Coherent manipulation of an Andreev spin qubit
Two promising architectures for solid-state quantum information processing are electron spins in semiconductor quantum dots and the collective electromagnetic modes of superconducting circuits. In some aspects, these two platforms are dual to one another: superconducting qubits are more easily coupled but are relatively large among quantum devices $(\sim\mathrm{mm})$, while electrostatically-confined electron spins are spatially compact ($\sim \mathrm{\mu m}$) 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 $T_S = 17~\mathrm{\mu s}$ and a spin coherence time $T_{2E}=52~\mathrm{ns}$. 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.