Characterizing the noise affecting superconducting qubits is essential for improving their performance. Existing noise-sensing techniques use the qubit itself as a detector, but itsshort coherence time limits both sensitivity and accessible frequency range. Here, we demonstrate a method for measuring qubit frequency noise by converting it into photon loss in a coupled high-quality superconducting cavity. We prepare a single photon in the cavity and perform repeated mid-circuit qubit measurements with post-selection to isolate noise-induced loss from intrinsic cavity decay, placing an upper bound on the intrinsic dressed-dephasing rate of (0.29s)−1 at 508 MHz, corresponding to a qubit frequency-noise power spectral density below 5.4×103Hz2/Hz. By exploiting the cavity’s millisecond-scale lifetime, this technique provides access to high-frequency noise processes that are beyond the reach of conventional qubit-based spectroscopy and that may impose previously unexplored limits on qubit coherence.
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.