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"A" illustrates the separation of either left/right or up/down spin components of electrons. If we take either the left or right spin electrons and pass them through the up/down separator, we lose our information about left/right spin. Similarly if we pass up or down spin electrons through a left/right separator, electrons in either the left or right stream will be found to have randomized the up/down component.
In "B," we take right spin electrons, feed them through an up/down separator, but reflect the streams back into a mixing box thus losing any information on up/down spin. If the electrons from the mixing box are then passed through a left/right separator they are found to be 100% right spin.
However blocking the path of the "up" stream electrons so they do not enter the mixing box, gives only "down" electrons in the mixing box. But then passing these electrons from the box through our left/right separator gives us a 50% mix of left/right spin electrons. It appears that we are not permitted to have knowledge of both kinds of spin at the same time. (reference: David Z. Albert, Scientific American 270 (5) 32 (1994))
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