> Given perfect information, in this situation, they'll produce, somewhat miraculously, a complete description of the state of the system. I don't think that t

> Given perfect information, in this situation, they'll produce, somewhat miraculously, a complete description of the state of the system. I don't think that the complete description of the system when the atom is going from a pure excited state to a pure ground state will include pure states of the atom which are superpositions of the excited state and the ground state. If we have a complete description of the system, the atom may be part of a larger system which is in a pure state and in that case the quantum state of the atom may be an improper mixture of the excited state and the ground state. The atom may also be in a pure state on its own, but then it will be either in the excited state or in the ground state. That's all I said. I may be wrong but I fail to see in your comments a reason to think so. Edit: By the way, I know an atom can in principle be in a state which is a superposition of states with different energies (I said so in my first comment). It's just that I don't think that happens during the spontaneous transition from one state to another. If it does happen, I would be glad to learn about it. Edit2: Looking again at some quantum optics papers I see people argue that stochastic equations have a physical meaning and are not just a calculation device. Anyway, those interaction models are quite complex and full of approximations so it's not clear what "pure" means anymore...