What is a cat qubit? What is it good for?

Cat qubits are superconducting qubits which are particularly well-suited to implementing error correction, since they can advantageously trade bit-flip errors for phase-flip errors.

When tuning a specific parameter (the average number of the photons in the cat qubit), the frequency of bit-flip errors decreases exponentially, while the frequency of phase-flip errors increases only linearly.

This “biased noise” makes it possible to virtually eliminate bit-flip errors, while keeping phase-flip errors below the error correction threshold.

This in turn enables much simpler error correction schemes: because there’s virtually only one type of error to correct, you can use a simple repetition code, instead of a surface code that requires far more qubits.

Several physical qubits running a repetition code can then become a “logical qubit”, featuring a much lower error rate than any of the physical qubits it is made of.

Using cat qubits, we estimate we can build a fault-tolerant quantum computer requiring 60 times fewer or even 200 times fewer qubits than if we were using transmons.

To learn more about the physics of cat qubits, you can read our seminal paper at https://www.nature.com/articles/s41567-020-0824-x or https://arxiv.org/abs/1907.11729.