Long-distance transmon coupler with CZ gate fidelity above $99.8\%$

TL;DR

This paper presents a scalable, long-distance tunable coupler for superconducting qubits that enables high-fidelity CZ gates above 99.8% by reducing crosstalk and allowing flexible qubit placement.

Contribution

The work introduces a novel floating transmon-based tunable coupler design that maintains strong coupling over 2 mm distance, improving scalability and gate fidelity in quantum processors.

Findings

Achieved CZ gate fidelity of 99.81%

Enabled qubit placement at least 2 mm apart

Reduced non-nearest neighbor coupling and crosstalk

Abstract

Tunable coupling of superconducting qubits has been widely studied due to its importance for isolated gate operations in scalable quantum processor architectures. Here, we demonstrate a tunable qubit-qubit coupler based on a floating transmon device which allows us to place qubits at least 2 mm apart from each other while maintaining over 50 MHz coupling between the coupler and the qubits. In the introduced tunable-coupler design, both the qubit-qubit and the qubit-coupler couplings are mediated by two waveguides instead of relying on direct capacitive couplings between the components, reducing the impact of the qubit-qubit distance on the couplings. This leaves space for each qubit to have an individual readout resonator and a Purcell filter needed for fast high-fidelity readout. In addition, the large qubit-qubit distance reduces unwanted non-nearest neighbor coupling and allows multiple control lines to cross over the structure with minimal crosstalk. Using the proposed flexible and scalable architecture, we demonstrate a controlled-$Z$ gate with $(99.81 \pm 0.02)\%$ fidelity.