  We present experimental demonstrations of two-photon interference involving
temporally separated photons within two types of interferometers: a
Mach-Zehnder interferometer and a polarization-based Michelson interferometer.
The two-photon states are probabilistically prepared in a symmetrically
superposed state within the two interferometer arms by introducing a large time
delay between two input photons; this state is composed of two temporally
separated photons, which are in two different or the same spatial modes. We
then observe two-photon interference fringes involving both the Hong-Ou-Mandel
interference effect and the interference of path-entangled two-photon states
simultaneously in a single interferometric setup. The observed two-photon
interference fringes provide simultaneous observation of the interferometric
properties of the single-photon and two-photon wavepackets. The observations
can also facilitate a more comprehensive understanding of the origins of the
interference phenomena arising from spatially bunched/anti-bunched two-photon
states comprised of two temporally separated photons within the interferometer
arms.
