Alice & Bob: Cat Qubits and Elevator Codes

Alice & Bob: Cat Qubits and Elevator Codes

Summary

Alice & Bob is a quantum computing company (Paris/Boston, founded 2020) focused on building a fault-tolerant universal quantum computer using cat qubits—bosonic qubits engineered to have strongly biased noise. Their recent work on Elevator Codes aims to dramatically reduce logical error rates with relatively low overhead by stacking (concatenating) codes tuned to that bias.

Notes

What are cat qubits?

• Implemented in superconducting microwave cavities (bosonic modes) coupled to nonlinear elements.
• Logical states are superpositions of coherent states (“Schrödinger cat” states) in phase space.
• Key property: noise can be made highly biased:
  • Bit flips (logical X errors) are exponentially suppressed as you tune parameters.
  • Phase flips (logical Z errors) increase more gently.

Why this matters:

• Traditional qubit architectures often have roughly symmetric X/Z error rates.
• If one error is much rarer, you can design codes that:
  • focus correction on the dominant error type
  • achieve lower logical error with fewer physical resources.

Alice & Bob has reported record bit-flip lifetimes (minutes+ region) for their cat qubits.

Bosonic codes vs qubit codes

• Bosonic codes encode logical qubits into oscillator states instead of collections of two-level systems.
• Examples: cat codes, GKP codes, binomial codes.
• Advantages:
  • Can map natural noise (e.g., photon loss) into correctable patterns.
  • Potentially reduce the number of physical components per logical qubit.

Cat qubits are a particular bosonic code where the structure of the code matches the device physics.

Elevator Codes (high-level)

Problem: even with biased noise and cat codes, you still need additional coding to reach very low logical error rates (e.g. ~(10^{-12}) per logical gate for chemistry).

Elevator Codes idea (simplified):

• Use code concatenation: a bosonic code (cat qubit) at the bottom, then additional repetition-like codes on top.
• Introduce a logical ancilla qubit that is reused and moved through different code blocks—like an elevator:
  • It interacts with different encoded blocks to extract syndrome information about logical bit flips.
  • Because bit flips are already rare, the overhead of this scheme is manageable.

Reported benefits (per Alice & Bob’s announcements, early 2026):

• Up to 10,000× reduction in logical error rates.
• Overhead of only about 3× more qubits compared to their previous architecture.

Where this sits in the landscape

• Many players pursue surface-code-based FTQC, which is general but has large overhead.
• Alice & Bob’s approach:
  • Leverages hardware-level bias (cat qubits) and hardware-friendly codes.
  • Aims to reduce total qubit count per useful logical qubit by orders of magnitude.
• If successful, this could:
  • Lower the hardware scale needed for early useful FTQC.
  • Influence how others think about co-design of hardware + codes.

What to watch for

• Demonstrations of:
  • Stable logical qubits with error decreasing as the code scales (true fault tolerance).
  • Benchmarking against surface-code-style logical qubits on similar hardware budgets.
  • Real algorithms or chemistry simulations using logical cat qubits + Elevator Codes.

References

• Company site and technical blog: https://alice-bob.com/
• Elevator Codes overview: https://alice-bob.com/blog/elevator-codes-pushing-logical-bit-flips-to-near-extinction/