  We revisit Mandel's notion that the degree of coherence equals the degree of
indistinguishability by performing Hong-Ou-Mandel- (HOM-)type interferometry
with single photons elastically scattered by a cw resonantly driven excitonic
transition of an InAs/GaAs epitaxial quantum dot. We present a comprehensive
study of the temporal profile of the photon coalescence phenomenon which shows
that photon indistinguishability can be tuned by the excitation laser source,
in the same way as their coherence time. A new figure of merit, the coalescence
time window, is introduced to quantify the delay below which two photons are
indistinguishable. This criterion sheds new light on the interpretation of HOM
experiments under cw excitation, particularly when photon coherence times are
longer than the temporal resolution of the detectors. The photon
indistinguishability is extended over unprecedented time scales beyond the
detectors' response time, thus opening new perspectives to conducting quantum
optics with single photons and conventional detectors.
