Short answer: A Kerr-cat qubit stores a qubit in superpositions of two opposite-phase coherent states of a superconducting microwave resonator, which protects it against phase-flip errors. A 2020 Nature paper reports stabilizing such a qubit with more than tenfold longer transverse relaxation than single-photon encoding, a 2023 Nature Communications paper reports generating cat states on a chip with 89.1% fidelity, and a 2025 paper reports coupling a Kerr-cat qubit to a cavity. These are small laboratory demonstrations.
The idea
The 2020 Nature abstract says Schrödinger cat states, superpositions of macroscopically distinct classical states, are a resource for quantum metrology, communication and computation. Superpositions of two opposite-phase coherent states in an oscillator encode a qubit protected against phase-flip errors. The authors say several challenges remain: the protection must be maintained by stabilizing these highly excited states, while the system stays compatible with fast gates and non-demolition readout of the encoded information.
2020: stabilization and operation
The authors report generating and stabilizing cat states through the interplay of Kerr nonlinearity and single-mode squeezing in a superconducting microwave resonator. They report an increase in the transverse relaxation time of the stabilized, error-protected qubit of more than one order of magnitude compared with single-photon Fock-state encoding. They perform all single-qubit gates on timescales more than sixty times faster than the shortest coherence time, and demonstrate single-shot readout of the protected qubit while it is stabilized.
2023: cat states on a chip
The 2023 Nature Communications paper says cat states are normally generated in three-dimensional cavities or with strong multi-photon drives, which face scalability and controllability challenges. The authors present a strategy to generate and preserve cat states in a coplanar superconducting circuit by fast modulation of Kerr nonlinearity. At the Kerr-free work point the cat states are passively preserved because the Kerr effect vanishes. They prepare a two-component cat state with a fidelity reaching 89.1% under a 96 ns gate time, and describe this as a route toward a chip-based bosonic quantum processor.
2025: coupling to a cavity
The 2025 paper starts from the observation that bosonic codes encode information redundantly in a harmonic oscillator, but ancilla errors propagating to the logical qubit during syndrome measurement limit experiments. The Kerr-cat qubit has been proposed as an ancilla because of a theoretically exponential noise bias, but the coupling needed has not been demonstrated. The authors realize driven parametric coupling of a Kerr-cat qubit to a high-quality-factor microwave cavity and demonstrate a gate set that would enable universal quantum control of the cavity. They found excess dephasing of the cavity caused by heating of the Kerr-cat to excited states, and removed it with frequency-selective dissipation. They say the results pave the way toward fault-tolerant error syndrome measurements of bosonic codes.
Limits
The protection discussed is against phase-flip errors, and the 2025 paper's own wording is that the exponential noise bias is theoretical and that the fault-tolerant syndrome measurement is a future goal. All three results are from single devices.
FAQ
What is a Kerr-cat qubit?
A qubit encoded in a superposition of two opposite-phase coherent states of a resonator, protected against phase-flip errors, per the 2020 paper.
How much longer did the 2020 qubit last?
The authors report more than one order of magnitude longer transverse relaxation than single-photon Fock-state encoding.
What fidelity was reported on a chip?
89.1% for a two-component cat state with a 96 ns gate time, in the 2023 paper.
What was new in 2025?
Driven parametric coupling of a Kerr-cat qubit to a microwave cavity, with a gate set aimed at universal control of the cavity.
Is it a fault-tolerant system?
No. The 2025 authors describe fault-tolerant syndrome measurement as the direction their results pave the way toward.
All field notes