Error Correction
Cat Qubit
A bosonic qubit stored in a microwave resonator, engineered so that bit-flip errors become exponentially rare.
Definition
A cat qubit stores quantum information in the state of a microwave resonator, in a superposition of two opposite classical-looking waves, named after Schrödinger's cat. A carefully engineered circuit keeps the resonator in that two-state space and makes one kind of error, the bit flip, almost impossible. The remaining error, the phase flip, can then be fixed with a very simple code, so cat qubits could cut the number of physical qubits needed for error correction.
In depth
Cat qubits exploit noise bias. In most qubits, bit flips and phase flips occur at similar rates, and codes must correct both. In a cat qubit, increasing the photon number in the oscillator stretches the bit-flip time exponentially, while the phase-flip rate grows in proportion to the photon number from photon loss. Alice and Bob demonstrated bit-flip times of more than seven minutes in 2024 and reported much longer times in 2025, while phase flips remain at microsecond scale. With such a strong bias, a repetition code that only corrects phase flips can suffice, using a linear chain of cat qubits instead of a two-dimensional surface code.
The approach has attracted major players. Alice and Bob in Paris lead the field with a roadmap to 100 logical qubits by 2030, and Amazon's Ocelot chip, announced in February 2025, combines cat qubits with a repetition code and reports about a 90 percent reduction in error-correction overhead compared with a surface code, according to the company. Nord Quantique pursues a related bosonic approach, and Yale and French research groups introduced and refined the basic ideas.
Challenges remain. Gates on cat qubits that preserve bias are harder than gates on transmons, in particular the entangling gates between cat qubits and the operation needed for universal computation. The photon number must be large enough for bias but small enough to keep phase flips low, which sets a tradeoff, and the dissipation engineering must be reliable at the scale of many qubits.
Technical Definition
A cat qubit encodes information in the span of two coherent states of an oscillator with opposite phase, alpha and minus alpha. Two-photon dissipation, engineered with a nonlinear buffer mode and a microwave drive, stabilizes this manifold. Bit-flip errors are suppressed exponentially in the mean photon number |alpha|^2, while phase-flip errors grow only linearly.
Visual Explanation: An Analogy
Imagine a pendulum with two stable resting positions separated by a tall wall. Noise rarely pushes it over the wall, so it stays in its position for a long time. But the wall does nothing against a different kind of disturbance that blurs which side it is on, and that one still needs correcting.
Key facts
| Item | Detail |
|---|---|
| Encoding | Superposition of two coherent states, alpha and minus alpha, of a microwave resonator |
| Bit-flip suppression | Exponential in mean photon number |
| Phase-flip rate | Grows linearly with photon number |
| Records | Bit-flip times above 7 minutes (2024) and tens of minutes reported in 2025 |
| Developers | Alice and Bob, Amazon (Ocelot), Nord Quantique, and academic groups at Yale and in France |
Where the idea comes from
Cat states were discussed by Schrödinger in 1935, and Mirrahimi, Leghtas, and colleagues proposed the dissipative stabilization of cat qubits in 2014. Experiments by Lescanne and Leghtas in 2020 showed exponential bit-flip suppression, and Alice and Bob was founded in 2020 to commercialize the approach.
Real-World Use Cases
- Building error-corrected qubits with repetition codes and low overhead.
- Studying bosonic error-correction and noise-biased gates.
- Hardware roadmaps aiming for fewer physical qubits per logical qubit.
- Cloud access to cat-qubit devices for research.
How it differs from related ideas
- A GKP qubit is another bosonic encoding that protects against small shifts, while a cat qubit protects against bit flips.
- A transmon has similar bit-flip and phase-flip rates, and a cat qubit has strongly biased noise.
- Concatenating a cat qubit with an outer code gives a hybrid scheme with lower overhead.
Common Misconceptions
- A cat qubit is not immune to errors. It suppresses bit flips and still suffers from phase flips.
- The name does not mean a live cat. It refers to the superposition of two macroscopic-looking states.
- Cat qubits are not yet a replacement for transmons in large machines. They are an emerging approach.
Frequently asked questions
What is a cat qubit?
A qubit encoded in two opposite-phase coherent states of a microwave resonator, engineered to suppress bit-flip errors exponentially.
Why does it reduce error-correction overhead?
With bit flips nearly eliminated, only phase flips need correcting, which a simple repetition code can do with fewer qubits.
Who is building them?
Alice and Bob, Amazon with its Ocelot chip, Nord Quantique, and research groups in France and at Yale.