  We theoretically discuss one-photon and two-photon double-slit interferences
for spontaneous and stimulated parametric down-conversions. We show that the
two-photon sub-wavelength interference can exist in a general spontaneous
parametric down-conversion (SPDC) for both type I and type II crystals. We
propose an alternative way to observe sub-wavelength interference by a
joint-intensity measurement which occurs for only type I crystal in a higher
gain of SPDC. When a signal beam injects into the crystal, it may create two
interference patterns by two stimulated down-converted beams, showing no
sub-wavelength interference effect.
