Microwave-Optical Transduction Mediated by a Single Quantum-Dot Molecule

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. We show that a quantum transducer made of an optically active quantum dot molecule (QDM) can solve the above issue thanks to the large single-photon coupling strength between an exciton and a microwave mode. In the first part of the thesis, we outline the theoretical concept of the transducer where single optical photon absorption by an InAs/GaAs QDM results in the generation of a large electric dipole which, in turn, efficiently couples to a microwave field. In order to realize this QDM based transducer, we fabricate a device incorporating a QDM in the field antinode of a superconducting coplanar waveguide resonator on a GaAs substrate. With this prototypical on-chip device, we demonstrate a sizeable single-photon coupling strength of 16 MHz between an exciton and a microwave resonator photon. Thanks to the high exciton decay rate in the QDM, the transduction bandwidth between an optical and microwave resonator photon reaches several 100s of MHz. In the later part of this thesis, we present a design that should improve the single-photon coupling strength to 100s of MHz. We fabricate such a device by integrating a gate tunable QDM into a high-impedance superconducting resonator. Furthermore, we achieve a low microwave photon loss rate of 1.8 MHz by utilizing a distributed-element filter to suppress microwave leakage and heterogeneous integration of a QDM into the resonator on a low-loss substrate. The demonstrated microwave loss rate is much lower than the expected single-photon coupling strength. This feature will facilitate efficient and fast quantum conversion mediated by single microwave and optical photon interactions