  Counter-propagating parametric conversion processes in non-linear bulk
crystals have been shown to feature unique properties for efficient narrowband
frequency conversion. In quantum optics, the generation of photon pairs with a
counter-propagating parametric down-conversion process (PDC) in a waveguide,
where signal and idler photons propagate in opposite directions, offers unique
material-independent engineering capabilities. However, realizing
counter-propagating PDC necessitates quasi-phase-matching (QPM) with extremely
short poling periods. Here, we report on the generation of counter-propagating
single-photon pairs in a self-made periodically poled lithium niobate waveguide
with a poling period on the same order of magnitude as the generated
wavelength. The single photons of the biphoton state bridge GHz and THz
bandwidths with a separable joint temporal-spectral behavior. Furthermore, they
allow the direct observation of the temporal envelope of heralded single
photons with state-of-the art photon counters.
