Fast Microwave-Activated CZ Gates with the Quarton Coupler

High-fidelity two-qubit gates are essential for minimizing overhead in quantum error correction. Achieving higher gate fidelities requires faster gate operations relative to qubit coherence times, which in turn demands stronger interactions. Recently, the quarton coupler was experimentally shown to enable a 580 MHz ZZ interaction between two qubits [1] and was proposed as a method to enable 5 ns qubit readout via a 234 MHz cross-Kerr between the qubit and resonator modes [2]. In this work, we propose a scheme to harness the quarton coupler's large ZZ interaction to implement a CZ gate. The gate is fully microwave-activated, which eliminates the need for fast flux control and avoids tuning qubit frequencies over wide ranges which could contain two-level systems. Simulations indicate that the proposed scheme achieves above 99.9% CZ gate fidelity within 10 ns and suppresses the ZZ interaction below 10 KHz when idling. Additionally, we explore strategies to mitigate the impact of fabrication variance on gate performance.

[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).