Patrice Bertet: Microwave photon counting and its application to single-spin magnetic resonance
Microwave photon counting and its application to single-spin magnetic resonance
Abstract: Detecting weak microwave signals is of interest in several fields of science, including magnetic resonance, quantum computing, and dark matter search. Microwave detection usually proceeds by measuring the field quadratures using amplification followed by demodulation, but the signal-to-noise ratio is then fundamentally limited by vacuum field fluctuations even at the lowest temperatures. Microwave photon counting, on the other hand, escapes this limit because photon vacuum is an energy eigenstate. Microwave photon counting can therefore lead to large gain in signal-to-noise ratio compared to quadrature detection. I will present our efforts to develop low- noise and high-efficiency microwave photon counters at 10mK, based on superconducting transmonqubits [1]. I will then show how these novel detectors can be applied to measure individual electron spins in solids, using as a model system paramagnetic Er3+ ions in a crystal of CaWO4 [2]. These electron spins moreover act as nano-antenna able to sense their immediate nuclear spin environment at the individual nuclear spin level [3]. I will report an experiment where we use this principle to identify chemically and structurally an individual 93Nb atom impurity located near the Er3+, and harness the high spectral resolution of the NMR measurements to detect previously unobserved terms in the Hamiltonian of this coupled spin system [4].
[1] L. Balembois et al., Phys. Rev. Applied 21, 014043 (2024)
[2] Z. Wang et al., Nature 619, 276 (2023)
[3] J. Travesedo et al., Sci. Adv. 11, eadu0581 (2025)
[4] J. Travesedo et al., in preparation
Bio: Prof. Patrice Bertet from CEA Saclay and Université Paris-Saclay is awarded the IES Medal in Physics for his groundbreaking contributions to the quantum physics of electron spin resonance (ESR), including seminal observations of fundamental phenomena in spin resonance as well as profound technological developments in spin sensitivity. His pioneering results include the first observation of cavity-induced relaxation of spins, as originally predicted by Purcell in 1946, and most recently, the demonstration of single-spin pulsed ESR achieved using novel single microwave photon detectors. Bertet’s research is driven by pushing ESR spectroscopy into new domains and extremes, down to millikelvin temperatures. He has used Josephson parametric amplifiers to reach ESR sensitivity at the quantum limit of noise and employed vacuum-state squeezing to surpass these noise limits. By bridging quantum optics, superconducting circuits, and spin physics, Bertet has repeatedly redefined state-of-the-art ESR sensitivity through new paradigms for spin manipulation and detection. He has used these capabilities to study NV centers in diamond, dopants in silicon, andrare-earth ions in low-nuclear spin oxides, obtaining electron spin coherence times of tens to hundreds of milliseconds for near-surface spins. His sustained and transformative contributions to the field over the past 15 years make him a highly deserving recipient of this award.
