  Single photons exhibit inherently quantum and unintuitive properties such as
the Hong-ou-Mandel effect, demonstrating their bosonic and quantized nature,
yet at the same time may correspond to single excitations of spatial or
temporal modes with a very complex structure. Those two features are rarely
seen together. Here we experimentally demonstrate how the Hong-Ou-Mandel effect
can be spectrally-resolved and harnessed to characterize a complex temporal
mode of a single-photon \textendash{} a zero-area pulse \textendash{} obtained
via a resonant interaction of a terahertz-bandwidth photon with a narrow
gigahertz-wide atomic transition of atomic vapor. The combination of bosonic
quantum behavior with bandwidth-mismatched light-atom interaction is of
fundamental importance for a deeper understanding of both phenomena, as well as
their engineering offering applications in the characterization of ultra-fast
transient processes.
