  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.

Mesoscale and Nanoscale Physics;Superconductivity;Quantum Physics