HISTORY AND BASIS OF QUANTUM COMPUTING

Transcription

1 CHAPTER 1 HISTORY AND BASIS OF QUANTUM COMPUTING 1.1 HISTORY Idea and concept of quantum computing was introduced way backs in 1970s and 1980s, by Richard Feynmann, David Deutsch and Paul Benioff. First breakthrough of the concept was reported in 1994 in Peter Shor s factoring algorithm, followed by Lov Grover, searching algorithm in Isaac L. Chuang, developed the world s first 2-qubit, 3-qubit, 5-qubit and 7-qubit quantum computer in In classical computing, information unit is a bit. The physical states of bits are two logically known as bit 1 and bit 0. These are commonly realized by physical ON state and OFF state of transistor. In quantum computing, equivalently two quantum states define basic information processing unit known as qubit. The representation of qubit 0 and 1 are as shown below: FIGURE 1.1 The major features of quantum computing revolve around followings: 1. Quantum nature: a combination of both the qubits 2. In preparing the initial state: only one of the two qubits 3. On measurement: only one state found 4. Probability: the state s component in the mix 5. Both preparation and measurement in contact with a macro system The basic of quantum computing starts with the preliminary knowledge of quantum beam splitting, quantum interference and quantum entanglement. An account of the topics are given in Appendix-A. 1

2 2 INTRODUCTION TO QUANTUM COMPUTING Quantum computing is a subject of Nano-system that is supposed to rule the world of future computing. How do we quantify nano world? How small is a nanometer? It is as below: 1. 1 meter µm = Size of red blood cell 3. 1 µm = Millionth of a meter nm = Size of polio virus 5. 1 nanometer = A billionth of a meter nm = Size of the hydrogen atom 7. 1 picometer = A trillionth of a meter 8. 1 femtometer = m, size of a proton. The necessity of nano world of computing is due to possible failure of further level of chip integration in near future. The quantum effect heavily stands on the way of going on with integration in tune with Moore s law. Since the inception of digital electronic in the brand name of ENIAC in 1948, the computer has gone through a number of generations, and it is now in the fifth generation. The so vast and rapid changes of five generations of the computer technology just over a period of 50 years results in one hand the reduction of size & cost of computers and on the other hand, the tremendous increase in the processing power & capacity of computers. The credit for these is due to IC (Integrated Circuit) technology. Out of many others, the famous empirical laws known as Moore s laws, basically, govern the pattern of growth of computers and that of IC technology. Mr Gordon Moore, Head of Research and Development of Fairchild coined these laws around Moore s laws state that (a) the number of components on an IC would double every year (this is the original Moore s law), (b) the doubling of circuit complexity on an IC every 18 months (this is known as revised Moore s law), and (c) the processing power of computer will double every year and a half (Moore s second law). Presently ICs are made of around 250 million transistors. Table 1.1 illustrates the growth of transistors integration on chip over years. If Moore s law continues to hold good, it is predicted that by 2010 ICs will be made of billion transistors. The threats to the survival of Moore s laws are heat dissipation and quantum effect that is a physical limit to IC integration. Several predictions were therefore earlier made for imminent death of Moore s laws. Contrary to these predictions, Moore s laws are

