  Quantum interference effects in rings provide suitable means for controlling
spin at mesoscopic scales. Here we apply such control mechanisms to coherent
spin-dependent transport in one- and two-dimensional rings subject to Rashba
spin-orbit coupling. We first study the spin-induced modulation of unpolarized
currents as a function of the Rashba coupling strength. The results suggest the
possibility of all-electrical spintronic devices. Moreover, we find signatures
of Berry phases in the conductance previously unnoticed. Second, we show that
the polarization direction of initially polarized, transmitted spins can be
tuned via an additional small magnetic control flux. In particular, this
enables to precisely reverse the polarization direction at half a flux quantum.
We present full numerical calculations for realistic two-dimensional ballistic
microstructures and explain our findings in a simple analytical model for
one-dimensional rings.
