  The characterization of quantum processes, e.g. communication channels, is an
essential ingredient for establishing quantum information systems. For quantum
key distribution protocols, the amount of overall noise in the channel
determines the rate at which secret bits are distributed between authorized
partners. In particular, tomographic protocols allow for the full
reconstruction, and thus characterization, of the channel. Here, we perform
quantum process tomography of high-dimensional quantum communication channels
with dimensions ranging from 2 to 5. We can thus explicitly demonstrate the
effect of an eavesdropper performing an optimal cloning attack or an
intercept-resend attack during a quantum cryptographic protocol. Moreover, our
study shows that quantum process tomography enables a more detailed
understanding of the channel conditions compared to a coarse-grained measure,
such as quantum bit error rates. This full characterization technique allows us
to optimize the performance of quantum key distribution under asymmetric
experimental conditions, which is particularly useful when considering
high-dimensional encoding schemes.
