  We discuss the transport properties of a quantum spin-Hall insulator with
sizable Rashba spin-orbit coupling in a disk geometry. The presence of
topologically protected helical edge states allows for the control and
manipulation of spin polarized currents: when ferromagnetic leads are coupled
to the quantum spin-Hall device, the ballistic conductance is modulated by the
Rashba strength. Therefore, by tuning the Rashba interaction via an
all-electric gating, it is possible to control the spin polarization of
injected electrons.
