COMPUTING WITH QUANTUMDOT CELLULAR AUTOMATA - Bio-Inspired and Nanoscale Integrated Computing - page 125

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Niether
polarization
is preferred

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(a) (b)

Figure 4.14. (a) Because of symmetry, when rotated and nonrotated cells are
placed adjacent to one another, they do not interact. (b) Recovering a digital
signal from the inverting chain is done by placing regular cells in between two of
the rotated cells. We recover either the signal or its inverse, depending on where
we tap off the inversion chain.

the two cells; the effect of one cell on the other does not break the energy
degeneracy between the polarization of the two cells. To convert a signal from
rotated to nonrotated wires the position of nonrotated cells has to be offset by a
half-cell, as illustrated in Figure 4.14b.
This effect can be exploited to implement wire crossovers. Consider two
standard, nonrotated cells placed at a separation of one cell. It is reasonable to
assume that the polarization effect of one cell is able to traverse this gap and
effectively break the energy degeneracy of the second cell. If we place a wire of
rotated cells in between these two cells, this interaction will not be disrupted
because of the geometric considerations discussed above. This property allows the
coplanar crossover to transmit information independently along the two different
cell wires: one wire comprised of regular cells and the other comprised of rotated
cells, as illustrated in Figure 4.15.
If there is no crosstalk between the two interconnects, a signal is able to
propagate across the gap. However, this building block is very sensitive to
fabrication variations [62, 63]. Variations in the position of cells or dots in cells
of this building block introduce crosstalk between the two interconnects, which
quickly dominate its operation. The problem stems from the cell gap between
input and output in the non-rotated cell wire. The coupling between the two wires
is 1/32 that of adjacent cells and the output wire is essentially floating.

4.9.2. Coplanar Crossover with Continuous Clocking
Blair et al. proposed a new approach to the coplanar crossover that does not
involve the use of rotated cells [30]. By using an eight-phase clocking scheme and
generating ACTIVE domains that were shorter than the NULL domains, Blair