Physics / mechanism
Bosonic qubits encode quantum information in the continuous degrees of freedom of a harmonic oscillator—typically a superconducting microwave cavity—rather than a two-level spin. Cat qubits specifically exploit superpositions of coherent states (|α⟩ ± |−α⟩), where the code space lives in a degenerate subspace protected against single-photon-loss errors by design. The key mechanism is parametric two-photon driving (Kerr-cat or dissipative stabilisation), which suppresses bit-flip errors exponentially in |α|² while phase-flip errors grow only linearly—enabling biased-noise qubits. Current state of the art: Alice & Bob report bit-flip suppression >10⁵× at moderate mean photon number (~4–6), with T₂ phase-flip times ~1–10 µs. AWS/CQC (Quantinuum collaboration) and Yale groups are pursuing similar architectures. Fabrication relies on high-Q 3D or planar aluminium/niobium cavities at millikelvin temperatures.
Competitive landscape
The primary competition is transmon-based superconducting qubits (IBM, Google), which are mature but require full active error correction on both error axes. Trapped-ion qubits offer high gate fidelity (>99.9%) but slow clock speeds. Photonic qubits (PsiQuantum, QuiX) exploit low decoherence but face deterministic gate challenges. Neutral atoms (Atom Computing, QuEra) offer scalable connectivity. Cat qubits sit between: hardware-level noise bias reduces classical control overhead substantially versus transmons, but require specialised cavity hardware not yet in foundry-ready processes.
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Frontier (open questions)
- To be added.