  We study how the information of the beam path is related to the
Hong-Ou-Mandel interference with two pulsed light sources. Through a simple
model in which two photons in the form of pulses pass a beamsplitter and are
observed at two detectors, we investigate how, during the measurement process,
information about the paths of the two photons can be erased. There are two
ways to clear the information of the beam path, the first being that from the
beginning, during the physical measurement process, the time information is not
obtained. The other is after measuring the information, to erase the temporal
information in the data analyzing process. We show that Hong-Ou-Mandel
interference can be obtained only when the beam path inofrmation is cleared
from the physical measurement process.
