Characterization of Quantum Frequency Processors

TL;DR

This paper reviews the current methods for characterizing quantum frequency processors, emphasizing experimental techniques like quantum process tomography, to advance scalable quantum networking using frequency-bin qubits.

Contribution

It provides a comprehensive overview of experimental characterization techniques for quantum frequency processors, including the first full process tomography of a frequency-bin operation.

Findings

Comparison of open box and black box approaches

First full process tomography of a frequency-bin operation

Guidance for future integrated device characterization

Abstract

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.