Cavity-enhanced optical readout and control of nuclear spin qubits

Their exceptional coherence makes nuclear spins in solids a prime candidate for quantum memories in quantum networks and repeaters. Still, the direct all-optical initialization, coherent control, and readout of individual nuclear spin qubits have been an outstanding challenge. Here, this is achieved by embedding 167Er dopants in yttrium orthosilicate in a cryogenic Fabry-Perot cavity, whose linewidth of 65 MHz is much smaller than the 0.9 GHz separation of neighboring hyperfine levels. Frequency-selective emission enhancement thus enables a single-shot readout fidelity of 91(2) %. Furthermore, a large magnetic field freezes paramagnetic impurities, leading to coherence times exceeding 0.2 s. The combination of nuclear-spin qubits with frequency-multiplexed addressing and lifetime-limited photon emission in the minimal-loss telecommunications C-band establishes 167Er as a leading platform for long-range, fiber-based quantum networks. Despite the promise of nuclear spin qubits in solids as candidates for quantum memories, combining long enough storage times with coherent control remains a formidable challenge. Here, the authors study 167Er-doped yttrium orthosilicate crystals (Er:YSO) in a cavity scheme with frequency-selective addressing, achieving 91% single-shot readout fidelity and 0.2 s coherence times at cryogenic temperatures.

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