If I am not mistaken, (contrary to what has been expressed in previous posts) coherence very often involves a single particle. Coherence very often means a stable phase shift between two states of the same particle, which in turn implies indistinguishability of both states. In the case of an electron or neutron, after going through a very small double slit on a screen, its wave would start to spread and we would have a large uncertainty in its position. Up until this point, and assuming some conditions on the way the particle approached the double slit, there would be coherence between the states corresponding to having passed through each slit. As soon as the particle interacts with something macroscopic, in a way that path information can be recorded, coherence is lost. In the case of the bubble chamber, decoherence would happen very often and we observe a narrow path.
In the case of photons, the interactions we normally think about are emission and absorption of the photon. So in this case we don't have a situation in which the photon bumps into something in such a way that we can detect where it is and the photon keeps going. Once we know where it is, it has been "destroyed" (absorbed). If we shoot the photon into a space where there are absorbing particles, then the photon will be absorved very quickly, but if we had the photon traveling through an optical fiber with low absorption for that wavelength, it would have a better chance to survive and mantain coherence until it is detected. Probably possible interactions that would not absorbe the photon, such as bending of the path along the fiber or passing through a beam splitter, don't record (in a permanent way) information such as path information or polarization, and therefore don't imply decoherence. So it seems that there is a close relationship between distinguishability of the states and coherence between them. It would be interesting to have some simple example showing how distinguishability implies dephasing.
