Single-spin-resolved magnetic resonance spectroscopy of a nanocrystal using microwave photon counting

Electron paramagnetic resonance (EPR) spectroscopy is the standard technique for characterizing paramagnetic centers, providing crucial insights into materials science, chemistry, and biology. However, conventional EPR is limited to ensemble-averaged measurements, and developing a practical, system-agnostic single-spin detection method remains an open challenge.

We present a versatile single-spin EPR platform based on microwave photon counting at millikelvin temperatures. In contrast to previous efforts where spins are intrinsic to the resonator substrate, we study independent spin systems placed on top of the inductive element of a planar superconducting resonator. This method enables the study of a wide variety of paramagnetic spin species and host materials. To demonstrate the platform’s capabilities, we study Er³⁺ ions embedded in sub-micron-sized yttrium oxide (Y₂O₃) nanocrystals.

We characterize individual paramagnetic Er³⁺ ions by detecting single microwave fluorescence photons, and demonstrate coherent control of single spins. The fluorescence signal exhibits antibunching, confirming its origin from a single emitter.



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