Rare-Earth ions spin detected with a microwave photon counter

We report the spin resonance spectroscopy of rare-earth ions using the newly developed method of spin microwave fluorescence detection at millikelvin temperature [1]. The rare-earth ion is Er3+ in a CaWO4 crystal, which behaves as an effective electron spin-½ with high gyromagnetic ratio and long coherence time [2]. The spins are magnetically coupled to a micron-size superconducting microwave resonator deposited on top of the crystal, which enhances their radiative relaxation rate via the Purcell effect [3]. The ions are excited by a resonant microwave pulse, and their radiative relaxation is detected at 10mK using a Single Microwave Photon Counter based on a transmon qubit [4].

We observe an excitation-power-dependent spin relaxation time that we attribute to the competition between radiative and non-radiative relaxation [2]. The high sensitivity of fluorescence detection allows to measure the spectrum of ions close to the surface and to the resonator. In this regime, the erbium spectroscopy becomes asymmetric, with a tail at high magnetic field. In this tail, the erbium spin relaxation time is particularly short and coherent oscillations are observed. This indicates that the ions are located right below the resonator inductance and supports the interpretation that the frequency shift is due to the mechanical strain [5]. Approximately 20 erbium ions are detected; our experiment is thus a meaningful step towards the detection of a single ion.