Sequential measurements
Suppose an electron is passed through a Stern-Gerlach
device, and is found to have spin up in the Z direction. If
we pass it through a second Stern-Gerlach device, it will
always be found to still have spin up in the Z direction.
So this seems like an actual property of the spin, which
we are measuring with the Stern-Gerlach device.
In a similar way, we can tilt the Stern-Gerlach apparatus
90 degrees on its side and measure the component of
spin in the X direction. Here again, that component of
the spin is discrete: it is either up or down in the X
direction. In fact, we can rotate it by any angle that we
like, and measure the component of the spin in any
direction; and it will always be found to have a discrete
value, up or down, in that direction.
– p. 4/25