  We show that optically active coupled quantum dots embedded in a
superconducting microwave cavity can be used to realize a fast quantum
interface between photonic and transmon qubits. Single photon absorption by a
coupled quantum dot results in generation of a large electric dipole, which in
turn ensures efficient coupling to the microwave cavity. Using cavity
parameters achieved in prior experiments, we estimate that bi-directional
microwave-optics conversion in nanosecond timescales with efficiencies
approaching unity is experimentally feasible with current technology. We also
outline a protocol for in-principle deterministic quantum state transfer from a
time-bin photonic qubit to a transmon qubit. Recent advances in quantum dot
based quantum photonics technologies indicate that the scheme we propose could
play a central role in connecting quantum nodes incorporating cavity-coupled
superconducting qubits.
