Modular Superconducting Qubit Architecture with a Multi-chip Tunable Coupler

TL;DR

This paper presents a modular superconducting qubit architecture using a floating tunable coupler to connect qubits on separate chips, demonstrating high-fidelity two-qubit gates without degrading qubit coherence.

Contribution

It introduces three designs of multi-chip tunable couplers with vacuum gap capacitors or indium bump bonds, enabling modularity without sacrificing performance.

Findings

Zero-coupling condition achieved between chips

Qubit relaxation rates unaffected by extra circuit elements

Two-qubit gate fidelity comparable to single-chip implementations

Abstract

We use a floating tunable coupler to mediate interactions between qubits on separate chips to build a modular architecture. We demonstrate three different designs of multi-chip tunable couplers using vacuum gap capacitors or superconducting indium bump bonds to connect the coupler to a microwave line on a common substrate and then connect to the qubit on the next chip. We show that the zero-coupling condition between qubits on separate chips can be achieved in each design and that the relaxation rates for the coupler and qubits are not noticeably affected by the extra circuit elements. Finally, we demonstrate two-qubit gate operations with fidelity at the same level as qubits with a tunable coupler on a single chip. Using one or more indium bonds does not degrade qubit coherence or impact the performance of two-qubit gates.