The perfect entangler spectrum as a tool to analyze crosstalk

TL;DR

The paper introduces the perfect entangler spectrum as a novel tool to detect and analyze dynamic crosstalk in quantum computers, aiding in the scaling of quantum systems by identifying undesired entanglement.

Contribution

It presents a new spectroscopy method based on perfect entanglers to identify and analyze dynamic crosstalk in quantum gates, demonstrated on transmon qubits.

Findings

Peaks in the spectrum indicate crosstalk during frequency scans.

Analysis of peaks reveals mechanisms causing crosstalk.

Method applicable to fixed-frequency and parametrically driven gates.

Abstract

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 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.