Realizing ultrafast superconducting qubit readout using the quarton coupler

Fast and high-fidelity qubit readout is crucial for quantum information processing. In superconducting qubits, state-of-the-art readout relies on a dispersive cross-Kerr coupling between the qubit and its readout resonator, typically limited to a few megahertz. In this work, we use the quarton coupler to achieve a significantly stronger cross-Kerr interaction—on the order of hundreds of megahertz [1]. We first establish a theoretical framework for readout using the quarton coupler [2]. Including internal modes of the series junctions and assuming a quantum efficiency of 50%, full stochastic master equation simulations indicate that qubit readout above 99% fidelity can be achieved in less than 20 ns using 2 readout photons. We present progress toward an experimental implementation of ultrafast readout using the quarton coupler.



[1] Y. Ye, J. B. Kline, A. Yen, G. Cunningham, M. Tan, A. Zang, M. Gingras, B. M. Niedzielski, H. Stickler, K. Serniak, M. E. Schwartz, K. P. O’Brien, “Near-ultrastrong nonlinear light-matter coupling in superconducting circuits.” arXiv:2404.19199.

[2] Y. Ye, J. B. Kline, S. Chen, A. Yen, K. P. O’Brien, “Ultrafast superconducting qubit readout with the quarton coupler.” Sci. Adv. 10, eado9094 (2024).