As a scientific pursuit, the search for a viable successor to silicon computer technology has garnered considerable curiosity in the last decade. The latest idea, and one of the most intriguing, is known as molecular computers, or moletronics, in which single molecules serve as switches, "quantum wires" a few atoms thick serve as wiring, and the hardware is synthesized chemically from the bottom up.
?????? The central thesis of moletronics is that almost any chemically stable structure that is not specifically disallowed by the laws of physics can in fact be built. The possibility of building things atom by atom was first introduced by Richard Feynman in 1959.
?????? An "assembler", which is little more than a submicroscopic robotic arm can be built and be controlled.? We can use it to secure and position compounds in order to direct the precise location at which chemical reactions occur. This general approach allows the construction of large, atomically precise objects by initiating a sequence of controlled chemical reactions. In order for this to function as we wish, each assembler requires a process for receiving and executing the instruction set that will dictate its actions. In time, molecular machines might even have onboard, high speed RAM and slower but more permanent storage. They would have communications capability and power supply.??????????????????????????????????????????????????????????????? ????????????????????????????????????????????? Moletronics is expected to touch almost every aspect of our lives, right down to the water we drink and the air we breathe. Experimental work has already resulted in the production of molecular tweezers, a carbon nanotube transistor, and logic gates. Theoretical work is progressing as well. James M. Tour of Rice University is working on the construction of a molecular computer. Researchers at Zyvex have proposed an Exponential Assembly Process that might improve the creation of assemblers and products, before they are even simulated in the lab. We have even seen researchers create an artificial muscle using nanotubes, which may have medical applications in the nearer term.
??????? Teramac computer has the capacity to perform? 1012 operations in one seconds but it has 220,000 hardware defects and still has performed some tasks 100 times faster than single-processor .The defect-tolerant computer architecture and its implications for moletronics is the latest in this technology.? So the very fact that this machine worked suggested that we ought to take some time and learn about it.
??????? Such a 'defect-tolerant' architecture through moletronics could bridge the gap between the current generation of microchips and the next generation of molecular-scale computers.
Recently, there have been some significant advances in the fabrication and demonstration of individual molecular electronic wires and diode switches. Some novel designs for several such simple molecular electronic digital logic circuits: a complete set of three fundamental logic gates: (AND, OR, and XOR gates), plus and adder function built up from the gates via the well-known combinational logic, was demonstrated. This means in coming future, this technology could be a replacement for VLSI. However, currently, this technology is only available under lab condition. How to mass product moletronic chips is still a big problem.
Currently, integrated circuits by etching silicon wafers using beam of light. It's the VLSI lithography-based technology makes mass production of Pentium III processor possible. But as the size of logic block goes to nano-scale, this technology no long available. As wavelength get too short, they tend to become X-rays and can damage the micro structure of molecules. On the other hand, the mask of lithography of Pentium III is so complex, and the shape and the dimension of its logic block varies so much. Looking at currently available integrated circuits, the transistor density of memory chip are much higher than processor chip, the reason is that the cell of memory is much more simple than circuit of processor. Because, except the decoding logic, most of the memory bit cell is the same. Could we find a way to fabricate complex logic circuit as Pentium processor using million of same logic units? The PLD(Programmable Logic Devices) is the answer. The paper is organized as following: section II presents some basic of moletronic gate circuit. section III uses PLD technology to build more complex blocks. section IV shows the nanotube can be used for interconnection wires.
We discuss an approach to computing with quantum dots, Quantum-dot Cellular Automata (QCA), which is based on encoding binary information in the charge configuration of quantum-dot cells. The interaction between cells is Coulombic, and provides the necessary computing power. No current flows between cells and no power or information is delivered to individual internal cells. Local interconnections between cells are provided by the physics of cell-cell interaction. The links below describes the QCA cell and the process of building up useful computational elements from it. The discussion is mostly qualitative and based on the intuitively clear behavior of electrons in the cell.
A QCA cell consists of 4 quantum dots positioned at the vertices of a square and contains 2 extra electrons. The configuration of these electrons is used to encode binary information. The 2 electrons sitting on diagonal sites of the square from left to right and right to left are used to represent the binary "1" and "0" states respectively. For an isolated cell these 2 states will have the same energy. However for an array of cells, the state of each cell is determined by its interaction with neighboring cells through the Coulomb interaction.
