  A design for a quantum gate performing transformations of a single electron
spin is presented. The spin rotations are performed by the electron going
around the closed loops in a gated semiconductor device. We demonstrate the
operation of NOT, phase-flip and Hadamard quantum gates, i.e. the single-qubit
gates which are most commonly used in the algorithms. The proposed devices
employ the self-focusing effect for the electron wave packet interacting with
the electron gas on the electrodes and the Rashba spin-orbit coupling. Due to
the self-focusing effect the electron moves in a compact wave packet. The
spin-orbit coupling translates the spatial motion of the electron into the
rotations of the spin. The device does not require microwave radiation and
operates using low constant voltages. It is therefore suitable for selective
single-spin rotations in larger registers.
