Curator's Take
AI Commentary
This article introduces a “bang‑bang” nondispersive readout that swaps the slow parameter sweeps of conventional schemes for instantaneous quenches of the qubit–resonator coupling, allowing measurement rates to approach the full interaction strength g instead of being limited by g²/Δ. By analytically showing that the single‑shot fidelity improves as 1/N with the number of resonator photons, it offers a path to faster, high‑QND measurements that could tighten the feedback loop in error‑corrected superconducting processors. The approach builds on recent efforts to accelerate readout while preserving qubit coherence, though practical deployment will require precise timing of coupling jumps or strong flux drives to avoid residual state transitions.
— Mark Eatherly
Summary
Fast, precise, and quantum-non-demolition (QND) readout of superconducting qubits is a fundamental component of high-fidelity quantum sensing and computation. Conventional approaches typically operate in the dispersive regime, where the qubit-resonator coupling $g$ is weak compared to the detuning $Δ$. While exhibiting good QND properties, the readout rate is limited to $\sim g^2\sqrt{N}/Δ\ll g$, where $N$ is the number of photons in the resonator. QND readout in the nondispersive regime, where the readout rate reaches its full potential $\sim g$, relies on parameter sweeps that may encounter resonances, leading to measurement-induced state transitions (MIST). In this work, we study a nondispersive readout protocol that replaces these sweeps by sudden quenches of the coupling constant, using a resonator that is preloaded with photons. We call this protocol bang-bang readout, and show that it realizes single-shot projective measurements. The fidelity and QNDness of the qubit post-measurement are remarkably high, with an error that decreases like $1/N$. To arrive at these findings, we develop an analytical theory for the dynamics and measurements of the Jaynes-Cummings (JC) model, including a systematic expansion of correction terms in powers of $1/\sqrt{N}$. We show that the protocol can also be implemented without preloading the resonator by instead strongly driving the qubit, e.g., with a classical flux drive.