Qubit Zoo

FerBo: a noise resilient qubit hybridizing Andreev and fluxonium states

2 min read

The FerBo qubit is a proposed superconducting quantum circuit that achieves simultaneous protection against relaxation and dephasing by hybridizing the fermionic Andreev degree of freedom in a nanowire weak link with the bosonic mode of a high-impedance LC circuit. Unlike other dual-protected designs (0-π, cos(2φ), bifluxon) that require multiple bosonic modes, FerBo uses a single mode, opening a wider parameter window for experimental realization.

Abstract

We propose a novel superconducting quantum circuit that should be robust against both relaxation and dephasing over a wide and experimentally accessible parameter range. The circuit consists of a parallel arrangement of a large inductance, a small capacitor, and a well-transmitting Josephson weak link. Protection against relaxation arises from the hybridization between the fermionic degree of freedom associated with Andreev levels in the weak link and the bosonic electromagnetic mode of the LC circuit, hence its name: FerBo. Furthermore, as in the fluxonium qubit, delocalization of the wavefunctions in phase space provides resilience against dephasing.

Key Findings

  • Proposes the FerBo qubit: a light fluxonium with the tunnel junction replaced by a highly transmissive Andreev weak link
  • Qubit states |0⟩ and |1⟩ reside in different Andreev manifolds (|−⟩ and |+⟩), providing disjoint support that suppresses charge relaxation by ~4 orders of magnitude
  • Phase delocalization across multiple wells (as in fluxonium) simultaneously suppresses flux-noise dephasing
  • Protection boundary: Z/R_Q ≈ 2E_C/(πε_r), defining a sharp transition to the protected regime
  • Operates at zero external flux (φ_ext = 0), eliminating the need for precision flux biasing
  • First dual-protected qubit design using a single bosonic mode + fermionic Andreev degree of freedom (vs. multiple bosonic modes in 0-π, cos(2φ), bifluxon)
  • Proposed implementation: InAs/Al nanowire weak link + Josephson junction array superinductance

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