Bang-bang protocol for nondispersive qubit readout

• URL: http://arxiv.org/abs/2608.23681v1
• Date: Mon, 24 Aug 2026 18:00:01 GMT
• Title: Bang-bang protocol for nondispersive qubit readout
• Abstract summary: Fast, precise, and quantum-non-demolition (QND) readout of superconducting qubits is a fundamental component of high-fidelity quantum sensing and computation.<n>In this work, we study a nondispersive readout protocol that replaces these sweeps by sudden quenches of the coupling constant.<n>We call this protocol bang-bang readout, and show that it realizes single-shot projective measurements.
• Score: 2.8621162328602723
• License: http://creativecommons.org/licenses/by/4.0/
• Abstract: 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.

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