3 HISTORY AND BASIS OF QUANTUM COMPUTING 3 surviving and hold true for IC integration. Recent two research reports have further showed confidence of survival of Moore s laws for at least another few years. A survey conducted jointly by IEEE (Institute of Electrical and Electronics Engineers) and the Response Center Inc of USA (a market research firm) over the fellows of IEEE showed that 17%, 52% and 31% respondents respectively predicts the Moore s laws continuation for more than 10 years, 5-10 years and less than 5 years. The average predicted life term for the laws is then about 6 years. Moore s laws existence if then guaranteed up to 2009, by the time of which following the laws the billion transistors IC will be a reality. The expectation of realizing billion transistors IC by 2010 has been further brightened by the current research of Intel expanding Moore s laws. Mr. Pat Gelsinger s vision of expanding Moore s laws includes Intel s 90 nanometer fabrication process. Although a several alternative technologies, namely quantum computing, bio computing, molecular electronics and chemical computing are under investigation as possible replacement digital computing, the year 2010 may achieve the landmark of billion transistor IC, an another leap forward in IC technology really a high hope and not a hype. In the chip level integration till date, Moore s laws say the last word. From SSI to ULSI, the trend set (Table 1.1) by Moore s law is followed. But beyond ULSI, what is there? The extrapolation of the trend predicts that the future will be the age of molecular dimension inherited by the already established subject of molecular electronics that is based on organic materials rather than inorganic semiconductor. Beyond ULSI, the further integration on a chip will face serious problem from physical constrain like the quantum effect. This may lead to the death of Moore s law. But another interesting dimension may be added to the cause of the death of Moore s law. This is based on the law of Price and Power. It is said that: The price per transistor will bottom out sometime between 2003 and From that point on, there will be no economic point in making transistors smaller. So Moore s law ends in a few years. In fact, economies may constrain Moore s law before physics does. If you do see an end [to the law], it will be an economic end, not a technical end.we may one day may have to go to some sort of nano-technology, with selfassembly of molecules, and so on, and that might not show the same economics. But we have a long time; semiconductors will be around for another 15 years at least experts say. Time will give the correct answer. CHAPTER 1

4 4 INTRODUCTION TO QUANTUM COMPUTING TABLE 1.1. Generation of IC Integration Generation Number of Components Small Scale Integration (SSI) 2 64 Medium Scale Integration (MSI) Large Scale Integration (LSI) ,000 Very Large Scale Integration (VLSI) 64,000 2,000,000 Ultra Large Scale Integration (ULSI) 2,000, ,000,000 To go beyond the conventional laws mentioned above, the computer technology is believed to take to new directions: molecular electronics and quantum computers. The subject of nano computing started with the concept of molecular electronics followed by chemical computing. The subject of molecular electronics has emerged as an important area of research and application during 1980 s. The definition of molecular electronics is not unique and simple. Even within a country scientists differ. A leading scientist of the field molecular electronics can be divided into two main themes: these are molecular materials for electronics (MME) and molecular scale electronics (MSE). The topic of molecular materials for electronics deals with the use of macroscopic properties of organic materials in devices, and includes current and near-term application. In the near-term it seems likely that conductive polymers will offer the prospect of novel electronic devices and that organic materials with pronounced non-linear optical properties will find application in up to electronics. A simplistic extrapolations of the reduction of time leads eventually to the molecular scale i.e., molecular scale electronics. Prof. Bloor further observed many regard the quest for molecular scale devices as true molecular electronics. However, it can be argued that the distinction between MME and MSE is somewhat arbitrary and that both need to be considered as constituent parts of molecular electronics if the topic is to grow and prosper. Ashwell, Sage and Trundle defined that its definition has broadened from electronics at molecular level to include molecular materials with potential electronics and photonic applications. Peterson defined that in the most general sense, molecular electronics covers the use of molecular (and hence essentially organic) materials to perform signal processing or transformation function. However, the famous Link programme of Britain defines molecular electronics as systematic exploitation of molecular,

