A Multi-Resonator Architecture for Long-Range Coupling of Superconducting Qubits

Flux tunable couplers are the dominant architecture for two-qubit gate implementation in superconducting quantum processors with state-of-the-art gate fidelities exceeding 99.9% in 50 ns [1]. A limitation of these coupler designs is that qubit-qubit separation is limited to several millimeters. This is in part because larger coupling circuits have low-frequency modes that can be thermally populated- decohering the coupled qubits. Short-ranged couplers heavily restrict the achievable connectivity of superconducting quantum processors, which in turn limits the style and ultimately hardware efficiency of the error correction that can be implemented. In addition, future quantum computers will likely need to be distributed across multiple chiplets, which may pose a difficult engineering challenge for traditional single-mode couplers. In this work, we investigate a coupling element made of a series of strongly coupled resonant CPW resonators. The lowest-frequency mode of this element is limited by the coupling strength between the resonators, which can be chosen to avoid thermal population. At the same time, the hybridized modes of the series resonators maintain significant dispersive coupling to each of the coupled qubits with strength inversely proportional to number of resonators . We use this series-resonator coupler to implement a microwave-activated two-qubit gate. With this architecture, gates between transmon qubits spaced multiple centimeters apart should be achievable.

[1] L. Ding, M. Hays, et al Phys Rev. X 13 031035 (2023)