Could you comment this optical device? (I can provide a link) A polarized photon a H> + b V> begins to hit a beam splitter. Then In the device components can rotate its phases, mix paths and so on. And all this depends on parameters (possibly functions of time). There are two output channels. in the first it is in the state with the probability output by the other case it is in the state [tex] a cos \alpha H> +b sin \beta V>[/tex] with the probability [tex] a^2 cos^2 \alpha +b^2 sin^2 \beta [/tex] In the second path it is in the state [tex] a sin \alpha H> +,b cos \beta V>[/tex] with the probability [tex] a^2 sin^2 \alpha +b^2 cos^2 \beta [/tex] The POVM is easily seen with two effects summing to Id. If alpha and beta are equal to pi / 4 the initial state is not modified. both effects are equal to Id/2 and the probability for each arm is 1/2 in the opposite case if alpha and beta = pi/2 the measuresure is like in a projective Stern Gerlach. As these angles may depend on time, we have continuous way to evolve from a set of equal effects to a set of orthonormal effects which define a PVM with a privileged basis (H> <H and V> <V) Is this peculiar to this device or is this a general procedd?