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 intwo 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.
Quantum jumps, the collapse of a quantum system upon measurement, are among the most striking consequences of observation in quantum mechanics. While recent experiments have revealedthe continuous nature of individual jumps, the crossover from coherent dynamics to measurement-dominated behaviour has remained elusive. Here, we tune the measurement strength of a continuously monitored superconducting qubit, and observe that quantum jumps emerge not through a gradual crossover, but via a cascade of three distinct dynamical transitions. The first transition manifests as an exceptional point where coherent oscillations abruptly cease, giving way to jumps towards a stable eigenstate. The second transition marks the onset of dynamical state freezing, where the qubit’s dwell time near the eigenstate diverges. A third threshold signals entry into the quantum Zeno regime, where stronger measurement paradoxically suppresses relaxation. Strikingly, we find that decoherence does not blur these transitions but rather fundamentally restructures the dynamical phase diagram, notably inverting their order. These results map measurement-induced transitions in a monitored qubit, revealing that the interplay between coherent driving, measurement, and decoherence gives rise to a hierarchy of distinct dynamical phases.
Decoherence in qubits, caused by their interaction with a noisy environment, poses a significant challenge to developing reliable quantum processors. Monitoring the qubit’s environmentenables not only to flag decoherence events but also to reverse these errors, thereby restoring the qubit coherence. This approach is particularly beneficial for superconducting cavity qubits, whose unavoidable interaction with auxiliary transmons impacts their coherence. In this work, we uncover the intricate dynamics of cavity qubit decoherence by tracking the noisy trajectory of a transmon acting as the cavity’s environment. Using real-time feedback, we successfully recover the lost coherence of the cavity qubit, achieving a fivefold increase in its dephasing time. Alternatively, by detecting transmon errors and converting them into erasures, we improve the cavity phase coherence by more than an order of magnitude. These advances are essential for using cavity qubits with low photon loss rates as long-lived quantum memories with high-fidelity gates and can enable more efficient bosonic quantum error correction codes.
Storing quantum information for an extended period of time is essential for running quantum algorithms with low errors. Currently, superconducting quantum memories have coherence timesof a few milliseconds, and surpassing this performance has remained an outstanding challenge. In this work, we report a qubit encoded in a novel superconducting cavity with a coherence time of 34 ms, an improvement of over an order of magnitude compared to previous demonstrations. We use this long-lived quantum memory to store a Schrödinger cat state with a record size of 1024 photons, indicating the cavity’s potential for bosonic quantum error correction.