Two promising architectures for solid-state quantum information processing are electron spins in semiconductor quantum dots and the collective electromagnetic modes of superconductingcircuits. In some aspects, these two platforms are dual to one another: superconducting qubits are more easily coupled but are relatively large among quantum devices (∼mm), while electrostatically-confined electron spins are spatially compact (∼μm) but more complex to link. Here we combine beneficial aspects of both platforms in the Andreev spin qubit: the spin degree of freedom of an electronic quasiparticle trapped in the supercurrent-carrying Andreev levels of a Josephson semiconductor nanowire. We demonstrate coherent spin manipulation by combining single-shot circuit-QED readout and spin-flipping Raman transitions, finding a spin-flip time TS=17 μs and a spin coherence time T2E=52 ns. These results herald a new spin qubit with supercurrent-based circuit-QED integration and further our understanding and control of Andreev levels — the parent states of Majorana zero modes — in semiconductor-superconductor heterostructures.
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 mostfermionic 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 μs 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.
We report the realization of quantum microwave circuits using hybrid superconductor-semiconductor Josephson elements comprised of InAs nanowires contacted by NbTiN. Capacitively-shuntedsingle elements behave as transmon qubits with electrically tunable transition frequencies. Two-element circuits also exhibit transmon-like behavior near zero applied flux, but behave as flux qubits at half the flux quantum, where non-sinusoidal current-phase relations in the elements produce a double-well Josephson potential. These hybrid Josephson elements are promising for applications requiring microwave superconducting circuits operating in magnetic field.