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arXiv:2012.09463 · Physical Review Applied

Ring-Resonator-Based Coupling Architecture for Enhanced Connectivity in a Superconducting Multiqubit Network

Sumeru Hazra, Anirban Bhattacharjee, Madhavi Chand, Kishor V. Salunkhe, Sriram Gopalakrishnan, Meghan P. Patankar, Rajamani Vijay

Published Aug 11, 2021• 37 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1103/physrevapplied.16.024018

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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∘{30}^{\ensuremath{\circ}}30\ensuremath∘ 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.
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We report entangling two-qubit experiments implemented in a novel ring resonator architecture in 2D planar geometry. The ring resonator acts as a multi-path coupler between qubits and can provide beyond nearest neighbour interactions. We demonstrate pairwise coupling between three fixed-frequency transmon qubits connected to the ring resonator with measured coupling strengths (4.70 MHz, 2.80 MHz, and 2.65 MHz) in good agreement with those predicted from finite-element simulations. We implement an all-microwave controlled phase (CPHASE) gate between a pair of qubits with a gate time of 196 ns and demonstrate a two-qubit Bell state with a measured state fidelity of [Formula: see text]. Our results demonstrate the ability to entangle two qubits using the ring resonator and pave the way for creating highly connected multi-qubit networks in this architecture.
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.
As systems for quantum computing keep growing in size and number of qubits, challenges in scaling the control capabilities are becoming increasingly relevant. Efficient schemes to simultaneously mediate coherent interactions between multiple quantum systems and to reduce decoherence errors can minimize the control overhead in next-generation quantum processors. Here, we present a superconducting qubit architecture based on tunable parametric interactions to perform two-qubit gates, reset, leakage recovery, and to read out the qubits. In this architecture, parametrically driven multielement couplers selectively couple qubits to resonators and neighboring qubits, according to the frequency of the drive. We consider a system with two qubits and one readout resonator interacting via a single coupling circuit and experimentally demonstrate a controlled- Z gate with a fidelity of 98.30 ± 0.23 % , a reset operation that unconditionally prepares the qubit ground state with a fidelity of 99.80 ± 0.02 % and a leakage recovery operation with a 98.5 ± 0.3 % success probability. Furthermore, we implement a parametric readout with a single-shot assignment fidelity of 88.0 ± 0.4 % . These operations are all realized using a single tunable coupler and a statically decoupled resonator, demonstrating the experimental feasibility of the proposed architecture and its potential for reducing the system complexity in scalable quantum processors.
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.