Two SG Detectors – at 90 degrees to each other.

First one (SG oriented along z axis) – exiting atom is +m (z axis) (or -m on the z axis). Ignore the -m ones and let the +m pass through the second analyzer

Second Analyzer (SG oriented along x axis)

Either they exit +m on the x axis or -m on the x axis.

Third  Analyzer  – same orientation (z axis) as first one. What happens if we let in all the +m (x) atoms?

Again, we get 50 – 50 pass through the two outlets of the SG. Because the z was measured waaaay back on appartus 1 . Since then, all info about that measurement is lost since it passsed through apparatus 2.

What most textbooks will descrube this as is ‘The atom is in NO particular Z state’ when it enters the third apparatus.

A more accurate way of saying this is that the atom is in the m (z) state – AND it is in ALL possible states on the x-axis. And all possible states on the y-axis.