COMPUTING WITH QUANTUMDOT CELLULAR AUTOMATA - Bio-Inspired and Nanoscale Integrated Computing - page 140

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Figure 4.34. Number of clock cycles through each stage of the processor.

the addition of two pieces of data, where the result of the addition was written
back to the internal memory. The total simulation time to perform this operation
was approximately five hours on the Pentium computer using a time-independent
simulation engine. In order to improve simulation performance, significant effort
is being invested in optimizing the available simulation engines, including the use
of multiprocessor computing clusters.

4.12. QCA FABRICATION

To date, QCA is a commercially unrealized technology. However, several ex-
perimental devices have been fabricated and are described in more detail in this
section. The different proposed implementations attempt to realize the required
bistable and locally interacting behavior required by the QCA paradigm. Of all the
implementations, four distinct classes of QCA implementations have emerged and
are listed below.

Semiconductor [3-5, 77-79]
Metal-Island [6, 7, 9-14, 16-26, 55]
Magnetic [42, 43, 80]
Molecular [27-35, 37-41, 81]

4.12.1. Semiconductor QCA
Room temperature QCA requires cells that are approximately 1-5 nm. Semiclas-
sical modeling of QCA cells shows that cells made any larger would be limited to
temperatures in the mK range [49, 50, 82]. Self-assembly of the quantum dots
offers a promising route to fabricating room temperature QCA; however, the
required homogeneity in quantum dot size and placement accuracy is not yet at a