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binary logic functions without the use of conventional current-based technology.
Within this scheme, the binary states “1” and “0” are encoded in the position
of electric charge. Variants exist but most commonly the basic cell consist of
a square (or rectangular) quantum dot ensemble occupied by 2 electrons. Elec-
trons freely tunnel among the quantum dots in a cell, while electron tunneling
between cells does not occur. Within a cell, two classically equivalent states
exist, each with electrons placed on the diagonal of the cell. Multiple cells cou-
ple and naturally mimic the electron configuration of nearest-neighbour cells. In
general, cell-cell interactions must be described quantum mechanically but to
a good approximation they are described simply by electrostatic interactions.
A line of coupled cells serves as a binary wire. When a terminal cell is forced by
a nearby electrode to be in one of its two polarized states, adjacent cells copy
that configuration to transfer that input state to the other terminus. This trans-
fer can happen spontaneously or can be zonally regimented by a clock signal
that controls inter-dot barriers, or some other parameter. The last key feature
of QCA is that three binary lines acting as computation inputs and one line
acting as output can converge on a node cell to create a majority gate. If two
of the three input lines are of one binary state, the fourth side of the node cell
will output the majority state. Variants of such an arrangement allow for the
realization of a full logical basis. To date, all manner of digital circuits have been
designed, from memories to multipliers to even a microprocessor.
While complex working circuits have not yet been realized, all the rudimen-
tary circuit elements have been already experimentally demonstrated [
3
,
4
]. Fur-
thermore, the input state of a QCA circuit has been externally controlled and
the output has been successfully read-out by a coupled single electron transistor
[
5
,
6
]. Until the present work, all available quantum dots, typically consisting of
thousands of atoms, had narrowly spaced energy levels requiring ultra-low tem-
perature to exhibit desired electronic properties. Moreover, approximately as
many wires as quantum dots were required to adjust electron filling, a scenario
that would greatly limit the complexity of circuitry that could be explored.
A prospect for highly complex and room temperature operational QCA cir-
cuitry suddenly emerged with the discovery of atomic silicon quantum dots.
Figure
1
shows a schematic 4-dot QCA cell on the left occupied with two elec-
trons (indicated by blackened circles). On the right is an STM image of a real
atom-scale cell made of 4 ASiQDs, the cell being less than 2 nm on a side. The
darker of the two dots are predominantly electron occupied.
The ultimate small size of the ASiQDs leads to ultimate wide spacing of
energy levels indeed suciently widely spaced to allow room temperature device
operation. The ASiQDs can be prepared in a native 1
charge state (charge is
expressed in elementary charge units henceforth). Close placement of dots causes
Coulombic repulsion and even removal of an electron to the silicon substrate
conduction band. By fabricating dots at an appropriate spacing, a desired level
of electron occupation can be predetermined, eliminating the need for many
wires. As all atomic dots are identical, and their placement occurs in exact
registry with the regular atomic structure of the underlying crystalline lattice,
−
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