Magnetic resonance is a non-invasive tool that plays a central role in a wide range of fi elds, from medical imaging (MRI) to analyticalchemistry, and more recently in quantum computing, where it is used to control and read spin-based qubits. However, this techniquesuff ers from low sensitivity, requiring the collective response of a large number of spins to produce a detectable signal. Recent advancesin superconducting quantum technologies have dramatically improved this sensitivity—by more than ten orders of magnitude—bycombining the Purcell eff ect with novel detectors such as microwave photon counters.
This project builds on these breakthroughs by developing an innovative superconducting platform for the fast and effi cient readout of

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single electron spins, based on enhancing the spin-resonator coupling through magnetic mode hyper-focusing.

Using a specially designed parallel-plate capacitor geometry, featuring a central nanowire, the magnetic fi eld of the microwave mode canbe concentrated within a region of just a few hundred nanometers. This signifi cantly increases the local interaction between the fi eld andthe electron spin located just beneath the nanowire. The central objective of the project is to boost the Purcell factor by two orders ofmagnitude, from 10¹³ to 10¹5, in order to drastically reduce spin detection time and potentially reach the strong coupling regime at thelevel of a single spin.

In a first phase, the project will focus on Er³? ions implanted in crystals such as CaWO4, Y2SiO5, or directly in silicon, with the aim ofintegrating them into hybrid quantum computing architectures combining superconducting circuits and spin-based quantum memories.In a second phase, the platform will be extended to more realistic paramagnetic systems, such as organic radicals or metallic centers inproteins, paving the way for quantum spectroscopy of complex molecular compounds, well beyond the scope of current model systems.
Building on the expertise of the Quantronics group at CEA Saclay in superconducting circuits, nanofabrication, cryogenics, andmicrowave single-photon detection, the project will provide the PhD student with comprehensive training at the intersection ofexperimental physics, nanoscience, and quantum information, within a world-class research environment.