Continuous monitoring of a trapped, superconducting spin

Readout and control of fermionic spins in solid-state systems are key primitives of quantum information processing and microscopic magnetic sensing. The highly localized nature of most fermionic spins decouples them from parasitic degrees of freedom, but makes long-range interoperability difficult to achieve. In light of this challenge, an active effort is underway to integrate fermionic spins with circuit quantum electrodynamics (cQED), which was originally developed in the field of superconducting qubits to achieve single-shot, quantum-non-demolition (QND) measurements and long-range couplings. However, single-shot readout of an individual spin with cQED has remained elusive due to the difficulty of coupling a resonator to a particle trapped by a charge-confining potential. Here we demonstrate the first single-shot, cQED readout of a single spin. In our novel implementation, the spin is that of an individual superconducting quasiparticle trapped in the Andreev levels of a semiconductor nanowire Josephson element. Due to a spin-orbit interaction inside 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 as well as a long-range interaction between the quasiparticle and a superconducting microwave resonator. Owing to the strength of this interaction in our device, measuring the resultant spin-dependent resonator frequency yielded QND spin readout with 92% fidelity in 1.9 $μ$s and allowed us to monitor the quasiparticle's spin in real time. These results pave the way for new "fermionic cQED" devices: superconducting spin qubits operating at zero magnetic field, devices in which the spin has enhanced governance over the circuit, and time-domain measurements of Majorana modes.