are probed with sequences of microwave pulses; they respond by emitting microwave signals through the detection cavity into the measurement line that are then detected.

In pulse EPR spectroscopy, the Hahn echo sequence leads to the emission of a burst of coherent microwave radiation called the spin-echo, which is detected by amplification and demodulation. We have recently proposed and demonstrated [1] an alternative method, where we apply a single pi pulse to the spins. While this does not lead to any coherent microwave emission, incoherent microwave photons are emitted upon radiative relaxation towards equilibrium at a rate enhanced by the Purcell effect [2], and these photons can be efficiently detected by a microwave photon counter [3].

I will present this new EPR spectroscopy by spin fluorescence method, and I will compare its sensitivity to the usual inductively-detected echo method.

[1] E. Albertinale et al., arxiv:2102:01415

[2] A. Bienfait et al., Nature 531, 74 (2016)

[3] R. Lescanne et al., PRX 10, 021038 (2020)