  Quantum process tomography, the standard procedure to characterize any
quantum channel in nature, is affected by a circular argument: in order to
characterize the channel, the tomographic preparation and measurement need in
turn to be already characterized. We break this loop by designing an
operational framework able to optimally characterize any given unknown quantum
channel in a device-independent fashion, namely, by only looking at its
input-output statistics, under the sole assumption that quantum theory is
valid. We provide explicit solutions, in closed form, for practically relevant
cases such as the erasure, depolarizing, and amplitude-damping channels.
