COMPUTING WITH QUANTUMDOT CELLULAR AUTOMATA - Bio-Inspired and Nanoscale Integrated Computing - page 121

Information flow

E

φ 1 φ 2 φ 3 φ 4

Figure 4.9. Submerged electrodes can be used to clock QCA cells by applying a
forward moving electric field at the level of the cells. The figure is not shown to
scale, as many cells can be placed between adjacent electrodes.

inherent clocked operation at the device level. Although CA architectures are not
normally implemented in most synchronous digital systems, we can look at
structures that have similar properties as being target candidates for QCA
implementation. Examples are bit-serial architectures and systolic arrays. Inter-
estingly, parallel designs, which require large fan-outs to the parallel logic
components, will introduce more latency because of the very nature of QCA
interconnects; thus, we will need to change our computing paradigms to match the
different cost functions inherent with using future QCA technology. Significant
work in these areas remains to be done.

4.5.2. Continuous Clocking
A method of clocking molecular QCA using a continuous clocking wave, realized
by generating a moving electric field by a network of submerged electrodes, has
been proposed in references [28, 31, 32]. Clocking networks using carbon
nanotubes have been investigated in [57]. The design of three-dimensional
molecular QCA cells enables a field in the vertical direction to modulate the
potential energy between the NULL state and the two ACTIVE states. By
applying phase-shifted sinusoidal potentials to each of the electrodes, a forward
moving wave can be generated at the level of the cells. Cells are latched at the
wavefront of this forward moving wave as illustrated in Figure 4.9. This enables
the cells to be clocked without influencing the cell-cell interactions. This method is
also valuable because there is no physical connection between the clocking
network and the QCA devices. The number of cells between any two electrodes
can be large, allowing the electrodes to be quite large when compared to the
individual cells.

4.6. INPUT/OUTPUT INTERFACING

Interfacing to QCA circuits requires a technology that can set and detect the
electronic configuration, or magnetization in the case of magnetic QCA, of single