Tunable Nonlocal ZZ Interaction for Remote Controlled-Z Gates Between Distributed Fixed-Frequency Qubits

TL;DR

This paper introduces a distributed superconducting qubit architecture using double-transmon couplers to enable high-fidelity remote controlled-Z gates over a 25 cm cable, addressing scalability challenges.

Contribution

It proposes a novel hardware scheme with synchronized couplers to suppress static coupling and activate non-local interactions, achieving over 99.99% fidelity for remote CZ gates.

Findings

Achieves an on/off ratio exceeding 10^6 for non-local interactions.

Demonstrates a remote CZ gate fidelity over 99.99% in simulations.

Uses realistic hardware parameters for system-level validation.

Abstract

Fault-tolerant quantum computing requires large-scale superconducting processors, yet monolithic architectures face increasing constraints from wiring density, crosstalk, and fabrication yield. Modular superconducting platforms offer a scalable alternative, but achieving high-fidelity entangling gates between distant modules remains a central challenge, particularly for highly coherent fixed-frequency qubits. Here, we propose a distributed hardware architecture designed to overcome this bottleneck by employing a pair of double-transmon couplers (DTCs). By synchronously controlling the two DTCs stationed at opposite ends of a macroscopic cable, our scheme strongly suppresses residual static inter-module coupling while enabling on-demand activation of a non-local cross-Kerr interaction with an on/off ratio exceeding $10^6$. Through comprehensive system-level numerical simulations incorporating realistic hardware parameters, we demonstrate that this mechanism can realize a remote controlled-Z (CZ) gate with a fidelity over 99.99\% between fixed-frequency transmons housed in separate packages interconnected by a 25 cm coaxial cable. These results establish a highly viable, hardware-efficient route toward high-performance distributed superconducting processors.