  The boson sampling problem has triggered a lot of interest in the scientific
community because of its potential of demonstrating the computational power of
quantum interference without the need of non-linear processes. However, the
intractability of such a problem with any classical device relies on the
realization of single photons approximately identical in their spectra. In this
paper we discuss the physics of boson sampling with non-identical single photon
sources, which is strongly relevant in view of scalable experimental
realizations and triggers fascinating questions in complexity theory.
