One-Photon and Two-Photon Double-Slit Interferences in Spontaneous and   Stimulated Parametric Down-Conversions
  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. 
De-Zhong Cao, Zhuan Li, Yan-Hua Zhai, Kaige Wang