Quantum Toys for Quantum Computing: Persistent Currents Controlled by the Spin Josephson Effect

Quantum devices and computers will need operational units in different architectural configurations for their functioning. The unit should be a simple “quantum toy,” an easy to handle superposition state. Here such a novel unit of quantum mechanical flux state (or persistent current) 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 (the spin Josephson effect). It is demonstrated that by the use of two connected rings, 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 operation gate, and may be useful in quantum computing.

Figure 1

The system of chirality-driven persistent current with three ferromagnetic dots.

Figure 2

Two rings coupled (a) with one spin in common and (b) with two spins in common. The current state $J_2$ in the second ring is a result of a unitary transformation of $J_1$ specified by $(\theta ,\varphi )$. (c),(d): An example of operation on the flux by controlling ${\mathit{S}}_4$. In (d), a superposition state of current in the second ring is created from the $R$ state in the first ring.