Quantum logic gate of a small ring with quantum dots

A novel quantum flux state in a conducting ring with three ferromagnetic quantum dots is presented. The state is labeled by the two directions of the persistent current, which is driven by the spin chirality of the dots, and is controlled by the spin. It is demonstrated that using two rings connected, one can carry out unitary transformations on the input flux state by controlling one spin in one of the rings, enabling us to prepare superposition states. The flux is shown to be a quantum XOR operation gate, and may be useful in quantum computing.