Dynamically suppressing cavity dephasing induced by frequency fluctuations of a coupled nonlinear mode

Superconducting microwave cavities are essential for quantum information processing due to their long coherence times. However, controlling these cavities typically requires coupling them to nonlinear Josephson elements, such as flux-tunable transmons (FTTs). These elements are susceptible to 1/f flux noise, which is inherited by the cavity as dephasing, potentially degrading the performance of quantum error-correction protocols. This paper investigates whether a hardware-efficient driving protocol can dynamically protect the cavity from this inherited noise.

The authors introduce Stark-Assisted Flux-noise Evasion (SAFE). This protocol applies a weak, off-resonant microwave drive to the nonlinear ancillary element. This drive induces an AC Stark shift on the cavity transition frequency. By selecting specific drive parameters, the authors create a dynamical sweet spot where the cavity transition frequency becomes first-order insensitive to fluctuations in the external magnetic flux. The researchers analyze this setup using a 3D superconducting cavity dispersively coupled to an FTT and validate their analytical predictions with Monte Carlo simulations.

The study demonstrates that SAFE can extend the cavity dephasing time by more than 20 times compared to an undriven system. The authors identify two regimes for the sweet spot: a large-detuning regime, which allows for the simultaneous protection of multiple cavity Fock states (ideal for bosonic error correction), and a small-detuning regime, which targets specific transitions. The analysis confirms that while the drive introduces new channels for decoherence, these effects remain subdominant to the suppression of the primary flux-induced dephasing.