Breaking the scalability barrier via a vertical tunable coupler in 3D integrated transmon system

TL;DR

This paper presents a 3D integrated superconducting quantum processor with vertical tunable couplers, enabling scalable interchip entanglement and high-fidelity quantum operations.

Contribution

Introduction of a vertically integrated 3D superconducting quantum processor with tunable couplers for scalable quantum computing.

Findings

Single-qubit gate fidelity of 99.87% with negligible crosstalk

Controlled-Z gate fidelity of 97.5% for intrachip and interchip operations

Successful generation of a four-qubit W state demonstrating interchip entanglement

Abstract

Scaling superconducting quantum processors beyond the constraints of monolithic planar architectures is essential for fault-tolerant quantum computation. Here we demonstrate a three-dimensional (3D) integrated superconducting quantum processor in which two qubit chips are vertically stacked on opposing sides of a carrier chip and galvanically connected via multilayer flip-chip bonding. Intrachip qubit coupling is mediated by planar tunable couplers, whereas interchip coupling is enabled by vertical tunable couplers embedded in the carrier chip. Randomized benchmarking reveals simultaneous single-qubit gate fidelities of 99.87 % with negligible crosstalk, and controlled-Z gates achieve an average fidelity of 97.5 % for both intrachip and interchip operations. We further demonstrate high-fidelity Bell-state preparation and coherent generation of a four-qubit $W$ state, confirming the architecture's capability for interchip entanglement distribution. These results establish vertical coupling as a promising pathway toward scalable quantum processors compatible with advanced quantum error-correcting codes.