Readout and control of electrostatically confined electrons in semiconductors are key primitives of quantum information processing with solid-state spin qubits^1,2. In superconductor–semiconductor heterostructures, localized electronic modes known as Andreev levels result from confinement that is provided by the pair potential^3,4. Unlike electronic modes confined exclusively via electrostatic effects, Andreev levels carry supercurrent. Therefore, they naturally integrate with the techniques of circuit quantum electrodynamics (cQED) that have been developed in the field of superconducting qubits and used to detect pairs of quasiparticles that are trapped in Andreev levels^5,6,7,8. Here, we demonstrate single-shot cQED readout of the spin of an individual quasiparticle trapped in the Andreev levels of a semiconductor nanowire Josephson element. Owing to a spin-orbit interaction in the nanowire, this ‘superconducting spin’ directly determines the flow of supercurrent through the element. We harnessed this spin-dependent supercurrent to achieve both a zero-field spin splitting and a long-range interaction between the quasiparticle and a superconducting microwave resonator^{9,10,11,12,13}. Measurement of the resultant spin-dependent resonator frequency yielded quantum non-demolition spin readout with 92% fidelity in 1.9 μs, which enabled us to monitor the quasiparticle spin in real time. These results pave the way for superconducting spin qubits that operate at zero magnetic field and for time-domain measurements of Majorana zero modes^{9,10,12,14,15}.

半导体中静电受限电子的读出和控制是固态自旋量子位^1,2的量子信息处理的关键原语。在超导体-半导体异质结构中，被称为安德列夫能级的局部电子模式是由对势^3,4提供的约束产生的。与仅通过静电作用限制的电子模式不同，安德列夫能级携带超电流。因此，它们自然地与超导量子位领域中开发的电路量子电动力学（cQED）技术集成在一起，并用于检测被困在Andreev ^{5、6、7、8}能级中的准粒子对。在这里，我们展示了捕获在半导体纳米线约瑟夫森元件的安德列夫能级中的单个准粒子的自旋的单次cQED读数。由于纳米线中的自旋轨道相互作用，这种“超导自旋”直接决定了超电流通过该元件的流量。我们利用这种与自旋有关的超电流来实现零场自旋分裂和准粒子与超导微波谐振器之间的长距离相互作用^{9,10,11,12,13}。测量所得的自旋相关谐振器频率可在1.9μs内产生92％保真度的量子非拆卸自旋读数，这使我们能够实时监控准粒子自旋。这些结果为在零磁场下工作的超导自旋量子位和马约拉纳零模^{9,10,12,14,15的}时域测量铺平了道路。