Patrice BERTET

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. The sensitivity needed to detect single electron spins has been reached so far using spin-dependent photoluminescence, transport measurements, or 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 spin resonance at millikelvin temperature by spin fluorescence detection [1], using a microwave photon counter at cryogenic temperatures based on a superconducting transmon qubit [2]. We detect individual paramagnetic erbium ions in a scheelite crystal coupled to a small-mode-volume, high-quality factor superconducting resonator to enhance their radiative decay rate [3], 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 [4]. Coherence times up to 3ms are measured, limited by the ion radiative lifetime. The method applies to arbitrary paramagnetic species with long enough non-radiative relaxation time, and offers large detection volumes; as such, it may find applications in magnetic resonance and quantum computing [4].

[1] E. Albertinale et al., Nature 600, 434 (2021) 

[2] R. Lescanne et al., Phys. Rev. X 10, 021038 (2020) 

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

[4] Z. Wang et al., arXiv:2301.02653 (2023)