During the past 70 years, enormous progress in the theoretical foundations of computer science, in materials science and integrated circuit fabrication, and in systems design and integration have led to fantastic increases in the computational power, flexibility, and affordability of computers, along with a surprising and equally fantastic decrease in the sizes of, and power consumption and dissipation by, modern computer systems. A currently available, inexpensive desktop personal computer provides far more computing power than a supercomputer of twenty years ago. Much of the progress in computing can be attributed to a steady increase in the density of circuitry that can be manufactured in integrated circuits resulting from a steady decrease in the widths of signal lines and dimensions of submicroscale electronic components that can be fabricated by photolithographic processes. Unfortunately, the tiny dimensions at which signal lines and submicroscale electronic components can be manufactured may be approaching physical limits to further size decreases. Further increases in the density of fabricated submicroscale electronic components may depend on using a very different fabrication strategy, rather than photolithography-based methods. Continued progress in computing may depend either on developing new integrated circuit fabrication methods and materials, or may instead depend on finding entirely new strategies for computing, such as quantum computing, massively parallel computer architectures, or other such innovations.
During the past decade, an entirely new fabrication method for nanoscale electronic circuits and nanoscale electronic components has begun to be developed, and has become a foundation of the emerging field of molecular electronics. One promising type of nanoscale-component fabrication process is based on nanoscale crossbars composed of nanowires, with passive and active electronic components, including resistors, diodes, and various types of transistors, fabricated at selected points of overlap between approximately perpendicular nanowires in stacked, orthogonally oriented layers of parallel nanowires. Working nanowire-crossbar circuits have been fabricated in research laboratories, and have been integrated with conventional submicroscale circuitry to produce tiny, high-density memories and logic circuits. Although nanowire crossbars represent an exciting and promising approach to fabrication of computer components at molecular dimensions, much additional research and development effort is needed for commercial production and integration of nanowire-crossbar-based computer components. Many issues remain concerning the reliability of fabrication of passive and active electronic components at nanowire junctions, and much effort will be needed to efficiently construct dense circuitry at molecular dimensions. Furthermore, it remains a challenge to fabricate fully functional processors using nanowire crossbars, because it is not currently easy to store computed values in nanowire crossbars and to route stored, computed values from one storage location to another within nanowire crossbars. For these reasons, researchers, developers, and manufacturers of submicroscale electronics have recognized the need for simple, universal computing devices, with the full computational power of Turing machines (simple theoretical computing devices used in theoretical studies of computability and undecidability that can solve the class of problems solvable by all currently known computing devices), that can be practically fabricated at molecular dimensions and that can be practically controlled and operated to perform general computation tasks.