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molecule, rather than by the flow of charge in and out of the cell, which is how
logic states are changed in standard CMOS-based logic. Since the movement of
charge during logic state switching gives rise to most of the power dissipation in
modern CMOS logic, QCA technology promises very low power dissipation.
4.2. THE CAD TOOL: QCADesigner
When commercial fabrication processes have reached a stage of advancement where
any emerging technology is an economic reality, the potential of that technology
should already have been explored and uses for the technology found. In fact, this
vertical approach may even change the amount of effort spent on developing a
technology based on, for example, the uses (or lack of them) found and the
fabrication tolerances required. The best way to evaluate a technology is to provide
tools for design and simulation and then to make these available to a large group of
future designers within that technology. These tools, with sufficiently accurate
simulation models and computational techniques, are able to predict the level of
success of nanocircuit designs, even though such nanotechnology is not available.
Throughout this chapter we will be illustrating the design concepts and showing
simulation results using the QCA CAD tool, QCADesigner [46]. QCADesigner
provides a comprehensive design and layout tool, along with a simulation engines
that uses two different quantum mechanical approximations. The tool is currently
available as a free download from http://www.qcadesigner.ca/ and provides an
excellent vehicle for exploring the concepts discussed here. The various elements of
QCADesigner will be introduced, where required, later in the chapter.
4.3. THE QCA CELL
QCA computing involves the interaction of many QCA cells, each constructed
from a square pattern arrangement of four quantum dots. It is important to note
that although initial work on QCA focused on semiconductor quantum dots,
recent research has included cells implemented from single molecules [27-41] as
well as magnetic nanoparticles [42, 43]. The electronic QCA cell is charged with
two mobile nonbonding electrons that repel each other as a result of their mutual
Coulombic repulsion, and, occupy one of the two diagonals of the cell in the
ground state. These two states are referred to as the ''ACTIVE'' states and are
used to encode binary information as depicted in Figure 4.1. Other higher energy
states can be ignored if the energy of those states is sufficiently high.
The quantum dots can be coupled through a quantum mechanical tunneling
barrier that enables the controlled transfer of electrons from one site to another.
The cells are typically illustrated in the literature as having a bounding box (shown
in Fig. 4.1). This bounding box does not necessarily have a physical structure; it is,
rather, used for illustration and design purposes to isolate one cell from the next.
Some implementations are based on double dots, two of which are required to
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