  Frequency-bin qubits possess unique synergies with wavelength-multiplexed
lightwave communications, suggesting valuable opportunities for quantum
networking with the existing fiber-optic infrastructure. Although the coherent
manipulation of frequency-bin states requires highly controllable
multi-spectral-mode interference, the quantum frequency processor (QFP)
provides a scalable path for gate synthesis leveraging standard telecom
components. Here we summarize the state of the art in experimental QFP
characterization. Distinguishing between physically motivated ''open box''
approaches that treat the QFP as a multiport interferometer, and ''black box''
approaches that view the QFP as a general quantum operation, we highlight the
assumptions and results of multiple techniques, including quantum process
tomography of a tunable beamsplitter -- to our knowledge the first full process
tomography of any frequency-bin operation. Our findings should inform future
characterization efforts as the QFP increasingly moves beyond
proof-of-principle tabletop demonstrations toward integrated devices and
deployed quantum networking experiments.
