Schrödinger’s cat” is a famous thought experiment that illustrates a quantum system in a superposition of states that are clearly different from each other: a cat that is both alive and dead. Inspired by this, cat qubits encode quantum information in superpositions of also clearly distinguishable states. Instead of animals, however, we use states of light called “coherent states.
Coherent states are specific patterns of electromagnetic waves in a superconducting cavity. The “logical states” – ∣0⟩ and ∣1⟩ – are defined by these coherent states. In honor of Leonard Nimoy, we do not have illogical states.
Superpositions of the logical states are called “cat states.” The popular states |+⟩ and |-⟩ have well-defined photon parity, which means that the ∣+⟩ state (“even cat state”) has an even number of photons (0, 2, 4, …) and the ∣-⟩ state (“odd cat state”) has an odd number of photons (1,3, 5, …).
Photon parity and resilience against photon loss help reduce errors during computation. Adding photons significantly decreases bit-flip errors while slightly increasing phase-flip errors. This overall reduction in errors means that fewer physical qubits will be needed for fault-tolerant computation.