  One of the most fundamental quantum features is the two-photon intensity
correlation on a beam splitter, resulting in photon bunching into either output
port. According to the conventional understanding of quantum mechanics, the
origin of photon bunching on a beam splitter is in the indistinguishable
characteristics between coincident photons, resulting in destructive quantum
interference even without a clear definition of phase information of the paired
photons. Here, a completely different approach is presented for the same
quantum feature, where a strong mutual phase dependency is the essential
requirement for the nonclassical feature of photon bunching. This definite
phase relationship is now understood as the origin of this quantum feature
resulting from the phase-basis superposition of the photon-BS system, resulting
in photon indistinguishability. On behalf of a coherent model with a single
input-port BS system, an extended scheme of a Mach-Zehnder interferometer is
additionally analyzed for the validity of forbidden phase-basis superposition
in the quantum feature.
