Microwave-activated gates between a fluxonium and a transmon qubit

TL;DR

This paper proposes microwave-activated cross-resonance and CPHASE gates between fluxonium and transmon qubits, demonstrating high fidelity and improved chip yield, addressing challenges due to their large frequency differences.

Contribution

It introduces and analyzes two novel microwave-activated gates between fluxonium and transmon qubits, with detailed simulations showing high fidelity and better chip yield.

Findings

Gate fidelities above 99% achieved in simulations.

Gate times between 100 and 300 ns.

Improved chip yield over transmon-only architectures.

Abstract

We propose and analyze two types of microwave-activated gates between a fluxonium and a transmon qubit, namely a cross-resonance (CR) and a CPHASE gate. The large frequency difference between a transmon and a fluxonium makes the realization of a two-qubit gate challenging. For a medium-frequency fluxonium qubit, the transmon-fluxonium system allows for a cross-resonance effect mediated by the higher levels of the fluxonium over a wide range of transmon frequencies. This allows one to realize the cross-resonance gate by driving the fluxonium at the transmon frequency, mitigating typical problems of the cross-resonance gate in transmon-transmon chips related to frequency targeting and residual ZZ coupling. However, when the fundamental frequency of the fluxonium enters the low-frequency regime below 100 MHz, the cross-resonance effect decreases leading to long gate times. For this range of parameters, a fast microwave CPHASE gate can be implemented using the higher levels of the fluxonium. In both cases, we perform numerical simulations of the gate showing that a gate fidelity above 99% can be obtained with gate times between 100 and 300 ns. Next to a detailed gate analysis, we perform a study of chip yield for a surface code lattice of fluxonia and transmons interacting via the proposed cross-resonance gate. We find a much better yield as compared to a transmon-only architecture with the cross-resonance gate as native two-qubit gate.