1 Introduction
The Stern–Gerlach experiment1–4 played an essential role in
the early development of quantum mechanics. The combination
of a simple experiment and a striking result inspired
Einstein, Pauli and many others. Due to its conceptual
importance the experiment is still discussed in contemporary
textbook accounts on quantum theory.5–8 An elaborated
version of the Stern–Gerlach experiment sometimes referred
to as a Stern–Gerlach filter was introduced by Wigner9 to
illustrate how one can utilize the Stern–Gerlach idea as a basis
for a measuring device. Sometimes the SG setup is referred to
as a ideal quantum measurement of a spin state of a particle.