  We study theoretically the spin transport of a Quantum Spin Hall round disk.
When an electron traverses the disk in virtue of the edge states, its spin's
in-plane component can be rotated by a magnetic flux through the disk. The spin
rotation occurs due to the interference of two helical edge states with
opposite spins, which is regarded as the Aharonov-Bohm effect in the spin space
and a manifestation of the Berry phase. Besides, the disk has a spin filter
effect on the tunneling current when we apply an appropriate magnetic field and
gate voltage on it. The spin polarization ratio can reach 100% when the
couplings between the disk and leads are weak.
