 Suppose, now, that the electrons interact with something else (call it the environment) on the way to the screen, that could serve as a “which-way” detector; that is, the state of this auxiliary system becomes entangled with the state of the electron in such a way that its state is correlated with \(\ket{\psi_{1}}\) and \(\ket{\psi_{2}}\). Then the state of the quantum system, \(s\), and its environment, \(e\), is