Scaling superconducting quantum processors beyond single dilution refrigerators requires efficient optical interconnects, yet integrating microwave-to-optical (M2O) transducers poses challenges due to frequency mismatches and qubit decoherence. We propose a modular architecture using SNAIL-based parametric coupling to interface Brillouin M2O transducers with long-lived 3D cavities, while maintaining plug-and-play compatibility. Through numerical simulations incorporating realistic noises, including laser heating, propagation losses, and detection inefficiency, we demonstrate raw entangled bit fidelities of F~0.8 at kHz-level rates over 30 km using the Duan-Lukin-Cirac-Zoller (DLCZ) protocol. Implementing asymmetric entanglement pumping tailored to amplitude damping errors, we achieve purified fidelities F~0.94 at 0.2 kHz rates. Our cavity-based approach outperforms transmon schemes, providing a practical pathway for distributed superconducting quantum computing.
Scaling superconducting quantum processors beyond single dilution refrigerators requires efficient optical interconnects, yet integrating microwave-to-optical (M2O) transducers poses challenges due to frequency mismatches and qubit decoherence. We propose a modular architecture using SNAIL-based parametric coupling to interface Brillouin M2O transducers with long-lived 3D cavities, while maintaining plug-and-play compatibility. Through numerical simulations incorporating realistic noises, including laser heating, propagation losses, and detection inefficiency, we demonstrate raw entangled bit fidelities of F0.8 at kHz-level rates over 30 km using the Duan-Lukin-Cirac-Zoller (DLCZ) protocol. Implementing asymmetric entanglement pumping tailored to amplitude damping errors, we achieve purified fidelities F0.94 at 0.2 kHz rates. Our cavity-based approach outperforms transmon schemes, providing a practical pathway for distributed superconducting quantum computing.
Superconducting quantum processors operate at microwave frequencies in millikelvin environments, making it challenging to interconnect distant nodes using conventional microwave wiring. Coherent microwave-to-optical (M2O) transduction enables superconducting quantum networks by interfacing itinerant microwave photons with low-loss optical fiber. However, many state-of-the-art transducers provide efficient conversion only over a narrow frequency span, complicating deployment with heterogeneous superconducting devices that are detuned by gigahertz-scale offsets. Here we demonstrate a frequency-agile microwave-optical interface that overcomes this bandwidth mismatch by cascading an electro-optic M2O transducer with a multimode microwave-to-microwave (M2M) frequency converter, with in situ tunability of the microwave resonances in both stages. Using this architecture, we realize continuous frequency coverage from 5.0 to 8.5 GHz within a single system. As an application relevant to superconducting-qubit networking, we use the cascaded M2M-M2O interface to optically read out a superconducting qubit whose readout resonator is detuned by 1.7 GHz from the native M2O microwave resonance, demonstrating a scalable route toward fiber-linked superconducting quantum nodes.
Superconducting quantum circuits are a leading hardware candidate for realizing a large-scale fault-tolerant quantum computer. However, scaling superconducting quantum computers will soon approach the physical limits imposed by their cryogenic operating environments. Consequently, interconnects between multiple processor modules will become increasingly necessary for scaling quantum computers. Optical fiber is a particularly promising candidate for these interconnects due to its low loss and negligible thermal noise at room temperature. However, interfacing microwave-frequency superconducting circuits with optical fibers presents several challenges. In this thesis, we present a modular architecture which addresses many of the current challenges in building this interface. We develop an optomechanical quantum transducer which is capable of generating microwave- and optical-frequency photons in pairs, and demonstrate quantum entanglement between these microwave-optical photon pairs. Further, we develop a superconducting qubit module optimized for efficient absorption of itinerant microwave photons, and demonstrate a high-efficiency photon catch protocol. Finally, we connect the superconducting qubit module to the transducer module via a superconducting microwave cable. We drive the microwave-optical photon pair generation process in the transducer module and employ the photon catch protocol in the qubit module to facilitate state transfer between the qubit and the flying microwave photon from the transducer. We observe correlations between the final qubit state and the detection of an optical photon, demonstrating a crucial step toward establishing quantum entanglement between superconducting qubits and light. We discuss ongoing progress and challenges in realizing hybrid entanglement and in extending this approach to entangle remote superconducting qubits over optical networks.
