  Interference between different photons occurs and has been observed under
diverse experimental conditions. A necessary condition in order to obtain
interference fringes is the existence of at least two possible paths and
unknown which-path information. If the photon beams have different frequencies,
stability of the sources and fast enough detectors are also required.First
order interference between two truly independent CW laser sources is observed.
Contrary to what is expected, interference is observed although the photon
beams are distinguishable and the path is unequivocally known for each photon
beam. Segments of the CW wavetrains are selected with an acousto optic
modulator. Temporal and spatial interference are integrated in a single
combined phenomenon via streak camera detection. The fringes displacement in
the time-space interferograms reveal the trajectories of the labeled photons.
These results indicate that in non-degenerate frequency schemes, the ontology
has to be refined and the which path criterion must be precisely stated. If
reference is made to the frequency labeled photons, the path of each photon is
known, whereas if the query is stated in terms of the detected photons, the
path is unknown.
