COMPUTING WITH QUANTUMDOT CELLULAR AUTOMATA - Bio-Inspired and Nanoscale Integrated Computing - page 124

these cells. The first involves a QCA network that distributes a constant
polarization to all the AND and OR gates, inverting where necessary. In this
case, the fixed polarization is provided as a separate input to the circuit. Only one
such input is required. The second approach involves a dot-level manipulation
of cells within the circuit or the controlled placement of fixed charges. It is
theoretically possible to implement a fixed polarization cell by simply removing
two quantum dots from the cell, leaving only the two dots associated with the
desired diagonal. The use of fixed polarization rather than a distribution network
results in circuits that consume far less area and are not prone to errors associated
with the large network required by the first approach. Significant work remains to
be done in this area.

4.9. WIRE CROSSING

The majority gate, inverter, and wire are insufficient to design and build a complex
QCA circuit. Two other building blocks are required: the fanout and a method of
wire crossing. The fanout can be implemented by joining two QCA wires to the
same node. The wire crossing represents a far more difficult problem and is still
being investigated. In standard technologies, wire crossings are implemented with
several layers of metallic interconnects. The situation is quite different for QCA
circuits, where such metal layers are will probably not be available and converting
between the electronic configuration of QCA cells and voltages in wire inter-
connects is not practical.

4.9.1. Coplanar Crossover
In earlier research, Tougaw et al. proposed an additional type of cell to create a
coplanar crossover [61]. The cell is the same size as the standard cell except the
dots are rotated by 45

as shown in Figure 4.13, where we also show the
information encoding in these rotated cells.
As a result of the geometry of these cells, each cell in a wire will relax to the
opposite polarization of its nearest neighbors. When placed directly adjacent to
one another, rotated and nonrotated cells do not interact with each other, as
shown in Figure 4.14a. This lack of interaction is a result of the symmetry between

1

Binary value = 1
Polarization = 1

Binary value = 0
Polarization = 1

−

Figure 4.13. 451-rotated QCA cells, showing the definition of polarization and
binary encoding.