Spin qubit with coherence exceeding one second measured by microwave photon counting. Part 2/3

Electron spin resonance (ESR) spectroscopy is the method of choice for characterizing parama-

gnetic 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 nee-

ded 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.

In this second part, I will present the spectroscopy of individual Erbium ions by microwave photon counting with an improved detector sensitivity.

[1] Albertinale, E. et al. Detecting spins by their fluorescence with a

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 mi-

crowave photon counting. Nature 619, 276–281 (2023).

[4] R. Lescanne et al. Irreversible Qubit-Photon Coupling for the De-

tection 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).