Piezo-optomechanical signal transduction using hybridized mechanical supermodes

: Microwave-to-optical transduction in suspended GaAs PIC platform

Abstract

A hybrid system approach is necessary to build effective quantum information systems, where resources from different physical platforms are combined to leverage their individual strengths. An example of such a hybrid system is a distributed quantum network, where superconducting qubits form the physical network nodes used for computation (quantum processors) and the quantum information is transmitted and received between these nodes via low-loss optical fibers, with optical photons being the carriers of quantum information. To interconnect such two physical platforms, a microwave-to-optical transducer is necessary which can mediate quantum states efficiently between two systems with high fidelity. The fundamental challenge in achieving high efficiency for such a transducer lies in the vast difference of the frequencies (wavelengths) of the two domains. While microwave qubits tend to work around 1 - 10 GHz, the frequency of the telecom photons lies around 200 THz. A workaround for this problem has been proposed where the microwave signal is first converted to an acoustic wave with the help of a piezoelectric resonator. The wavelengths of the acoustic waves travelling on the surface of a piezoelectric material lies in the range of wavelengths of the telecom photons, providing strong overlap with the optical fields.

Piezoelectric optomechanical transduction has been shown with two types of optical cavities, bulk and 1D photonic crystals. The bulk approach allows one to achieve high electromechanical cooperativity and large number of intracavity photons, but is limited in the optomechanical interaction strength that is needed for high efficiency. On the other hand, 1D photonic crystals have shown excellent single photon optomechanical strength but are limited in the number of intracavity photons and the electromechanical cooperativity. There is a need for a middle ground between the two, which I demonstrate with microring optical cavity and Lamb wave resonators. By hybridizing the Lamb wave resonance mode with the mechanical mode of the microring waveguide to form a supermode, high transduction efficiency can be achieved.

I start with microwave-to-optical transduction with a SAW mode, which despite being leaky results in a photon number conversion efficiency of ≈ 1.5 × 10⁻¹⁰. The efficiency is very small but it is still higher than the previously achieved efficiency in GaAs for 1D photonic crystals (i.e. ≈ 10⁻¹²). Once the fabrication process is established with SAW, piezo-optomechanical transduction with Lamb waves supermodes is performed, where the transduction efficiency is significantly enhanced to 1.17 × 10⁻⁷ for the mechanical mode at 2 GHz, which is the highest achieved efficiency demonstrated for III-V platforms. Not only that, using Lamb wave supermodes, the microwave-to-optical transduction was also achieved for higher frequency mechanical modes, up to 7 GHz.

Date of Award	27 Sept 2022
Original language	English
Awarding Institution	University of Bristol
Supervisor	Krishna Coimbatore Balram (Supervisor) & John G Rarity (Supervisor)