  Crosstalk occurs in most quantum computing systems with more than one qubit.
It can cause a variety of correlated and nonlocal crosstalk errors that can be
especially harmful to fault-tolerant quantum error correction, which generally
relies on errors being local and relatively predictable. Mitigating crosstalk
errors requires understanding, modeling, and detecting them. In this paper, we
introduce a comprehensive framework for crosstalk errors and a protocol for
detecting and localizing them. We give a rigorous definition of crosstalk
errors that captures a wide range of disparate physical phenomena that have
been called "crosstalk", and a concrete model for crosstalk-free quantum
processors. Errors that violate this model are crosstalk errors. Next, we give
an equivalent but purely operational (model-independent) definition of
crosstalk errors. Using this definition, we construct a protocol for detecting
a large class of crosstalk errors in a multi-qubit processor by finding
conditional dependencies between observed experimental probabilities. It is
highly efficient, in the sense that the number of unique experiments required
scales at most cubically, and very often quadratically, with the number of
qubits. We demonstrate the protocol using simulations of 2-qubit and 6-qubit
processors.
