Fundamental speedup of the controlled-Z gate in superconducting qubits using a triple-state degeneracy

Useful quantum computation requires fast and high-fidelity entangling gates between qubits. In superconducting quantum circuits, many common realizations of the two-qubit controlled-Z (CZ) gate involve an interaction between a pair of states in the two-excitation subspace. It has been proposed theoretically [1] that increasing the number of involved states from two to three would yield a CZ gate faster by a factor of √2. This scheme requires two qubits with exactly opposite anharmonicities. Here, we experimentally demonstrate such a system using a transmon coupled capacitively to an inductively shunted transmon (IST) [2]. We observe that the dynamics of the triple-state degeneracy in the two-excitation subspace closely match theoretical predictions, including the √2 interaction speedup. This allows us to execute a CZ gate in less than 25 ns using fast flux control, while also maintaining low levels of coherent (ZZ) error during idling thanks to the opposite anharmonicities. Crucially, this gate scheme does not require tunable coupling between qubits, minimizing hardware complexity.

[1] P. Zhao, P. Xu, D. Lan, J. Chu, X. Tan, H. Yu, and Y. Yu, Phys. Rev. Lett. 125, 200503 (2020)

[2] S. D. Fasciati, B. Shteynas, G. Campanaro, M. Bakr, S. Cao, V. Chidambaram, J. Wills, and P. J. Leek, arXiv:2410.10416