  Quality of individual photons and their ability to interfere is traditionally
tested by measuring the Hong-Ou-Mandel photon bunching effect. However, this
phase insensitive measurement only tests the particle aspect of the quantum
interference. Motivated by these limitations, we formulate a witness capable of
recognizing both the quality and the coherence of two photons from a single set
of measurements. We exploit the conditional nonclassical squeezing which is
sensitive to both the quality of the single photons and their
indistinguishability and we show that it can be used to irectly reveal both the
particle and the wave aspects of the quantum interference. Finally, we
experimentally test the witness by applying it to a pair of independent single
photons generated on demand.
