High-resolution single nuclear spin spectroscopy using microwave photon counting

We introduce an all-microwave platform for single-spin NMR based on single-microwave-photon counting. A ⁹³Nb impurity in CaWO₄ serves as a magnetic probe, read out by monitoring the fluorescence of a coupled Er³⁺ electron spin. Using this probe, we measure couplings to nine surrounding ¹⁸³W nuclear spins with spin echo double resonance sequences, then concatenate these sequences to extract pairwise ¹⁸³W–¹⁸³W couplings. We resolve couplings down to ~100 mHz for the most weakly coupled ¹⁸³W spins, with times as long as 28 s. Finally, we perform high-resolution spectroscopy on the ¹⁸³W nuclei and observe relative Larmor frequency offsets between these nuclei of ~10,000 ppm. We discuss the possible origin of these large offsets.

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