  The study of entangled states has greatly improved the basic understanding
about two-photon interferometry. Two-photon interference is not the
interference of two photons but the result of superposition among
indistinguishable two-photon amplitudes. The concept of two-photon amplitude,
however, has generally been restricted to the case of entangled photons. In
this letter we report an experimental study that may extend this concept to the
general case of independent photons. The experiment also shows interesting
practical applications regarding the possibility of obtaining high resolution
interference patterns with thermal sources.
