Scalable Low-overhead Superconducting Non-local Coupler with Exponentially Enhanced Connectivity

TL;DR

This paper demonstrates a scalable superconducting qubit coupler with exponentially improved connectivity using a binary-tree mapping, enabling high-fidelity non-local entanglement and advancing quantum error correction implementation.

Contribution

It introduces a novel on-chip coupler and a binary-tree mapping layout that significantly enhances connectivity in superconducting qubit systems.

Findings

Achieved 99.37% fidelity in entangling gates.

Reduced average entangling distance from O(N) to O(logN).

Maintained low ZZ rate of 144 Hz without active cancellation.

Abstract

Quantum error correction codes with non-local connections such as quantum low-density parity-check (qLDPC) incur lower overhead and outperform surface codes on large-scale devices. These codes are not applicable on current superconducting devices with nearest-neighbor connections. To rectify the deficiency in connectivity of superconducting circuit system, we experimentally demonstrate a convenient on-chip coupler of centimeters long and propose an extra coupler layer to map the qubit array to a binary-tree connecting graph. This mapping layout reduces the average qubit entangling distance from O(N) to O(logN), demonstrating an exponentially enhanced connectivity with eliminated crosstalk. The entangling gate with the coupler is performed between two fluxonium qubits, reaching a fidelity of 99.37 % while the system static ZZ rate remains as low as 144 Hz without active cancellation or circuit parameter targeting. With the scalable binary tree structure and high-fidelity non-local entanglement, novel quantum algorithms can be implemented on the superconducting qubit system, positioning it as a strong competitor to other physics systems regarding circuit connectivity.