The modern understanding of the Josephson effect in mesosopic devices derives from the physics of Andreev bound states, fermionic modes that are localized in a superconducting weak link. Recently, Josephson junctions constructed using semiconducting nanowires have led to the realization of superconducting qubits with gate-tunable Josephson energies. We have used a microwave circuit QED architecture to detect Andreev bound states in such a gate-tunable junction based on an aluminum-proximitized InAs nanowire. We demonstrate coherent manipulation of these bound states, and track the bound-state fermion parity in real time. Individual parity-switching events due to non-equilibrium quasiparticles are observed with a characteristic timescale $T_\mathrm{ parity} = 160\pm 10~\mathrm{ \mu s}$. The $T_\mathrm{ parity}$ of a topological nanowire junction sets a lower bound on the bandwidth required for control of Majorana bound states.
Categories: cond-mat.supr-con cond-mat.mes-hall quant-ph