A High-Efficiency, Low-Noise Platform for Microwave-to-Optical Quantum Transduction

Citation

Sonar, Sameer Anil (2026) A High-Efficiency, Low-Noise Platform for Microwave-to-Optical Quantum Transduction. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/1cbe-vs09. https://resolver.caltech.edu/CaltechTHESIS:06272025-211326959

Abstract

Quantum computing platforms based on superconducting qubits have achieved remarkable progress in recent years, with significant advancements in quantum error correction, coherence times, and gate fidelities. However, the path to large-scale, fault-tolerant quantum computing faces a critical scaling bottleneck: the physical limits of single-chip architectures. Integrating millions of qubits on a single superconducting chip presents formidable engineering challenges, including increased thermal load, crosstalk, and complex wiring within the dilution refrigerator.

A promising approach to overcome these limitations is to interconnect multiple smaller superconducting quantum processors via a quantum network, allowing for distributed quantum computation. In this context, telecom-wavelength optical photons (around 1550 nm or 200 THz) are particularly attractive for transmitting quantum information across long distances due to their low propagation loss in optical fiber and negligible thermal occupation at room temperature. However, superconducting qubits typically operate at microwave frequencies (around 5-10 GHz), leading to a fundamental mismatch in operating frequencies that prevents direct coupling between these two domains.

This five-orders-of-magnitude frequency mismatch poses a major challenge for coherent quantum transduction, requiring a highly efficient, low-noise interface to faithfully convert quantum states between microwave and optical photons. A leading approach for transduction involves piezo-optomechanical platforms, where an intermediary acoustic resonator facilitates the conversion between microwave photons and microwave acoustic phonons, which are then converted to optical photons. However, existing designs often suffer from poor conversion efficiency and added noise due to geometric constraints and substrate heating, limiting their scalability for real-world quantum networks. In the first part of this thesis, I will introduce an optimized two-dimensional optomechanical crystal platform with a side-coupled optical waveguide. This geometry significantly improves the noise-efficiency metric for optical photon-acoustic phonon conversion. I will then discuss the integration of piezo-acoustic circuits into these two-dimensional crystals to realize a full microwave-to-optical transducer. I will cover the underlying design principles, fabrication processes, and preliminary measurement results, highlighting the potential of this platform for enabling future quantum communication and distributed quantum computing.

Another critical challenge in quantum networking is the frequency mismatch that arises when attempting to interfere photons emitted by different quantum nodes. This mismatch is primarily caused by variations in fabrication processes. In the second part of this thesis, I will present a novel post-fabrication tuning technique for piezo-optomechanical transducers, based on atomic force microscope (AFM) nano-oxidation. By applying a voltage bias to the AFM tip, we can selectively oxidize the surface of the dielectric device, introducing a controlled, localized change in refractive index and mechanical properties. This allows for precise tuning of both optical and acoustic resonance frequencies. I will demonstrate the effectiveness of this technique through experimental results at both room and cryogenic temperatures, highlighting its potential for scaling quantum networks.