  The large capacity and robustness of information encoding in the temporal
mode of photons is important in quantum information processing, in which
characterizing temporal quantum states with high usability and time resolution
is essential. We propose and demonstrate a direct measurement method of
temporal complex wavefunctions for weak light at a single-photon level with
subpicosecond time resolution. Our direct measurement is realized by ultrafast
metrology of the interference between the light under test and self-generated
monochromatic reference light; no external reference light or complicated
post-processing algorithms are required. Hence, this method is versatile and
potentially widely applicable for temporal state characterization.
