ZZ-Free Two-Transmon CZ Gate Mediated by a Fluxonium Coupler

TL;DR

This paper introduces a novel TFT architecture with fluxonium couplers that effectively eliminate residual ZZ interactions in transmon qubits, enabling high-fidelity CZ gates even at large detunings.

Contribution

The paper presents a new fluxonium-mediated coupling scheme that suppresses residual ZZ interactions at large qubit detunings, improving quantum gate performance.

Findings

Zero residual ZZ interactions at 409 MHz and 616 MHz detunings.

Achieved CZ gate fidelities of approximately 99.64% and 99.68%.

Demonstrated effective suppression of crosstalk in superconducting qubits.

Abstract

Eliminating residual ZZ interactions in a two-qubit system is essential for reducing coherent errors during quantum operations. In a superconducting circuit platform, coupling two transmon qubits via a transmon coupler has been shown to effectively suppress residual ZZ interactions. However, in such systems, perfect cancellation usually requires the qubit-qubit detuning to be smaller than the individual qubit anharmonicities, which exacerbates frequency crowding and microwave crosstalk. To address this limitation, we introduce TFT (Transmon-Fluxonium-Transmon) architecture, wherein two transmon qubits are coupled via a fluxonium qubit. The coupling mediated by the fluxonium eliminates residual ZZ interactions even for transmons detuned larger than their anharmonicities. We experimentally identified zero-ZZ interaction points at qubit-qubit detunings of 409 MHz and 616 MHz from two distinct TFT devices. We then implemented an adiabatic, coupler-flux-biased controlled-Z gate on both devices, achieving CZ gate fidelities of 99.64(6)% and 99.68(8)%.