If the barriers between cells are sufficiently high, the electrons will be well localized on individual dots. The Coulomb repulsion between the electrons will tend to make them occupy antipodal sites. For an isolated cell there are two energetically equivalent arrangements of the extra electrons which we denote as a cell polarization P = +1 and P = -1. The term "cell polarization" refers only to this arrangement of charge and does not imply a dipole moment for the cell. The cell polarization is used to encode binary information - P = +1 represents a binary 1 and P = -1 represents a binary 0.
The two polarization states of the cell will not be energetically equivalent if other cells are nearby. Consider two cells close to one another. The figure inset illustrates the case when cell 2 has a polarization of +1. It is clear that in that case the ground-state configuration of cell 1 is also a +1 polarization. Similarly if cell 2 is in the P = -1 state, the ground state of cell 1 will match it.
The Programmable Logic Devices(PLD) are nothing new, they have been around for almost 20 years. Since PLD device exists, it makes the life of a lot of Electronic designer's life easy. It is well known that in order to design a digital system, besides microprocessors and peripheral ICs there are needed several other devices, such as lots of logic gates to glue these chips together. This circuits make our life and our printed boards very hard and complex. It exists a way to dramatically improve this way of design digital devices that, although it is not completely different from the others, brings the desired results more efficiently: in a shorter time and with fewer expenses. The way abovementioned is Programmable logic devices (PLD), they permit the customizing of one or more logic functions on a chip in contrast to the designer being restricted to defining a logic function with specific chips. The programmability aspect permits the logic designer to spend more time on the development and validation of high level functionality.
Unlike the traditional fully customised VLSI circuits, Field Programmable Gate Array(FPGAs) represent a technical breakthrough in the corresponding industry. Before they were introduced, an electronic designer had only a few options for implementing digital logic. These options included discrete logic devices (VLSI or SSI); programmable devices (PALs or PLDs); and Masked Programmed Gate Arrays(MPGA) or Cell-Based ASICs.
Today, one way to pack transistors more densely on a chip is to make the already microscopic wires smaller and thinner. But the wires are approaching the thickness of a few hundred atoms. Once wires get down to only several atoms thick, says IBM researcher Phaedon Avouris, they blow up when you try to send electrical signals through them. Nanotubes don't. IBM and others are racing to use nanotubes to make the first carbon chips, perhaps the successor to silicon chips, though the program is only in the earliest stages. A carbon nanotube is a tubular form of carbon with a diameter as smaller as 1 nm. The length can be from a few nanometers to several microns. (1 micron is equal to 1,000 nanometers.) It is made of only carbo atoms. To understand the CNT's structure, it helps to imagine folding a two-dimensional graphene sheet. Depending on the dimensions of he sheet and how it is folded, several variations of nanotubes can arise. Also, just like the singel or the multilayer nature of graphene sheets, the resulting tubes may be a single- or a multiwall type. The tube's orientation is denoted by a roll-up vector(See Fig.8) 
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Figure: Carbon nanotubes: their structure, properties  and uses in nano-electronic devices |
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Teramac is a massively parallel experimental computer built at Hewlett-Packard Laboratories to investigate a wide range of different computational architectures. It is a true supercomputer, capable of operating 100 times faster than a high-end workstation for some configurations. Teramac also contains about 220,000 defects, any one of which could prove fatal to a more conventional machine. The architecture of Teramac, the philosophy behind its construction, and its ability to tolerate large numbers of defects have significant implications for any future nanometer-scale computational paradigm. It is not necessary to chemically synthesize perfect devices with a 100% yield and assemble them into a completely deterministic network in order to obtain a reliable and powerful system. Future computers may not have a central processing unit, but may instead be an extremely large configurable memory that is trained for specific tasks by a tutor. Several concepts related to the logical architecture of Teramac are graphically presented here. (A) The Cross Bar represents the heart of the configurable wiring network that makes up Teramac. The inset shows a configuration bit (a memory element) that controls a switch. The bit is located and configured using the address lines, and its status is read using the data lines. The cross bar provides not only a means of mapping many configuration bits together into some desired sequence, but it also represents a highly redundant wiring network. Between any two configuration bits, there are a large number of pathways, which implies a high communication bandwidth within a given cross bar. Logically, this may be represented as a 'fat tree.' Such a 'fat tree' is shown in (B), where it is contrasted with a standard tree architecture. Note that both trees appear the same from the front view, but from an oblique view, the fat tree has a bandwidth that the standard tree does not. Color coded dots and a dashed box are included to show the correspondence between a given level of the fat tree and the cross bar in (A). See figure.9.
Figure: The Majority Gate |
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CONCLUSION
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