Superconducting circuits are a leading platform for quantum computation and quantum simulation. Interactions are typically mediated by pairwise coupling elements between nearest neighbor qubits. The development of tunable couplers has been essential to achieving high fidelity entangling gates alongside high fidelity single qubit gates in quantum processors. In quantum simulators, tunable coupling allows flexibility in emulating different systems and the exploration of different parameter regimes of those models in a single physical device. Distributed element circuitry has enabled long range coupling and leveraging of interference effects and spatial mode variations to enhance device performance. In this thesis I will discuss work towards developing a distributed element tunable coupling circuit: a transmission line cavity terminated by SQUIDs, which serve as flux tunable impedances, at either boundary. With this device we hope to integrate the dynamic programmability of interactions mediated by a tunable coupler with the long-range, multi-qubit coupling enabled by distributed transmission lines in a single system. We experimentally demonstrate the core functionality, including tunability of the coupling rates, programmability of the connectivity graph, and parallel operation via multiplexing. Additionally, we utilize this device to explore photon blockade effects in the Tavis-Cummings Model, highlighting the value of tunability and controllability when simulating novel light-matter interactions in highly connected systems. These results contribute to ongoing research towards designing, calibrating, and operating highly connected superconducting circuit systems. Devices with high connectivity enable more efficient circuit compilation and are useful as quantum routers in quantum networks. In recent years, understanding the role of connectivity in quantum error correction has emerged as an important research focus. There is ongoing work evaluating the tradeoff between performance and connectivity in real devices. The development of coupling architectures that enable denser and/or longer range connectivity will play a critical role in optimizing this tradeoff.
Superconducting circuits are a leading platform for quantum computation and quantum simulation. Interactions are typically mediated by pairwise coupling elements between nearest neighbor qubits. The development of tunable couplers has been essential to achieving high fidelity entangling gates alongside high fidelity single qubit gates in quantum processors. In quantum simulators, tunable coupling allows flexibility in emulating different systems and the exploration of different parameter regimes of those models in a single physical device. Distributed element circuitry has enabled long range coupling and leveraging of interference effects and spatial mode variations to enhance device performance. In this thesis I will discuss work towards developing a distributed element tunable coupling circuit: a transmission line cavity terminated by SQUIDs, which serve as flux tunable impedances, at either boundary. With this device we hope to integrate the dynamic programmability of interactions mediated by a tunable coupler with the long-range, multi-qubit coupling enabled by distributed transmission lines in a single system. We experimentally demonstrate the core functionality, including tunability of the coupling rates, programmability of the connectivity graph, and parallel operation via multiplexing. Additionally, we utilize this device to explore photon blockade effects in the Tavis-Cummings Model, highlighting the value of tunability and controllability when simulating novel light-matter interactions in highly connected systems. These results contribute to ongoing research towards designing, calibrating, and operating highly connected superconducting circuit systems. Devices with high connectivity enable more efficient circuit compilation and are useful as quantum routers in quantum networks. In recent years, understanding the role of connectivity in quantum error correction has emerged as an important research focus. There is ongoing work evaluating the tradeoff between performance and connectivity in real devices. The development of coupling architectures that enable denser and/or longer range connectivity will play a critical role in optimizing this tradeoff.
Tunable couplings are a central requirement for scalable quantum processing with superconducting quantum circuits. Practical quantum processors must dynamically reconfigure their interaction landscape to accommodate different quantum operations, including idle, single-qubit gates, two-qubit gates, readout, and reset, while maintaining high coherence and suppressing spurious couplings. While widely used tunable interaction approaches rely on single-mode mediation, they are often constrained by nearest-neighbor connectivity, limited on-off ratios, and stringent requirements on qubit frequency allocation. This thesis develops and studies novel strategies for tailoring tunable interactions by moving beyond single-mode mediation, spanning multimode engineered interactions and modeless interaction schemes ("many to no modes"), while accounting for realistic constraints in superconducting hardware. Using a microwave metamaterial waveguide realized by coupled resonator arrays, I demonstrate tunable dissipative interactions that allow on-demand, fast, and high switching-ratio photon emission from a transmon used as a multi-level quantum emitter. This capability is leveraged to achieve deterministic generation of multidimensional photonic cluster states, as well as unconditional reset and leakage reduction of frequency-tunable superconducting qubits. Complementary to these multimode dissipative tunable interactions, this thesis also introduces architectures that realize tunable interactions mediated by many coherent modes or by no mediating modes. I discuss a long-range interaction scheme between superconducting dual-rail qubits mediated by spatially extended eigenmodes of a coupled-resonator array bus. Finally, I propose and analyze a novel modeless coupling architecture based on a SQUID coupler which provides intrinsic cross-Kerr interactions, enabling fast, hybridization-free CZ gates for far-detuned pairs, and discuss its implications for miniaturization of superconducting quantum processors.
The path toward practical superconducting quantum processors requires the integration of a large number of high-performance qubits. Modular architectures could offer a way to address the scaling limitations of monolithic designs by partitioning a large quantum processor into physically separated modules, or chiplets, linked through long-range interconnects. In this context, although fluxonium qubits have emerged as a compelling platform for quantum computing due to their long coherence times and high-fidelity gates, existing coupling schemes remain restricted to qubits in close proximity on a single chip. This limitation inherently precludes the long-range interconnects essential for modular integration. In this work, we propose a long-range tunable coupler designed to interconnect fluxonium qubits separated by more than one centimeter, thereby supporting the realization of modular fluxonium quantum processors. Under realistic assumptions, the proposed coupler has the potential to achieve inter-module two-qubit gate performance, specifically sub-100-ns gates with intrinsic errors below 10−4, comparable to that of intra-module (intra-chiplet) gates, while enabling modular integration with low quantum crosstalk, a key requirement for scalable systems. We further discuss the integration of this coupler into modular fluxonium lattices and explore its feasibility for achieving the higher connectivity and longer coupling range required for complex quantum error correction codes. This work could contribute to the development of large-scale fluxonium quantum processors by bridging their demonstrated potential with modular scalability.
