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contacts and the molecular channel, it is very difficult to achieve high coupling
from the three contacts to the molecular channel. If the source and drain are
tightly coupled to the molecule, the gate remains very weakly coupled. As a result,
the on/off ratio of the device is very poor. Remember that the current success
of the metal-oxide-semiconductor transistor has been due to its nearly ideal
properties, which have been maintained in a succession of highly controllable
silicon technology advances. Finding a replacement that has all of these features
and is also faster and/or cheaper is a formidable task.
There has been great research interest in finding the ''next switch'' that will
replace the current transistor [1], but there have also been alternative proposals
to replace the underlying paradigm on which computers operate. One of these
paradigms was introduced to the research community in 1993 by C. S. Lent; he
published the first work on a novel paradigm for computing at the nanoscale that
is based on cellular automata, which he called quantum-dot cellular automata
(QCA) [2]. Rather than encoding information in the on/off state of a current
switch, the QCA paradigm encodes information in the electronic configuration
of coupled quantum dots [3-5], metallic islands [6-26], molecules [27-41], or
alternatively the magnetization in anisotropic nanomagnets [42, 43]. Information
is transmitted and processed via the electrostatic or magnetic exchange interaction
between individual QCA cells [44]. The potential advantages of QCA include high
device density, high operating speed, and low power dissipation; the degree to
which these advantages can be realized depends on the particular choice of
physical implementation. The lack of physical connection (at the molecular scale)
to each molecule and the fact that no current passes through each molecule makes
the paradigm more suitable as a molecular implementation of the switch [27].
The cellular automata computing architecture paradigm has been studied for
many years; in fact, John von Neumann, who is credited with inventing the stored
program concept for building computers—the concept on which most of today's
computer processors are still based—conceived the first cellular automaton in the
late 1940s [45]. Cellular automata (CA) are essentially discrete dynamic systems
whose behavior is completely specified in terms of local interactions with adjacent
cells. The cell is usually assumed to be a finite-state machine driven by a central
clock that synchronously advances time in all cells in the automata array. The
outputs of neighboring cells at a given clock pulse are used, by each cell, to
compute the cell's state following the next clock pulse. Most research has looked
at fairly simple cells that can be implemented with little hardware cost. The QCA
cell originally introduced by Professor Lent and his colleagues has a very simple
structure with only two states provided by the two energetically low-lying
electronic configurations of non-bonding electrons; the process by which electrons
move about in the cell is quantum mechanical tunneling. The cell is designed such
that these two states are energetically degenerate in the absence of any external
influence. The configuration becomes deterministic under the perturbing influence
of the electronic configuration of adjacent cells. A major feature of the technology
(besides having to be built at nanometer scales) is that the logic state of the cell is
determined by the position of extra charge in a fixed location on the substrate or
 
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