Abstract Resonators in circuit quantum electrodynamics systems naturally carry multiple modes, which may have non-negligible influence on qubit parameters and device performance. While new theories and techniques are under investigation to deal with the multi-mode effects in circuit quantum electrodynamics systems, novel engineering designs are also proposed featuring multi-mode resonators to achieve enhanced functionalities of superconducting quantum processors. In this work, multi-mode bus coupling architecture is proposed, in which superconducting qubits are coupled to multiple bus resonators to gain larger coupling strength. Application of multi-mode bus coupler can help improve iSWAP gate fidelity and gate speed beyond the limit of single-mode scenario. The proposed multi-mode bus coupling architecture is compatible with a scalable variation of the traditional bus coupling architecture. It opens up new possibilities for realization of scalable superconducting quantum computation with circuit quantum electrodynamics systems.
Abstract Resonators in circuit quantum electrodynamics systems naturally carry multiple modes, which may have non-negligible influence on qubit parameters and device performance. While new theories and techniques are under investigation to deal with the multi-mode effects in circuit quantum electrodynamics systems, novel engineering designs are also proposed featuring multi-mode resonators to achieve enhanced functionalities of superconducting quantum processors. In this work, multi-mode bus coupling architecture is proposed, in which superconducting qubits are coupled to multiple bus resonators to gain larger coupling strength. Application of multi-mode bus coupler can help improve iSWAP gate fidelity and gate speed beyond the limit of single-mode scenario. The proposed multi-mode bus coupling architecture is compatible with a scalable variation of the traditional bus coupling architecture. It opens up new possibilities for realization of scalable superconducting quantum computation with circuit quantum electrodynamics systems.
Resonators in circuit quantum electrodynamics systems naturally carry multiple modes, which may have non-negligible influence on qubit parameters and device performance. While new theories and techniques are under investigation to deal with the multi-mode effects in circuit quantum electrodynamics systems, researchers have proposed novel engineering designs featuring multi-mode resonators to achieve enhanced functionalities of superconducting quantum processors. Here, we propose multi-mode bus coupling architecture, in which superconducting qubits are coupled to multiple bus resonators to gain larger coupling strength. Applications of multi-mode bus couplers can be helpful for improving iSWAP gate fidelity and gate speed beyond the limit of single-mode scenario. The proposed multi-mode bus coupling architecture is compatible with a scalable variation of the traditional bus coupling architecture. It opens up new possibilities for realization of scalable superconducting quantum computation with circuit quantum electrodynamics systems.
Superconducting circuit quantum electrodynamics is the main architecture for implementation of quantum processors. This work presents a proposal to miniaturize quantum memory devices by combining the quantum memory qubits encoded directly in the first two Fock states ${|0\ensuremath{\rangle},|1\ensuremath{\rangle}}$ and the bus channel in the same specially designed multimode cavity. One key feature of the resonant modes used for quantum memory is that the microwave current is localized, which facilitates the storage and retrieval of information in superconducting qubits while preventing crosstalk from other resonant modes. Frequency design and distinct coupling strengths in this kind of device are analyzed with electromagnetic simulations and numerical calculations. The quantum memory properties, e.g., storage speed, fidelity, and stability, are examined in four cases: (1) memory operation of single-qubit state, (2) disturbing effects, (3) coupling operation, and (4) entangled states storage. Decoherence time of the quantum processor can be extended to more than one order of magnitude longer than that without the memory. By integrating transmons with optimally designed coplanar waveguide multimode quantum memories, the present study could provide an optimistic perspective for the development of large-scale superconducting quantum processors.
Qubit coherence and gate fidelity are typically considered the two most useful metrics for characterizing a quantum processor. An equally useful metric is interqubit connectivity as it minimizes gate count and allows implementing algorithms efficiently with reduced error. However, interqubit connectivity in superconducting processors tends to be limited to nearest neighbor due to practical constraints in typical planar realizations. Here, we introduce a superconducting architecture that uses a ring resonator as a multiqubit coupling element to provide beyond nearest-neighbor connectivity without compromising on coupling uniformity or introducing fabrication complexities. We theoretically analyze the interqubit coupling as a function of frequency for a pair of qubits placed at different positions along the ring resonator and show that for carefully chosen operating frequency and angular spacing between the qubits, the variation of coupling can be minimized. For an operating frequency between the first two resonances of the ring resonator and a ${30}^{\ensuremath{\circ}}$ angular spacing for qubits, we compute interqubit coupling for a device capable of supporting up to 12 qubits with each qubit connected to nine other qubits. Using four qubits positioned strategically in the ring-resonator coupler, we experimentally verify the theoretical prediction for all possible angular spacings between the two qubits and demonstrate good agreement. Just like the standard bus resonator coupler, the coupling in the ring-resonator coupler is mediated via virtual photons since the operating frequency is far away from the resonant modes of the ring coupler. This ensures that any small internal loss in the ring resonator does not introduce decoherence during the coupling operation. We also compute extensions of this idea involving larger ring resonators and a multiring system and show the possibility of highly connected networks with larger number of qubits. Apart from being plug and play for existing superconducting architectures, our concept is scalable, adaptable to other platforms and has the potential to significantly accelerate progress in quantum computing, annealing, simulations, and error correction.
