  To improve the performance of multi-qubit algorithms on quantum devices it is
critical to have methods for characterizing non-local quantum errors such as
crosstalk. To address this issue, we propose and test an extension to the
analysis of simultaneous randomized benchmarking data -- correlated randomized
benchmarking. We fit the decay of correlated polarizations to a composition of
fixed-weight depolarizing maps to characterize the locality and weight of
crosstalk errors. From these errors we introduce a crosstalk metric which
indicates the distance to the closest map with only local errors. We
demonstrate this technique experimentally with a four-qubit superconducting
device and utilize correlated RB to validate crosstalk reduction when we
implement an echo sequence.

Quantum Physics