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What Is the Maximum Density of Microwave Control Lines in a Superconducting Quantum Computer?

Nikolas Klemola Tango*, Mar Francis De Guzman, Slawomir Simbierowicz, Yuri Henriqson Accordi, Ville Nuutinen, Massimo Borrelli†, and Russell E. Lake

  • *Present address: Aalto University School of Science, P.O. Box 15400, FI-00076 AALTO, Finland.
  • †Contact author: Massimo.Borrelli@bluefors.com
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Phys. Rev. Lett. 137, 110202 – Published 9 September, 2026

DOI: https://doi.org/10.1103/m9p9-z6hl

Abstract

Scaling up superconducting quantum computers requires denser control wiring, introducing microwave crosstalk along the entire cryogenic signal path rather than only near the chip. Here, we introduce a framework that combines microwave transmission-line theory with a quantum Hamiltonian description to quantify the wiring-induced microwave crosstalk and the resulting gate infidelity for an all-microwave universal gate set. Using this approach, we determine the equal-level far-end crosstalk (ELFEXT) required to operate fixed-frequency transmons at arbitrary target fidelities and find that achieving 99.99% fidelity requires ELFEXT levels of approximately −43  dB for the RX gate and −73  dB for the cross-resonance gate. Finally, we use these results to bound the number of physical qubits on a chip.

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Phys. Rev. Lett. 137, 110202– Published 9 September, 2026
  • Received 19 December 2025
  • Accepted 16 July 2026
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