We theoretically discuss one-photon and two-photon double-slit interferencesfor spontaneous and stimulated parametric down-conversions. We show that thetwo-photon sub-wavelength interference can exist in a general spontaneousparametric down-conversion (SPDC) for both type I and type II crystals. Wepropose an alternative way to observe sub-wavelength interference by ajoint-intensity measurement which occurs for only type I crystal in a highergain of SPDC. When a signal beam injects into the crystal, it may create twointerference patterns by two stimulated down-converted beams, showing nosub-wavelength interference effect.
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