Hierarchical design of quantum-dot cellular automata devices

We use hierarchical layout methods to combine fundamental quantum-dot cellular automata (QCA) devices into more complex devices, including a 2-to-1 multiplexer, a 4-to-1 multiplexer, and an eight-function bit-slice arithmetic logic unit (ALU). The ALU unit is implemented using 1245 cells, making it the largest QCA device ever designed and simulated. This hierarchical design method is possible because the Coulombic interaction between QCA cells is very local in nature. Since distant cells have very little effect on each other, the state of a cell can be very well approximated by considering only those cells that are near it. In a QCA device, nearby cells are usually part of the same functional unit, so we can design each functional unit separately, connect them with appropriate binary wires, and then simulate the entire device to verify its correct operation.