Bosonic and Cat Qubits
Bosonic qubits store information in the many levels of a microwave cavity mode, using redundancy in one physical element to resist errors.
Information in a mode
Instead of a two-level circuit, a bosonic qubit uses a harmonic oscillator, typically a high-quality superconducting microwave cavity, whose infinite ladder of photon-number states provides room for a redundant encoding. A nonlinear ancilla, often a transmon, supplies the control and measurement.
Cat codes
A cat qubit encodes 0 and 1 in two coherent states of opposite phase, superpositions that resemble Schroedinger's cat. Engineered two-photon dissipation pins the state to this manifold, so bit-flips between the two coherent states become exponentially rare as the photon number grows. Phase-flips remain and are handled by an outer code, turning a two-dimensional error problem into a nearly one-dimensional one.
Other bosonic codes
- GKP (Gottesman-Kitaev-Preskill) codes, which encode a qubit in grid states of an oscillator and protect against small shifts
- Binomial codes, which use specific photon-number superpositions to detect single-photon loss
- Cat codes, which bias errors toward a single correctable type
Why it matters
Hardware-efficient error correction is the goal: by putting redundancy inside one long-lived cavity rather than across many physical qubits, bosonic encodings may reduce the overhead of fault tolerance. Break-even experiments, where an encoded qubit outlives its best physical component, have been demonstrated.
Bosonic qubits sit between raw physical qubits and full error correction. They show that the qubit-modality question is not only which physical system, but how information is laid out within it.