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  • Featured in Physics
  • Open Access

Resource-Efficient Cross-Platform Verification with Modular Superconducting Devices

Kieran Dalton1,2,3,*, Johannes Knörzer1,2, Finn Hoehne1, Yongxin Song1,2,3, Alexander Flasby1,3, Dante Colao Zanuz1,2, Mohsen Bahrami Panah1,3, Ilya Besedin1,2,3, Jean-Claude Besse1,2,3 et al.

Andreas Wallraff1,2,3,†

  • *Contact author: kieran.dalton@phys.ethz.ch
  • †Contact author: andreas.wallraff@phys.ethz.ch
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PRX Quantum 6, 040365 – Published 16 December, 2025

DOI: https://doi.org/10.1103/czph-xpzs

Abstract

Large-scale quantum computers are expected to benefit from modular architectures. Validating the capabilities of modular devices requires benchmarking strategies that assess performance within and between modules. In this work, we evaluate cross-platform verification protocols, which are critical for quantifying how accurately different modules prepare the same quantum state—a key requirement for modular scalability and system-wide consistency. We demonstrate these algorithms using a six-qubit flip-chip superconducting quantum device consisting of two 3-qubit modules on a single carrier chip, with connectivity for intra- and intermodule entanglement. We examine how the resource requirements of protocols relying solely on classical communication between modules scale exponentially with qubit number, and demonstrate that introducing an intermodule two-qubit gate enables subexponential scaling in cross-platform verification. This approach reduces the number of repetitions required by a factor of 4 for three-qubit states, with greater reductions projected for larger and higher-fidelity devices.

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synopsis

Verifying a Quantum State More Efficiently

Published 16 December, 2025

A new method for checking the reliability of a quantum computer can be scaled up to devices of any size.

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Popular Summary

Let us say that you have two 100-qubit quantum computers and want to test that they both work. There are many ways to do this on a small scale, at the one-qubit or two-qubit level. This typically involves making sure single- and two-qubit gates work as we expect by applying them many times and measuring the proximity of the resulting state to the expected state in the absence of errors.

However, some errors, such as crosstalk, only appear when we try to run computations involving more than two qubits. On a larger scale, a holistic test is to check that the two computers can produce the same state with high fidelity: distributed inner-product estimation. If you restrict yourself to classical communication between the quantum computers, for a repetition period of 1 μs, it will take you longer than the lifetime of the Universe to verify an arbitrary 100-qubit state. The time scales exponentially with qubit number. However, if you allow the computers to be connected with a quantum link, the time is constant (in theory), no matter the qubit number.

In our paper, we experimentally test this. We build on our prior demonstration of an intermodule two-qubit gate by designing and fabricating a six-qubit modular device, which includes both intermodule and intramodule gates. We use this device to compare the measured inner products derived from protocols relying on classical communication and protocols relying on a quantum link. We also check the resource scaling of these protocols. We show that, even for two 3-qubit modules, using a quantum link can reduce the number of experimental repetitions needed by a factor of 4. We project greater improvements with larger devices and lower error rates.

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PRX Quantum 6, 040365– Published 16 December, 2025
  • Received 29 July 2025
  • Revised 7 October 2025
  • Accepted 3 November 2025
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