Bang-bang protocol for nondispersive qubit readout

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 $\Delta$. While exhibiting good QND properties, the readout rate is limited to $\sim g^2\sqrt{N}/\Delta\ll g$, 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.

22 pages, 15 figures. Supplementary: 29 pages, 1 figure

arXiv:2608.23681 [quant-ph]

(or arXiv:2608.23681v1 [quant-ph] for this version)

https://doi.org/10.48550/arXiv.2608.23681 Focus to learn more arXiv-issued DOI via DataCite (pending registration)

arXivLabs is a framework that allows collaborators to develop and share new arXiv features directly on our website.

Both individuals and organizations that work with arXivLabs have embraced and accepted our values of openness, community, excellence, and user data privacy. arXiv is committed to these values and only works with partners that adhere to them.

Have an idea for a project that will add value for arXiv's community? Learn more about arXivLabs.