  We study the Ruderman-Kittel-Kasuya-Yoshida (RKKY) interaction mediated by
helical edge states in quantum spin hall system. The helical edge states induce
an in-plane noncollinear exchange coupling between two local spins, in contrast
to the isotropic coupling induced in normal metal. The angle between the two
local spins in the ground state depends on the Fermi level. This property may
be used to control the angle of spins by tuning the electric gate.
