Quantum networks of superconducting circuit quantum processors are an exciting prospect but are unfeasible without an efficient quantum interface between optical light and superconducting qubits. This problem motivates the development of a quantum microwave-to-optical transducer, a device that can faithfully transfer the state of a superconducting circuit to a photonic flying qubit and vice versa. This thesis describes our progress towards building a microwave-to-optical transducer mediated by phonons in a high-overtone bulk acoustic resonator (HBAR). Novel aluminum nitride-on-quartz HBARs are designed, fabricated, and tested. The acoustic loss rates are found to be near the state-of-the-art. Transduction-compatible transmon qubits are designed and tested. These qubits are capable of operating inside an optical cavity as well as achieving electromechanical interactions in the strong-coupling regime. An initial experiment studied how superconducting qubits recover after exposure to infrared light and demonstrated that the transduction-compatible qubits should be able to operate in a transducer with a reasonable duty cycle. The concept of microwave-to-optical transduction using multiple acoustic modes is established, and new simulation methods are developed to rapidly optimize HBARs. These tools are used to create a transduction-compatible qubit-HBAR device operating in the multi-mode strong coupling regime. Separately, a transduction compatible optical cavity was built and used to perform optomechanical measurements of a transduction-compatible HBAR and confirm that this device can reach the high-cooperativity regime. Finally, a measurement for verifying that a transducer is operating as a source of single photons is discussed. This can be accomplished by measuring the optical second-order correlation function of transduced photons. A filter cavity cascade is constructed to perform this measurement without being overwhelmed by optical pump leakage. These results establish bulk acoustic transduction as a promising platform for low-noise quantum microwave-to-optical transduction.
Surface acoustic waves (SAWs) coupled to quantum dots (QDs), trapped atoms and ions, and point defects have been proposed as quantum transduction platforms, yet the requisite coupling rates and cavity lifetimes have not been experimentally established. Although the interaction mechanism varies, small acoustic cavities with large zero-point motion are required for high efficiencies. We experimentally establish the feasibility of this platform through electro- and optomechanical characterization of tightly focusing, single-mode Gaussian SAW cavities at approximately 3.6 GHz on
GaAs. We explore the performance limits of the platform by fabricating SAW cavities with mode volumes approaching $6{\ensuremath{\lambda}}^{3}$ and linewidths less than 1 MHz. Employing strain-coupled single
InAs QDs as optomechanical intermediaries, we measure single-phonon optomechanical coupling rates ${g}_{0}\ensuremath{\approx}2\ensuremath{\pi}\ifmmode\times\else\texttimes\fi{}1.2$ MHz. Sideband scattering rates thus exceed intrinsic phonon loss, indicating the potential for quantum optical readout and transduction of cavity phonon states. To demonstrate the feasibility of this platform for low-noise ground-state quantum transduction, we develop a fiber-based confocal microscope in a dilution refrigerator and perform single-QD resonance fluorescence sideband spectroscopy at millikelvin temperatures. These measurements show conversion between microwave phonons and optical photons with sub-natural linewidths.
A device capable of converting single quanta of the microwave field to the optical domain is an outstanding endeavor in the context of quantum interconnects between distant superconducting qubits, but likewise can have applications in other fields, such as radio astronomy or, in the classical realm, microwave photonics. A variety of transduction approaches, based on optomechanical or electro-optical interactions, have been proposed and realized, yet the required vanishing added noises and an efficiency approaching unity, have not yet been attained. Here we present a transduction scheme that could in theory satisfy the requirements for quantum coherent bidirectional transduction. Our scheme relies on an intermediary mechanical mode, a high-overtone bulk acoustic resonance (HBAR), to coherently couple microwave and optical photons through the piezoelectric and strain-optical effects. Its efficiency results from the combination of integrated
Si3N4 photonic circuits with ultra-low loss sustaining high intracavity photon numbers with the highly efficient microwave to mechanical transduction offered by piezoelectrically coupled HBAR. We develop a quantum theory for this multipartite system by first introducing a quantization method for the piezoelectric interaction between the microwave mode and the mechanical mode from first principles (which to our knowledge has not been presented in this form) and link the latter to the conventional Butterworth-Van Dyke model. The HBAR is subsequently coupled to a pair of hybridized optical modes from coupled optical ring cavities via the strain-optical effect. We analyze the conversion capabilities of the proposed device using signal flow graphs, and demonstrate that near quantum coherent transduction is possible, with realistic experimental parameters. Combined with the high thermal conduction via the device bulk, heating effects are mitigated, and the approach does not require superconducting resonators that are susceptible to absorption of optical photons.