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  • Open Access

Protected Phase Gate for the 0-π Qubit using its Internal Modes

Xanda C. Kolesnikow*, Thomas B. Smith, Felix Thomsen, Abhijeet Alase, and Andrew C. Doherty

  • Centre for Engineered Quantum Systems, School of Physics, University of Sydney, Sydney, NSW 2006, Australia

  • *Contact author: xkol5336@uni.sydney.edu.au
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PRX Quantum 7, 010306 – Published 12 January, 2026

DOI: https://doi.org/10.1103/bywc-cx98

Abstract

Protected superconducting qubits such as the 0-π qubit promise to substantially reduce physical error rates. However, a key challenge in the field is designing gates for these qubits that do not compromise their protection or become infeasibly slow as the protection of the qubit is improved. In this work we propose a protected phase gate that is compatible with the protected regime of the 0-π qubit and does not suffer from spurious coupling to additional circuit modes. Our gate utilizes an internal mode of the circuit as an ancilla and is achieved by varying the qubit-ancilla coupling via a tunable Josephson element. Through numerical simulations, we study how the gate error scales with the circuit parameters of the 0-π qubit and the tunable Josephson element that enacts the gate. Ultimately, we find that a protected gate with the 0-π qubit is possible with near-term circuit parameters. Our work opens up the possibility of performing protected gates on protected superconducting qubits, which may significantly reduce hardware overheads for quantum computation.

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Physics Subject Headings (PhySH)

Popular Summary

Quantum computers promise to solve interesting problems that can’t be solved on current classical computers. However, making a quantum computer at scale is challenging due to the way quantum systems naturally interact with their environment, leading to errors in the computation. At the same time, we must interact with these quantum systems in order to perform a quantum computation. In our work, we design a way to interact with superconducting qubits, a type of quantum system, while ensuring they do not interact with the environment in a way that leads to errors. This will allow quantum algorithms to be run with far fewer errors.

We achieved this protected interaction, known as a quantum gate, by using an extra degree of freedom in a type of superconducting qubit called the 0-π qubit. This extra degree of freedom can be manipulated in such a way that the information stored in the qubit remains protected while the quantum gate is occurring. Through numerical simulations, we find out what the experimental requirements are to achieve a certain error rate.

Looking forward, our scheme could be extended to other types of gates as well as other types of superconducting qubits.

Article Text

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Outline

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PRX Quantum 7, 010306– Published 12 January, 2026
  • Received 5 May 2025
  • Revised 2 November 2025
  • Accepted 9 December 2025
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

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