Chapter 3 QCA

QCA is a novel emerging technology in which logic states are not stored as voltage levels, but rather the position of individual electrons. Conceptually, QCA represents binary information by utilizing a bistable charge configuration rather than a current switch. A QCA cell can be viewed as a set of four “dots” that are positioned at the corners of a square. A quantum dot is a site in a cell in which a charge can be localized. The cell contains two extra mobile electrons that can quantum mechanically tunnel between dots, but not cells. In the ground state and in the absence of external electrostatic perturbation [1], the electrons are forced to the corner positions to maximize their separation due to Coulomb repulsion. As shown in Figure 3.1, the two possible charge configurations are used to represent binary “0” and “1”. Note that in the case of an isolated cell, the two polarization states are energetically degenerate. However the presence of other charges (neighbor cells) breaks the degeneracy and one polarization state becomes the cell ground state [1]. Polarization P measures the extent to which the charge distribution is aligned along one of the diagonal axes. If the charge density on a dot i is ρi, then the polarization is defined as [2] [3]: