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Coherence limits in interference-based cos(2φ) qubits

S. Messelot1,*, A. Leblanc2,†, J.-S. Tettekpoe1, F. Lefloch2, Q. Ficheux1,‡, J. Renard1, and É. Dumur2,§

  • *Present address: Quobly, 38000 Grenoble, France.
  • †Present address: Center for Quantum Information Physics, Department of Physics, New York University, New York, NY 10003, USA.
  • ‡Contact author: quentin.ficheux@neel.cnrs.fr
  • §Contact author: etienne.dumur@cea.fr
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Phys. Rev. Applied 26, 044021 – Published 7 October, 2026

DOI: https://doi.org/10.1103/x4vn-51dj

Abstract

We investigate the coherence properties of parity-protected cos(2φ) qubits based on interference between two Josephson elements in a superconducting loop. We show that qubit implementations of a cos(2φ) potential using a single loop, such as those employing semiconducting junctions, rhombus circuits, Flowermon, and KITE structures, can be described by the same Hamiltonian as two multiharmonic Josephson junctions in a SQUID geometry. We find that despite the parity protection arising from the suppression of single Cooper-pair tunneling, there exists a fundamental trade-off between charge and flux-noise dephasing channels. Using numerical simulations, we examine how relaxation and dephasing rates depend on external flux and circuit parameters, and we identify the best compromise for maximum coherence. With currently existing circuit parameters, the qubit lifetime T1 can exceed milliseconds, while the dephasing time Tφ remains limited to only a few microseconds due to either flux or charge noise. Our findings establish practical limits on the coherence of this class of qubits and raise questions about the long-term potential of this approach.

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Phys. Rev. Applied 26, 044021– Published 7 October, 2026
  • Received 28 January 2026
  • Revised 30 April 2026
  • Accepted 10 June 2026
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