You are collapsing the wave function at t1 and t2. At t1 explicitly by stating that the state is known, and by t2 implicitly by defining that you have a defined

You are collapsing the wave function at t1 and t2. At t1 explicitly by stating that the state is known, and by t2 implicitly by defining that you have a defined distance t2- t1. The emission of an photon at an explicit time t2 is not reversible, the emission of an photon according to the wave function of the atom leads to an photon entangled with the atom and the total has a reversible wave function. (For illustrative purposes, when you measure and you find the photon at a distance (t- t2)*c from the atom, then you will also find the atom in a state that has emitted a photon at t2, no Idea if that is especially illustrative.)