Tuning Hybrid Optomechanics for Remote Entanglement

Superconducting microwave circuits are a leading platform for quantum computing, offering high coherence and controllability. However, their reliance on microwave photons, which are highly susceptible to thermal noise at room temperature due to their relatively low frequencies, necessitates operation at millikelvin temperatures. This requirement presents a major scalability challenge, particularly for connecting distant processors within a distributed quantum network. Microwave-optical transducers offer a promising solution by enabling coherent links between the microwave and optical domains, allowing quantum information to be shared via telecom-wavelength photons that propagate efficiently through low-loss optical fibers at room temperature. Among the various transduction platforms, hybrid piezo-optomechanical crystals (OMCs) are particularly promising due to their strong optomechanical and piezoelectric coupling and the potential for high-efficiency, low-noise transduction mediated by microwave frequency phonons. Proposed architectures for remote entanglement distribution rely on the interference of indistinguishable photons emitted from individual transducers. Although state-of-the-art fabrication techniques provide nanometer-level precision, achieving identical OMCs remains challenging, leading to device-to-device variations in optical and mechanical resonance frequencies. To enable scalable quantum networks based on optically mediated remote entanglement, a robust, selective, and precise post-fabrication tuning method is essential. Here, we present an in situ, selective technique for tuning the optical and acoustic resonances of hybrid silicon optomechanical crystals through electric field-induced nano-oxidation using an atomic force microscope (AFM). The localized growth of a few-nanometer-thick silicon dioxide layer modifies the local permittivity, stiffness, and mass of the OMC at the oxidation region, consequently altering the optical and mechanical modes supported by the structure. Using this method, we demonstrate precise and targeted spectral alignment of both optical and mechanical modes across multiple devices within their respective mode linewidths. In addition, we extend this technique to achieve selective room-temperature pre-alignment of the optical mode of OMCs for precise wavelength alignment at millikelvin temperatures. This capability is essential for realizing indistinguishable photon emission from independently fabricated transducers toward entanglement of distant quantum processors in optically linked quantum networks. In the second part of this thesis, we present a side-coupled two-dimensional optomechanical cavity designed for high-efficiency, low-noise phonon–photon transduction. This architecture enables near-unity conversion efficiency between optical photons and microwave frequency phonons while maintaining thermal occupancy of the phonon mode well below unity, an essential requirement for quantum-enabled operations. Finally, we describe the design, fabrication, and preliminary characterization of a microwave-to-optical transducer based on this new side-coupled 2D OMC platform.