Perfect-entangler spectrum as a tool to analyze crosstalk

Crosstalk is a key obstacle to scaling up quantum computers. It may arise from persistent qubit-qubit couplings or dynamically during gate operation, with the latter being particularly difficult to detect. Here, we introduce the perfect-entangler spectrum as a means to identify dynamic crosstalk leading to undesired entanglement. It leverages the geometric classification of two-qubit gates in terms of perfect entanglers. We exemplify the application of the spectroscopy for fixed-frequency transmons and parametrically driven gates. When scanning the frequency of a spectator qubit, peaks in the perfect-entangler spectrum signal dynamic crosstalk, and analysis of the peaks reveals the mechanisms causing the crosstalk. We discuss the experimental implementation of the crosstalk spectroscopy, which requires two two-qubit gate tomographies.