COMPUTING WITH QUANTUMDOT CELLULAR AUTOMATA - Bio-Inspired and Nanoscale Integrated Computing - page 118

TABLE 4 . 2 . Theoretic Orders of QCA Cell Switching Frequencies.
Implementation Frequency

Molecular [44] THz

Semiconductor [51] THz

Metallic-Island [25] GHz

Magnetic [42] MHz

provided, the cell will relax to the correct ground state. Changes in the boundary
conditions (input values) raise the system energy and cause the system to relax to a
new ground state and a new output. Original work on QCA focused on an edge-
driven approach where the QCA circuit was provided inputs at the edge of a
computational array, thus raising the system energy by introducing kinks at these
inputs and allowing the system to relax to the correct ground state [2, 52, 53].
However, this approach limits the size of the circuit, since larger circuits will have
a tendency to stick in one of many unwanted metastable states, as illustrated in
Figure 4.6. The ability of the circuit to find the correct ground state can be
enhanced with the application of adiabatic clocking [44].

4.5. CLOCKING

As with standard technologies, QCA clocking provides a mechanism for syn-
chronizing information flow through the circuit. However, unlike standard

Application of new input

Metastable state

Ground state

Correct ground state

Time

Figure 4.6. In the edge-driven computation approach, inputs are applied to one
side of the circuit and the circuit is allowed to relax. Correct operation requires
that the circuit settles to the correct ground state and does not get stuck in a
metastable state.