  Quantum transduction between the microwave and optical domains is an
outstanding challenge for long-distance quantum networks based on
superconducting qubits. For all transducers realized to date, the generally
weak light-matter coupling does not allow high transduction efficiency, large
bandwidth, and low noise simultaneously. Here we show that a large electric
dipole moment of an exciton in an optically active self-assembled quantum dot
molecule (QDM) efficiently couples to a microwave resonator field at a
single-photon level. This allows for transduction between microwave and optical
photons without coherent optical pump fields to enhance the interaction. With
an on-chip device, we demonstrate a sizeable single-photon coupling strength of
16 MHz. Thanks to the fast exciton decay rate in the QDM, the transduction
bandwidth between an optical and microwave resonator photon reaches several
100s of MHz.
