Continuous monitoring of a trapped superconducting spin

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^5–8. 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^9–13. 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}.