  Two photons are said to be identical when they are prepared in the same
quantum state. Given the latter, there is a unique way to achieve this.
Conversely, there are many different manners to prepare two non-identical
photons: they may have different frequency, polarization, amplitude, etc.
Therefore, photon distinguishability depends upon the specific degree of
freedom being varied. By means of a careful analysis of the coincidence
probability distribution in a Hong-Ou-Mandel experiment, we can show that
photon distinguishability can be actually quantified by the rate of
distinguishability of photons, an experimentally measurable parameter that
crucially depends on both the photon quantum state and the degree of freedom
under control.
