Modelling Microwave Crosstalk in Planar Superconducting Quantum Devices

Microwave crosstalk poses a major challenge to scaling superconducting quantum devices as it introduces control errors and additional relaxation channels for qubits. While its strength and impact have been studied across several instances, physical models that quantitatively predict the measured crosstalk for a given geometry are not common. Here, we investigate microwave crosstalk in planar superconducting devices with cross-overs and identify two dominant mechanisms: stray capacitance between a drive line and a nearby qubit, and cross-coupling between a drive line and a qubit-qubit coupler. We have designed samples to elucidate these crosstalk mechanisms and have developed physical models that show quantitative agreement with the observed crosstalk. Our results shed light on the origin of microwave crosstalk, provide guidance for designing devices with lower crosstalk, and establish a foundation for studying next-to-leading crosstalk mechanisms. The methods we present motivate microwave crosstalk studies on more complex architectures, such as those employing through-silicon vias and flip-chip bonding.