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4
COMPUTING WITH QUANTUM-
DOT CELLULAR AUTOMATA
Konrad Walus and Graham A. Jullien
This chapter describes a promising emerging technology for nanoscale computing
based on cellular automata. Rather than relying on the on/off state of a current
switch to encode information, this paradigm, called quantum-dot cellular auto-
mata (QCA), represents binary information in the electronic configuration of a set
of coupled quantum dots, molecules, or magnetic nanoparticles. QCA logic and
circuits are implemented by connecting various logic gates using arrays of QCA
cells, which also make up the interconnect network. Control over information
flow and synchronization, as well as true power gain, is possible by clocking the
cells. Using the available building blocks, several circuit examples are provided,
together with a description of available computer-aided design tools. This chapter
also presents a description of recent experiments and several promising imple-
mentations of QCA.
4.1. INTRODUCTION
The seemingly endless progress of microelectronics is the result of the ability of the
semiconductor industry to continuously scale down the transistor. However, there
are challenges to this scaling as the lateral dimensions of transistors approach a
few tens of nanometers, and the gate oxides are reduced to just a handful of atoms.
These challenges include leakage currents through the gate oxide, the discreteness
of impurities, low on/off ratios due to drain induced barrier lowering (DIBL), and
very high power dissipation. When scaled down to the level of single mole-
cules, several additional problems emerge. A molecular transistor would require
a channel, implemented with a single molecule, and three contacts, most likely
implemented with metal. Given the significant size difference between the metallic
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