Typically when we consider any particle of matter, one is identical to another. There is no difference between an electron, proton nor atoms or molecules with identical properties. Certainly when we consider an experiment done in the lab, the properties of any particle are consistant. But what about entanglement? Aren't all particles unique in some aspect, regardless of whether they are photons or atoms? Can we really say that if a photon goes into an atom, causing an electron to move between energy levels, that this electron and the atom it's in is not in some way entangled to the source which produced the photon? Is there nothing, even in principal, to tell that the atom which absorbed the photon obtained it from the atom which emitted it? Needless to say, there is an argument which says these atoms are indistinguishable in practice to any other since their properties are identical, but aren't the two atoms now entangled in some way that in principal, can be discerned?