Spin qubit with coherence exceeding one second measured by microwave photon counting. Part 1/3

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.

In the first part of this talk, we will focus on a Single Microwave Photon Detector (SMPD), which

working principle is based on Circuit QED. A previous version of

this design, which enabled single electron spin detection for the first time, has reached a sensitivity

of 10−22W/√Hz [2]. We present here an improved version of this design, reaching a sensitivity of

10−23W/√Hz. This improvement has been achieved by enhancing the two figures of merit of the

SMPD : The dark count rate (rate of false-positive detection events) and the detection efficiency (probability of successful detection).

[1] Albertinale, E. et al. Nature 600, 434– 438 (2021).

[2] L. Balembois, et al. arXiv :2307.03614.

[3] Z. Wang, et al. Nature 619, 276–281 (2023).

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

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