QCADesigner: A CAD Tool for an Emerging Nano-Technology

This paper describes research on a novel design and simulation tool for quantum-dot cellular automata (QCA), which we refer to as QCADesigner. A typical design window is shown in Figure 1. QCADesigner facilitates rapid design of QCA circuits by implementing different simulation engines and many standard CAD capabilities [1][2]. The results of a simulation are displayed in graphs such as the one shown in Figure 2. The use of quantum-dots is a promising technology for implementing digital systems at the nano-scale. Recently studied computational paradigms for quantum-dot technology include the use of locally connected quantum-dot cellular automata. This technique is based on the confinement of electrons within quantum dots that take advantage of quantum phenomena the same phenomena that may prove problematic in future integrated circuit technologies as feature sizes continue to decrease. Information flow in QCA is based on the local interaction of switching cells. These cells, constructed from a set of four quantum-dots, are charged with two extra electrons and exhibit bi-stable behaviour as a result of the Coulombic repulsion of the two electrons. The two stable states are used to encode binary information as shown in Figure 3. Coulombic forces causes adjacent cells to interact. The state of the cells in the system will always tend to the lowest energy. Circuit design involves creating a layout of QCA cells where the lowest energy state represents the correct output of a given function. This energy minimization causes cells in a linear array to synchronize their polarization and is used to transmit information through a “wire” by forcing the polarization of a cell at one end as shown in Figure 4. Figure 5 shows the layout of a majority gate which can be used to implement AND and OR functions [3]. By properly arranging the cells, any logic function can be created, such as a memory cell of a random access memory (RAM) shown in Figure 6 [4] or a barrel shifter as shown in Figure 7. The dynamics of the QCA cells can be modeled by the many-particle Schrödinger equation. This exact approach is computationally intensive and becomes prohibitive when even very few cells are involved. It has been shown that it is possible to model larger QCA circuits using the Hartree-Fock approximation [5]. This approach models the intracellular dynamics quantum mechanically and intercellular interactions classically by assuming that these interactions are not coherent and purely Coulombic. This approach can sometimes lead to erroneous results, which can be detected. In order to facilitate rapid design QCADesigner includes two important QCA models. The first uses an approximation of the cell to cell response function given by eqn. (1),