Definition
A cat qubit is a type of superconducting quantum bit (qubit) that encodes quantum information in a superposition of two coherent states of a quantum harmonic oscillator — analogous to Erwin Schrödinger’s famous thought experiment of a cat in a superposition of alive and dead states (hence the name). Unlike conventional transmon qubits used in most superconducting quantum computers (IBM, Google), cat qubits exploit an ingenious design property: they are inherently protected against one of the two fundamental quantum error types (bit-flip errors) by their encoding scheme, while remaining sensitive to the other type (phase-flip errors). This asymmetric error bias dramatically reduces the overhead required for fault-tolerant quantum error correction — a cat qubit-based quantum processor potentially requires far fewer physical qubits per logical qubit than transmon-based approaches, offering a potentially faster path to practical fault-tolerant quantum computation.
Role in France 2030
Cat qubits are France’s most technically distinctive contribution to the global quantum computing race, and Alice & Bob — a Paris-based quantum hardware startup founded in 2020 by researchers from ENS Paris, Inria, and the Paris quantum research ecosystem — is the primary France 2030 company developing cat qubit technology. Alice & Bob has raised €30M+ in funding with Bpifrance participation, and is one of France’s National Quantum Strategy flagship companies. The company’s technical approach — which emerged from foundational research at ENS Paris and the laboratory of theoretical physicist Mazyar Mirrahimi — is considered one of the most theoretically compelling approaches to the fault-tolerant quantum computing problem by leading quantum computing researchers.
France’s National Quantum Strategy, announced in January 2021 with €1.8 billion in funding (of which approximately €800 million comes under France 2030), explicitly targets the development of two complementary quantum hardware approaches: neutral atom quantum processors (led by Pasqal in Massy) and superconducting qubit processors including the cat qubit approach (led by Alice & Bob). This dual-track strategy reflects a deliberate policy decision not to bet on a single qubit technology while the field remains uncertain — maintaining competitive positions in two of the most promising fault-tolerant quantum computing pathways.
The cat qubit’s connection to practical quantum computing timelines is significant. Conventional quantum computers based on transmon qubits (IBM, Google) are currently operating in the “NISQ era” (Noisy Intermediate-Scale Quantum): they can manipulate 100-1,000+ physical qubits but cannot perform error correction at scale, limiting them to short circuits and narrow problem classes. Fault-tolerant quantum computing — capable of running the long circuits required for drug discovery, cryptography, materials simulation, and optimization at commercially relevant scales — requires error-corrected logical qubits, which in transmon architectures require approximately 1,000-10,000 physical qubits per logical qubit. Cat qubits, by suppressing bit-flip errors by design, potentially require 10-100x fewer physical qubits for the same logical qubit quality, making fault-tolerant quantum computing achievable at smaller physical qubit counts and potentially faster timelines.
Key Facts
- Alice & Bob (Paris): primary cat qubit developer; founded 2020; raised €30M+; backed by Bpifrance as France 2030 quantum strategy company
- Cat qubit error bias: naturally suppress bit-flip errors (by up to 1,000x in laboratory demonstrations) while remaining susceptible to phase-flip errors — requiring only phase-flip error correction
- France National Quantum Strategy: €1.8B total (including France 2030 component); targets first fault-tolerant French quantum processor; Alice & Bob is hardware flagship
- Physical qubit advantage: cat qubit-based fault-tolerant computer projected to require ~100x fewer physical qubits than transmon-based equivalent — potentially compressing timeline to practical quantum advantage
- Theoretical foundation: cat qubit encoding developed at ENS Paris; Mazyar Mirrahimi (INRIA) is intellectual father of the approach
Why It Matters
For investors, cat qubits represent a genuine technical differentiation in the crowded quantum computing startup landscape — not merely another company building conventional transmon qubits in a different configuration, but a fundamentally different physical approach to the central engineering challenge of fault tolerance. The technical case for cat qubits is compelling, and Alice & Bob has demonstrated the key error suppression properties in laboratory conditions. The commercial question is whether the approach scales: cat qubit oscillators are more complex to build and control than transmon qubits, and the manufacturing challenges at scale (thousands to millions of qubits) are different from and potentially harder than transmon approaches.
France 2030’s bet on cat qubits is a high-risk, high-reward technology investment that reflects France’s strength in fundamental quantum physics research (multiple French Nobel laureates in quantum optics, including Serge Haroche) and its willingness to back scientifically distinguished but commercially unproven approaches. For global quantum computing investors, Alice & Bob represents one of the most technically credentialed alternative hardware approaches to the quantum computing problem — a serious contender for the fault-tolerant quantum computing milestone alongside, not merely behind, Google and IBM’s programs.