Autonomous cooling of Kerr-cat qubits to minutes-long bit-flip times

Abstract

Quantum error correction becomes more hardware efficient when physical noise is strongly biased towards one error channel. Cat qubits realize such a bias by encoding information in two opposite coherent states of a superconducting oscillator, so that bit flips are exponentially suppressed with photon number while phase-flip rates increase only linearly. In Kerr-cat qubits, a driven double-well potential confines these states and enables fast coherent control, but excitations can induce interwell switching, limiting bit-flip times to milliseconds. Here we show that the nonlinear interaction generating the confinement can also cool these excitations. When an intra-well transition becomes resonant with a lossy readout mode, excitations are transferred out of the qubit and dissipated, as established by spectroscopy and relaxation measurements. We observe the predicted exponential scaling of Kerr-cat bit-flip times with photon number, reaching 450 s, over five orders of magnitude beyond previous demonstrations. Phase-flip rates remain consistent with intrinsic photon loss at operating points with seconds-long bit-flip times. We further demonstrate fast Z gates without measurable degradation of bit-flip protection and single-shot readout with a bit-flip probability below $10^{-6}$ per measurement. Autonomous cooling thus brings minutes-long bit-flip protection to Kerr-cat qubits while retaining fast coherent control, without additional cooling drives or circuit elements.