Characterization of Quantum Frequency Processors

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