  Properties of entangled photon pairs generated in spontaneous parametric
down-conversion are investigated in interference experiments. Strong energy
correlations are demonstrated in a direct way. If a signal photon is detected
behind a narrow spectral filter, then interference appears in the Mach-Zehnder
interferometer placed in the route of the idler photon, even if the path
difference in the interferometer exceeds the coherence length of the light.
Narrow time correlations of the detection instants are demonstrated for the
same photon-pair source using the Hong-Ou-Mandel interferometer. Both these two
effects may be exhibited only by an entangled state.
