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Fast Gates for Bit-Flip Protected Superconducting Qubits

C. A. Siegele*, A. A. Sokolova, L. N. Kapoor, F. Hassani†, and J. M. Fink‡

  • *Contact author: christian.siegele@ista.ac.at
  • †Present address: Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
  • ‡Contact author: jfink@ista.ac.at
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PRX Quantum 7, 043008 – Published 14 October, 2026

DOI: https://doi.org/10.1103/1kmf-6fz9

Abstract

Superconducting qubits offer an unprecedentedly high degree of flexibility in terms of circuit encoding and parameter choices. However, in designing the qubit parameters, one typically faces the conflicting goals of long coherence times and simple control capabilities. Both are determined by the wavefunction overlap of the qubit basis states and the corresponding matrix elements. Here, we address this problem by introducing a qubit architecture with real-time tunable bit-flip protection. In the first, the “heavy” regime, the energy relaxation time can be on the order of hours for fluxons located in two near-degenerate ground states, as recently demonstrated in Ref. [Hassani et al., Nat. Commun. 14 (2023)]. The second, “light” regime, facilitates high-fidelity control on nanosecond timescales without the need for microwave signals. We propose two different tuning mechanisms of the qubit potential and show that base-band flux-pulses of around 10 ns are sufficient to realize a universal set of high-fidelity single- and two-qubit gates. This realizes a hardware-efficient and strongly noise-biased qubit platform with analogies to cat qubits with the potential for efficient quantum error correction. We expect that the introduced concept of fast and real-time wavefunction control can also be applied to other hardware-protected qubit designs.

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Popular Summary

Fluxon states in superconducting circuits can reach lifetimes of hours; however, coherent control is very challenging in this protected regime. In this article, we propose to encode a qubit in this fluxon subspace, in which gates are performed by temporarily breaking the underlying Hamiltonian symmetries. Physically, this can be realized by tuning the magnetic flux through the superconducting loops with base-band pulses. Our numerical results show that high-fidelity single- and two-qubit gates can be performed within a few nanoseconds, while bit-flip times are estimated to reach a few seconds and phase-flip times a few tens of microseconds in the idle regime. This strong inherent noise bias makes them promising candidates for fault-tolerant ancilla qubits and enables a hardware-efficient quantum error correction platform.

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PRX Quantum 7, 043008– Published 14 October, 2026
  • Received 14 April 2025
  • Revised 16 January 2026
  • Accepted 31 July 2026
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