Quantum technologies harness superposition and entanglement for computation, communication, and sensing. Superconducting quantum circuits (SQCs) are promising for scalable quantum information processing, but networking distant SQC nodes is challenging: microwave photons suffer severe thermal noise and loss at room temperature, while optical photons travel through fiber with negligible loss. A quantum transducer bridges this gap by converting microwave photons to optical photons with minimal added noise. This thesis presents a memory-assisted microwave-to-optical (M2O) transducer using an ensemble of Yb-ions doped in Y2SiO5 (YSO). Yb:YSO offers zero-first-order Zeeman (ZEFOZ) transitions at zero field, giving long coherence times, and GHz-range hyperfine splittings that interface directly with SQCs without an external magnetic field. The platform integrates a custom loop-gap resonator (LGR) with a free-space optical setup at 30 mK in a dilution refrigerator. The LGR, designed via CAD and COMSOL simulation and machined from oxygen-free copper, concentrates the microwave field for strong coupling to the spin ensemble. A Python-based automation framework synchronizes instrumentation for high-throughput acquisition of pulse sequences. Coherence is characterized via optical excited-state lifetime, spin population time, and optical coherence time. Optically detected magnetic resonance reveals coherent microwave-spin dynamics, including Rabi oscillations with collapse-and-revival features from spin inhomogeneity and cavity photon-number statistics. Continuous-wave M2O transduction at 2623 MHz achieves an internal efficiency of 2e-8 over 200 kHz bandwidth. Avoided-crossing measurements confirm strong spin-microwave coupling, with cooperativity exceeding unity. A theoretical framework links efficiency to microwave, optical, and atomic cooperativities, projecting ~1e-4 efficiency through improved doping, beam diameter, detuning, and cavity integration. A memory-assisted M2O protocol integrates quantum storage via rapid adiabatic passage phase imprinting, enabling on-demand retrieval separated from the pump and suppressing pump noise without lossy filtering. At 460 μs storage, this achieves 4e-8 efficiency with 0.4 noise photons, among the lowest noise figures reported for M2O transducers. Interference measurements confirm coherent information transfer, and multiplexing of up to ten transduced modes is demonstrated, exploiting Yb:YSO's broad inhomogeneous linewidths. This work, the first memory-assisted M2O interface, enables photon synchronization for path-erasure entanglement between remote SQCs, advancing scalable quantum repeaters and distributed quantum computing.
Superconducting quantum circuits operate at millikelvin temperatures, typically requiring independent microwave cables for each qubit for connecting room-temperature control and readout electronics. However, scaling to large-scale processors hosting hundreds of qubits faces a severe input/output (I/O) bottleneck, as the dense cable arrays impose prohibitive constraints on physical footprint, thermal load, wiring complexity, and cost. Here we demonstrate a complete optical I/O architecture for superconducting quantum circuits, in which all control and readout signals are transmitted exclusively via optical photons. Employing a broadband traveling-wave Brillouin microwave-to-optical transducer, we achieve simultaneous frequency-multiplexed optical readout of two qubits. Combined with fiber-integrated photodiode arrays for control signal delivery, this closed-loop optical I/O introduces no measurable degradation to qubit coherence times, with an optically driven single-qubit gate fidelity showing only a 0.19% reduction relative to standard microwave operation. These results establish optical interconnects as a viable path toward large-scale superconducting quantum processors, and open the possibility of networking multiple superconducting quantum computers housed in separate dilution refrigerators through a centralized room-temperature control infrastructure.
Superconducting quantum processors are a leading platform for implementing practical quantum computation algorithms. Although superconducting quantum processors with hundreds of qubits have been demonstrated, their further scaling up is constrained by the physical size and cooling power of dilution refrigerators. This constraint can be overcome by constructing a quantum network to interconnect qubits hosted in different refrigerators, which requires microwave-to-optical transducers to enable low-loss signal transmission over long distances. Despite that various designs and demonstrations have achieved high-efficiency and low-added-noise transducers, a coherent photonic link between separate refrigerators has not yet been realized. In this work, we experimentally demonstrate coherent signal transfer between two superconducting circuits housed in separate dilution refrigerators, enabled by a pair of frequency-matched aluminum nitride electro-optic transducers connected via a 1-km telecom optical fiber. With transducers at each node achieving >0.1% efficiency, an overall 80 dB improvement in transduction efficiency over commercial electro-optic modulators is attainable, paving the way towards a fully quantum-enabled link. This work provides critical design guidelines towards scalable superconducting quantum networks interconnected by photonic links.
One of the main thrusts of quantum science over the past few decades has been the developmentof quantum networks for the purposes of secure communication, enhanced detector sensitivity, and advanced computing. Realizing such a network of superconducting quantum processors that communicate via optical fibers would leverage the high-fidelity quantum signal processing of superconducting circuits and the thermal robustness of infrared light but requires a transducer capable of connecting these two sections of the electromagnetic spectrum separated by five orders of magnitude in energy. This thesis explores the optimization of a transducer architecture where the mechanical mode of a Si3N4 membrane mediates the coupling of a superconducting lumped element circuit and a Fabry-Perot optical cavity. We aim to maximize the coupling of these three harmonic oscillator to each other while shielding them from noisy processes that would decohere quantum signals. This architecture has led to transducers with unparalleled efficiency and continuous operation. Enhanced cooperativity between the optical cavity and mechanical oscillator enabled optically-detected readout of a superconducting qubit and optomechanical ground state cooling with negligible laser-induced heating of the superconducting qubit and microwave circuit. To surpass the threshold for quantum-enabled operation, we subsequently improved the cooperativity between the microwave circuit and mechanical oscillator by reducing the microwave loss and noise from two-level-system-like defects in the Si3N4 dielectric. When combined with enhanced coupling between the circuit and membrane or improved mechanical isolation, we project that this architecture will be capable of transducing quantum signals between the microwave and optical regimes with a signal-to-noise greater than one.