can coherence be restored (after de-coherence)? let's look at single particle (interference) in a double slit experiment: when we try to gain which way .... the ordering of the phase angles between the components of a photon in a quantum superposition is disturbed/changed (?) can we restore the coherence back? is this question conceptually similar to - can entropy be restored? on a separate note: We can introduce a phase difference, between say the two paths of a photon, say via placing a half/fully silvered mirror on one of the paths. We can zero out the phase difference via another half/fully silvered mirror on the other path. Thus we are able to introduce a phase difference and again reduce the phase difference back to zero as long as there is no measurement.