Spin qubit with coherence exceeding one second measured by microwave photon counting. Part 3/3

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 only, access 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 probes. These techniques are system-specific or sensitive only in a small detection volume, so that practical single spin detection remains an open challenge.

Using single-electron-spin-resonance techniques recently demonstrated [3] we characterize the magnetic environment of the single electron probe. The technique consists in measuring the spin fluorescence signal at microwave frequencies [1, 2] using a microwave photon counter based on a superconducting transmon qubit [3]. In our experiment, individual paramagnetic erbium ions in a scheelite crystal of CaWO4 are magnetically coupled to a small-mode-volume, high-quality factor superconducting microwave resonator to enhance their radiative decay rate [4]. The method applies to arbitrary paramagnetic species with long enough non-radiative relaxation time, and offers large detection volumes ( ∼ 10μm3) ; as such, it may find applications in magnetic resonance and quantum computing.

The third part of this talk will present the mechanisms by which the electron spin couples to the magnetic environment and discuss the techniques used to detect and characterize said environment.

[1] Albertinale, E. et al. Detecting spins by their fluorescence with am microwave photon counter. Nature 600, 434– 438 (2021).

[2] L. Balembois, et al. Practical Single Microwave Photon Counter with 10−22 W/√Hz sensitivity. arXiv :2307.03614.

[3] Z. Wang, et al. Single-electron spin resonance detection by microwave photon counting. Nature 619, 276–281 (2023).

[4] R. Lescanne et al. Irreversible Qubit-Photon Coupling for the Detection of Itinerant Microwave Photons. Phys. Rev. X 10, 021038 (2020).

[5] A. Bienfait et al. Controlling spin relaxation with a cavity. Nature 531, 74 (2016).