Send a beam through a vertical <Stern-Gerlach spin measurement> apparatus, spatially resolve its two <spin> components, and block the negative-$z$ beam. The transmitted particles are prepared in $\boxed{|\uparrow\rangle}$. For an initially unpolarized beam, half the intensity survives ideally. This is conditional <quantum state preparation> by a <projective measurement> and filtering.

Solution

Past exam of the short options paper of the physics Prelims of the University of Oxford / 2022 / S03 / 2 / d / Solution

 Solution (source code)