  We show that the edge of a two-dimensional topological insulator can be used
to construct a solid state Stern-Gerlach spin-splitter. By threading such a
Stern-Gerlach apparatus with a magnetic flux, Ahranov-Bohm like interference
effects are introduced. Using ferromagnetic leads, the setup can be used to
both measure magnetic flux and as a spintronics switch. With normal metallic
leads a switchable spintronics NOT-gate can be implemented. Furthermore, we
show that a sequence of such devices can be used to construct a single-qubit
$SU(2)$-gate, one of the two gates required for a universal quantum computer.
The field sensitivity, or switching field, $b$ is related to the device
characteristic size $r$ through $b = \frac{\hbar}{qr^2}$, with $q$ the unit of
electric charge.