In addition to striving to assemble more and more qubits in a single monolithic quantum device, taking a modular design strategy may mitigate numerous engineering challenges for achieving large-scalable quantum processors with superconducting qubits. Nevertheless, a major challenge in the modular quantum device is how to realize high-fidelity entanglement operations on qubits housed in different modules while preserving the desired isolation between modules. In this work, we propose a conceptual design of a modular quantum device, where nearby modules are spatially separated by centimeters. In principle, each module can contain tens of superconducting qubits and can be separately fabricated, characterized, packaged, and replaced. By introducing a bridge module between nearby qubit modules and taking the coupling scheme utilizing a tunable bus, tunable coupling of qubits that are housed in nearby qubit modules could be realized. Given physically reasonable assumptions, we expect that sub-100-ns two-qubit gates for qubits housed in nearby modules, which are spatially separated by more than two centimeters could be obtained. In this way, the inter-module gate operations are promising to be implemented with gate performance comparable with that of intra-module gate operations. Moreover, with the help of through-silicon vias technologies, this long-range coupling scheme may also allow one to implement inter-module couplers in a multi-chip stacked processor. Thus, the tunable longer-range coupling scheme and the proposed modular architecture may provide a promising foundation for solving challenges toward large-scale quantum information processing with superconducting qubits.
We propose a one-step scheme for implementing multi-qubit phase gates on microwave photons in multiple resonators mediated by a superconducting bus in circuit quantum electrodynamics (QED) system. In the scheme, multiple single-mode resonators carry quantum information with their vacuum and single-photon Fock states, and a multi-level artificial atom acts as a quantum bus which induces the indirect interaction among resonators. The method of pulse engineering is used to shape the coupling strength between resonators and the bus so as to improve the fidelity and robustness of the scheme. We also discuss the influence of finite coherence time for the bus and resonators on gate fidelity respectively. Finally, we consider the suppression of unwanted transitions and propose the method of optimized detuning compensation for offsetting unwanted transitions, showing the feasibility of the scheme within the current experiment technology.
Quantum computing holds great promise for both scientific discovery and commercial applications. Superconducting quantum systems have become one of the leading architectures for large-scale quantum computing. Towards realizing practical quantum computing, improving two-qubit gate fidelity is a crucial task. It is relevant for both near-term pre-fault-tolerant quantum computers and future fault-tolerant quantum computers. This dissertation aims to provide a comprehensive study of two-qubit gates in superconducting systems. The major focus is on improving gate fidelity via using different drive schemes and coupler designs. The dissertation contains two primary original contributions. First, a novel method for implementing continuous fSim gates is proposed and analyzed in detail. The fSim gate set is a family of gates that combines iSWAP gate and CPHASE gate into one gate with variable rotation angles and conditional phase. This gate set is particularly interesting for quantum simulation of fermionic systems. Second, a theoretical framework for multi-mode couplers is developed. A multi-mode coupler design is proposed to enable fully localized decoupling among qubits and independent control of qubit-qubit interactions in different excitation subspaces. An example of two qubits coupled by a two-mode coupler is studied in detail. Building on the fSim gate theory and multi-mode coupler design, several future research directions are outlined. Pulse-level algorithms for simulating electronic systems are discussed as a key application of fSim gates. A three-mode coupler architecture is further introduced. In addition, the importance of incorporating open-system dynamics for accurate modeling of coupler behavior and gate performance is emphasized. Finally, a unified framework for modeling superconducting quantum processors is proposed, providing a systematic way to compare gate fidelities across theoretical approaches and ultimately guide improvements in quantum hardware performance.