  Long-distance quantum communication relies on storing and retrieving photonic
qubits in orthogonal field modes. The available degrees of freedom for photons
are polarization, spatial-mode profile, and temporal/spectral profile. To date,
methods exist for decomposing, manipulating, and analyzing photons into
orthogonal polarization modes and spatial modes. Here we propose and
theoretically verify the first highly efficient method to carry out analogous
operations for temporally and spectrally overlapping, but field-orthogonal,
temporal modes. The method relies on cascaded nonlinear-optical quantum
frequency conversion.
