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Cat Qubits
Cat qubits are quantum bits encoded in Schrödinger “cat states,” which are superpositions of two coherent states of a harmonic oscillator, typically microwave photons in a superconducting resonator. Instead of using two discrete energy levels like a transmon, a cat qubit uses two opposite-phase coherent states ∣α⟩∣α⟩ and ∣−α⟩∣−α⟩ and their superpositions as its logical states, so the quantum information is distributed over many photons in a single bosonic mode. This bosonic encoding makes cat qubits a prime example of bosonic codes, where an infinite-dimensional oscillator Hilbert space is used to realize an effective two-level system. The main motivation for cat qubits is hardware-level protection against errors, especially bit-flip errors, by engineering the system so that transitions between the two coherent states are exponentially suppressed while phase-flip errors increase only linearly. In superconducting implementations such as Kerr-cat qubits, tailored drives and dissipation stabilize these cat states in resonators, leading to strongly biased noise where bit flips are rare, which in turn makes quantum error correction more efficient and has enabled long-lived quantum memories and steps toward fault-tolerant architectures.
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