Optically-interfaced spins in the solid state are a promising platform for quantum technologies. A crucial component of these systems is high-fidelity, projective measurement of the spin state. Here, we demonstrate single-shot spin readout of a single rare earth ion qubit, Er³⁺, which is attractive for its telecom-wavelength optical transition and compatibility with silicon nanophotonic circuits. In previous work with laser-cooled atoms and ions, and solid-state defects, spin readout is accomplished using fluorescence on an optical cycling transition; however, Er³⁺ and other rare earth ions generally lack strong cycling transitions. We demonstrate that modifying the electromagnetic environment around the ion can increase the strength and cyclicity of the optical transition by several orders of magnitude, enabling single-shot quantum nondemolition readout of the ion’s spin with 94.6% fidelity. We use this readout to probe coherent dynamics and relaxation of the spin.

固态光接口自旋是量子技术的一个有前途的平台。这些系统的一个重要组成部分是自旋态的高保真投影测量。在这里，我们演示了单个稀土离子量子位 Er ³⁺的单次自旋读出，它因其电信波长光学跃迁以及与硅纳米光子电路的兼容性而具有吸引力。在之前对激光冷却原子和离子以及固态缺陷的研究中，自旋读出是利用光学循环跃迁上的荧光来完成的。然而，Er ³⁺和其他稀土离子通常缺乏强循环跃迁。我们证明，改变离子周围的电磁环境可以将光学跃迁的强度和周期性增加几个数量级，从而实现离子自旋的单次量子非破坏读出，保真度为 94.6%。我们使用这个读数来探测自旋的相干动力学和弛豫。