Design and Test of Superconducting Parallel-Plate Resonators for the Detection of Single Electron Spins

Spin-bearing solid-state defects have proven their applicability in quantum communication, computation and sensing in the recent years [1]. However, the control and in particular the detection of the spin state is done predominantly using optical transitions to higher orbital states of the electron. The direct spin-flip transition, with frequencies typically in the microwave range, has such a low decay rate that until recently it was impossible to detect a single spin by its microwave fluorescence. Using a superconducting resonator with a Purcell factor of 10¹⁴ and a single-microwave-photon detector [2], we reduced the radiative lifetime of a single Er³⁺ ion in a CaWO₄ crystal to 1 ms [3] and coherently controlled nearby nuclear spin qubits [4]. In this talk, I will present superconducting parallel-plate resonators, which are expected to increase the Purcell factor by up to 2 orders of magnitude, as well as our progress towards the non-demolition readout of the spin state through the dispersive shift of the resonator's frequency.

[1] D. Awschalom et al., Nature Photonics, 12(9), 516-527 (2018)

[2] R. Lescanne et al., PRX 10, 021038 (2020)

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

[4] J. O'Sullivan, J. Travesedo et al., Nature Physics (2025)