Abstract: Coherent errors in a quantum system can, in principle, build up much more rapidly than incoherent errors, accumulating as the square of the number of qubits in the system rather than linearly.

I will characterize the types of channels that can exhibit such behavior and present a simple protocol that can detect and characterize coherent errors whenever they are present, no matter what their nature.

This allows us to identify coherent errors in gates and measurements to within a constant fraction of the maximum possible sensitivity to such errors.