Imagine a two-state quantum system in which a photon can be found only at two positions, (x1)(x_1) or (x2)(x_2) , with x2−x1=l.x_2-x_1=l. Suppose the expectation value of position evolves as ⟨x⟩=l2cos⁡(2πtT).\langle x\rangle=\frac{l}{2}\cos\left(\frac{2\pi t}{T}\right). How do we obtain a classical approximation of such a two-state system? As far as I understand, the usual correspondence 1iℏ[A^,B^]→{A,B}PB\frac{1}{i\hbar}[\hat A,\hat B]\rightarrow \{A,B\}_{\mathrm{PB}} is not by itself a complete prescription for obtaining the classical limit. More generally, I am trying to understand: what is the standard procedure for going from a quantum system to its classical approximation, especially when the quantum system has only two states?