The development of modular and versatile quantum interconnect hardware is a key next step in the scaling of quantum information platforms to larger size and greater functionality. For superconducting quantum systems, fast and well-controlled tunable circuit couplers will be paramount for achieving high-fidelity and resource-efficient connectivity, whether for performing two-qubit gate operations, encoding or decoding a quantum data bus, or interfacing across modalities. Here we propose a versatile and internally tunable double-transmon coupler (DTC) architecture that implements tunable coupling via flux-controlled interference in a three-junction dc superconducting quantum interference device. Crucially, the DTC possesses an internally defined zero-coupling state that is independent of the coupled data qubits or circuit resonators. This makes it particularly attractive as a modular and versatile design element for realizing fast and robust linear coupling in several applications such as high-fidelity two-qubit gate operations, qubit readout, and quantum bus interfacing.
Superconducting qubits have achieved remarkable progress in gate fidelity and coherence, yet their typical nearest-neighbor connectivity presents constraints for implementing complex quantum circuits. Here, we introduce a cavity-mediated coupling architecture in which a shared cavity mode, accessed through tunable qubit-cavity couplers, enables dynamically reconfigurable interactions between non-adjacent qubits. By selectively activating the couplers, we demonstrate that high-fidelity iSWAP and CZ gates can be performed within 50 ns with simulated coherent error below 10−4, while residual ZZ interaction during idling remains below a few kilohertz. Extending to a four-qubit system, we also simulate gates between every qubit pair by selectively enabling the couplers with low qubit crosstalk. This approach provides a practical route toward enhanced interaction flexibility in superconducting quantum processors and may serve as a useful building block for devices that benefit from selective non-local coupling.
This thesis explores novel strategies for advancing superconducting quantum computing through a series of experimental investigations and device innovations. Grounded in the development of circuit quantum electrodynamics, the work addresses the challenges posed by the limitations of coherent control, coherence time and scaling of superconducting qubits. Focusing on transmon qubits, the thesis first reviews the foundational principles of circuit quantum electrodynamics and the critical role of the transmon in achieving robust qubit performance. Three experimental studies form the core of this work. The first study demonstrates a universal quantum gate set for strongly coupled transmons. By exploiting the residual ZZ interaction, the work introduces a novel two-axis gate protocol for single-qubit operations and implements both CZ and CNOT gates with high fidelities, highlighting the potential for fast, low-error quantum operations in the strong coupling regime. The second study investigates a dual-rail qubit architecture using parametrically coupled transmons, showcasing a hardware-efficient approach to quantum error correction. This experiment converts single-photon loss errors into detectable erasures and leverages mid-circuit detection to enhance qubit coherence, thereby advancing fault-tolerant quantum computing schemes in the NISQ era. The final study focuses on the development of merged-element transmons (MET) and their FinMET variants. By integrating the Josephson junction and shunt capacitor into a single trilayer device, these innovations achieve a dramatic reduction in device footprint while addressing loss channels through advanced materials engineering, ultimately paving the way for scalable, high-coherence quantum circuits. Collectively, these contributions demonstrate significant progress in quantum gate design, error mitigation, and qubit architecture, offering promising routes toward the realization of largescale, fault-tolerant quantum computers
Quantum interactions are strikingly different from their classical counterparts, and they lead to counter-intuitive effects such as quantum entanglement. The advances in quantum science and technology over the past two decades have shown that such interactions can be harnessed in quantum devices to address hard computational problems. The field of quantum computation has emerged, and superconducting circuits have become a leading platform for quantum processors. Because of the large design space and the difficulty to simulate design prototypes on a classical computer, a key challenge in quantum processor engineering is to translate desired quantum characteristics into a circuit design. A particular obstacle has been the implementation of interactions between multiple quantum bits (qubits) in one coupling mechanism. Multi-qubit interactions are necessary for a number of applications in quantum annealing, analog quantum simulation, and digital quantum computation. In this work, we develop classical and quantum computational methods to address the challenge of implementing such interactions. We present a computational method for automated design of superconducting circuits. Our method performs a parallelized, closed-loop optimization to design superconducting circuit diagrams that match predefined properties, such as dispersion features and noise sensitivities. We employ it to design a coupler for superconducting flux qubits that mediates 4-qubit interactions. The coupler circuit is then realized experimentally. We find that it has the desired ground state dispersion properties versus flux and that it does not adversely affect the lifetime of a connected flux qubit. Moreover, the coupling concept can be extended by systematically adding loops to the coupler. In a second iteration, we implement a circuit architecture in which the coupler mediates interactions between three flux qubits. The system Hamiltonian is estimated via multi-qubit pulse sequences that implement Ramsey-type interferometry between all neighboring excitation manifolds. We verify that the multi-qubit system exhibits a 3-qubit coupling that is coherently tunable, which is important for practical applications. Finally, we address the challenge that complex multi-qubit circuits are difficult to simulate accurately on a classical computer as they become larger. As a solution, we propose quantum algorithms to design and test the performance of next-generation quantum hardware on a quantum computer. We have therefore addressed the challenge of quantum device design both from a classical optimization and from a quantum computational angle.