  Two-photon interference (TPI) lies at the heart of photonic quantum
technologies. TPI is generally regarded as quantum interference stemming from
the indistinguishability of identical photons, hence a common intuition
prevails that TPI would disappear if photons are distinguishable. Here we
disprove this perspective and uncover the essence of TPI. We report the first
demonstration of TPI between distinguishable photons with their frequency
separation up to $10^4$ times larger than their linewidths. We perform
time-resolved TPI between an independent laser and single photons with
ultralong coherence time ($>10\ \mu$s). We observe a maximum TPI visibility of
$72\%\pm 2\%$ well above the $50\%$ classical limit indicating the quantum
feature, and simultaneously a broad visibility background and a classical beat
visibility of less than $50\%$ reflecting the classical feature. These
visibilities are independent of the photon frequency separation and show no
difference between distinguishable and indistinguishable photons. Based on a
general wave superposition model, we derive the cross-correlation functions
which fully reproduce and explain the experiments. Our results reveal that TPI
as the fourth-order interference arises from the second-order interference of
two photons within the mutual coherence time and TPI is not linked to the
photon indistinguishability. This work provides new insights into the nature of
TPI with great implications in both quantum optics and photonic quantum
technologies.
