  Crosstalk is a leading source of failure in multiqubit quantum information
processors. It can arise from a wide range of disparate physical phenomena, and
can introduce subtle correlations in the errors experienced by a device.
Several hardware characterization protocols are able to detect the presence of
crosstalk, but few provide sufficient information to distinguish various
crosstalk errors from one another. In this article we describe how gate set
tomography, a protocol for detailed characterization of quantum operations, can
be used to identify and characterize crosstalk errors in quantum information
processors. We demonstrate our methods on a two-qubit trapped-ion processor and
a two-qubit subsystem of a superconducting transmon processor.
