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Above 99.9% Fidelity Single-Qubit Gates, Two-Qubit Gates, and Readout in a Single Superconducting Quantum Device

Fabian Marxer1,*,†, Jakub Mrożek1,*,‡, Joona Andersson1, Leonid Abdurakhimov1, Janos Adam1, Ville Bergholm1, Rohit Beriwal1, Chun Fai Chan1, Saga Dahl1 et al.

Soumya Ranjan Das2, Frank Deppe2, Olexiy Fedorets1, Zheming Gao1, Alejandro Gomez Frieiro1, Daria Gusenkova2, Andrew Guthrie1, Tuukka Hiltunen1, Hao Hsu2, Eric Hyyppä1, Joni Ikonen1, Sinan Inel1, Shan W. Jolin1, Azad Karis1, Seung-Goo Kim1, William Kindel2, Anton Komlev1, Miikka Koistinen1, Roope Kokkoniemi1, Snigdha Kumar1, Hsiang-Sheng Ku2, Julia Lamprich2, Sami Laine1, Alessandro Landra1, Lan-Hsuan Lee1, Nizar Lethif2, Per Liebermann1, Wei Liu1, Kunal Mitra1, Tuomas Mylläri1, Caspar Ockeloen-Korppi1, Tuure Orell1, Alexander Plyshch1, Jukka Räbinä1, Arthur Rebello1, Michael Renger2, Outi Reentilä1, Jussi Ritvas1, Sampo Saarinen1, Otto Salmenkivi1, Matthew Sarsby1, Mykhailo Savytskyi1, Ville Selinmaa1, Matthew Steggles1, Eelis Takala1, Ivan Takmakov1, Brian Tarasinski1, Jani Tuorila1, Alpo Välimaa1, Jeroen Verjauw2, Jaap Wesdorp1, Nicola Wurz2, Wei Qiu1, Lihuang Zhu1, Juha Hassel1, Johannes Heinsoo1, Attila Geresdi2, and Antti Vepsäläinen1,§

  • *These two authors contributed equally.
  • †Contact author: fabian@meetiqm.com
  • ‡Contact author: jakub@meetiqm.com
  • §Contact author: avepsalainen@meetiqm.com
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PRX Quantum 7, 020333 – Published 20 May, 2026

DOI: https://doi.org/10.1103/n86s-2b88

Abstract

Achieving high-fidelity single-qubit gates, two-qubit gates, and qubit readout is critical for building scalable, error-corrected quantum computers. However, device parameters that enhance one operation often degrade the others, making simultaneous optimization challenging. Here, we demonstrate that careful tuning of qubit-coupler coupling strengths in a superconducting circuit with two transmon qubits coupled via a tunable coupler enables high-fidelity single- and two-qubit gates, without compromising readout performance. Furthermore, we introduce a new calibration protocol for diabatic controlled-Z gates, phased-averaged leakage error amplification (PALEA), which enables efficient suppression of coherent gate errors and leakage to noncomputational states. Using PALEA in combination with careful tuning of qubit-coupler coupling strengths, we achieve a 40 h-averaged CZ gate fidelity of 99.93 %, simultaneous single-qubit gate fidelities of 99.98 %, and readout fidelities over 99.94 % in a single device. Our results demonstrate a viable path toward scaling up superconducting quantum processors while maintaining consistently high gate and readout fidelities.

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Physics Subject Headings (PhySH)

Popular Summary

Quantum computers promise to solve problems that are far beyond the reach of today’s classical machines. To reach that goal, however, the basic building blocks of quantum processors—single-qubit gates, two-qubit gates, and qubit readout—must operate with extremely high accuracy. Even small imperfections accumulate rapidly when running large quantum algorithms.

In this work, we demonstrate a complete quantum computing platform in which all key operations surpass the demanding 99.9% fidelity milestone: single-qubit rotations, controlled-Z two-qubit gates, and fast qubit readout. Achieving such performance simultaneously for every type of operation represents a crucial step toward making large-scale quantum computation practical. These advances rely on two key ingredients. First, we use an optimized device design, carefully tuning the qubit-coupler couplings to maximize the combined performance of single- and two-qubit gates while preserving high-fidelity readout. Second, we develop advanced calibration tools that minimize energy leakage to unwanted states during two-qubit operations—an error mechanism that poses a major challenge for quantum error correction.

The results show that it is now possible to build superconducting qubit processors where errors are reduced to levels needed for scalable fault-tolerant quantum computing. Beyond the specific architecture studied here, the methods we developed—both for improving fidelities and for diagnosing errors—can be readily applied to a wide range of quantum computing platforms.

Article Text

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Outline

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PRX Quantum 7, 020333– Published 20 May, 2026
  • Received 4 September 2025
  • Revised 3 February 2026
  • Accepted 26 March 2026
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