5 HISTORY AND BASIS OF QUANTUM COMPUTING 5 including macro-molecular, materials in electronics and related area such a photo-electronics. The molecular electronics is therefore to explore the potential application of organic materials and non-linear optics in the field of electronics. It is a highly interdisciplinary field and prospects lies on the successful interaction and cooperation of scientists of different fields like biology, chemistry, computing, physics and electronics History of Molecular Electronics CHAPTER 1 In history the concept of molecular electronics dates back to the last century. The familiar example is the use of organic materials in displays. The use of liquid crystals display found in watches, calculators and TV sets in historically patented over fifty years ago. As Prof. Bloor pointed out that molecular exhibit great variety in their structure and properties from simple diatomic species through to very large synthetic and bio-macromolecules. It is not surprising therefore that molecules can be found that process unique combination of properties which find application in fields of electronics and opto-electronics. This idea stimulated work on MME since 1950s. The reduction of size of active electronic device compound problems in regard to quantum effects. At this juncture, molecular electronics, the application of molecular materials in electronics, started exploiting some of the new advanced technologies that may be beyond the scope of the silicon chop. Prof. Bloor explained that the continuing development of silicon micro-electronic devices of smaller size and grater complexity has brought more compact and powerful instrumentation and computing facilities into the laboratory and office. Though silicon technology holds a dominant position the continuing reduction in dimensions of an individual device creates problems both at the fundamental and systems level. On one hand, quantum effects must ultimately come into play dissipation and the design of testable architectures are already with us. These pressures lead inevitably to a search for alternatives to current technology that can offer prospects for the realization of devices with even higher densities of active components. MSE is one avenue which is being explored with these targets in mind. The research and the interest in molecular electronics were mainly initiated by the late Forest Carter who conducted a series of international conferences on molecular electronics in 1980s. Prof. Bloor wrote that organic solids have attracted the interest of materials scientists and solidstate physicists since the 1950s both as alternative semiconductor and because of their optical properties. Strong research groups grew up in the USA, Russia, Germany and France at this time.

6 6 INTRODUCTION TO QUANTUM COMPUTING Although the progress of molecular electronics has not always been smooth, yet the prospects for the future are good. In this article, we shall review the present position and future aspects of molecular electronics Molecular Materials for Electronics (MME/M 2 E) The study of MME is to see the use of molecular materials in key and active roles in electronic and opto-electronic devices and systems. It is based on understanding and use of macroscopic properties of the bulk molecular materials i.e., of the organic materials. The main categories of MME are: Organic semiconductors and metals Liquid crystalline materials Piezo/ Pyro-electric materials Photo/Electro-chromic materials Non-linear optical materials/photonics. Organic Semiconductors and Applications: Organic semiconductors and metals have been much less studied than their inorganic counterpart. Under MME, a good study is gradually emerging. The major applications of organic semiconductor are in (1) electronic active devices and (2) xerography. Therefore before going to organic semiconductors, the process in amorphous materials is required to be studied. What are amorphous materials? In crystal, atoms or molecules are arranged in a regular structure with periodicity. But in amorphous materials there is no ordered structure. The developments of electronic devices in last few decades were tremendous because the electrical conductivity of crystalline semiconductors such as silicon can be controlled over much order of magnitudes by doping. But there are a number of areas where the expenses of preparing. These crystals and where the limited size to which they can be grown (at present about 25 cm in diameter) have prevented any very large area applications. For example, crystalline silicon solar cells are widely used in space vehicles for converting sunlight into electrical power, but the economics of their production is such that their use here on earth is relatively limited. Silicon can be prepared very cheaply in large areas by vacuum evaporation or by sputting, but the materials is then amorphous rather than crystalline sine (the) work on doping amorphous silicon (a-si) was published, there has been a considerable research into and development of this materials, leading to a member of commercial products.

7 HISTORY AND BASIS OF QUANTUM COMPUTING 7 MME makes a study with electronic processes as distinct from ionic processes, in organic crystals. What are organic crystals? By organic we usually mean a compound containing carbon. Almost 90% of 2 millions compounds known to us are organic. But for MME, there is choice and limitation that need a careful study. Till today organic materials have not presented to be a real competitor to the silicon/inorganic material in terms of active electronics devices. However, during last five years the progress in the synthesis of high purity semiconductor polymers and oligomers is note worthy. Experiments showed that conductive polymers could be employed as either metallic or the semiconducting component of metal-semiconductor junction devices. Semiconducting polymers can be used to produced Schottky diodes. Where the polymer has temperature dependent properties have been observed, with rectifying behavior at room temperature changing to ohmic behavior above 100 C. Burroughes et al., first reported an active polymer transistor in The important characteristic of this device were: (1) no chemical doping or side reactions and (2) the characteristic of the polymers device was insensitive to disorder. But the major disadvantage of the device was that its maximum operating frequency was limited. This is because the carrier mobility in the amorphous polyacetylene layer is very low. The mobility s of electrons in semiconducting polymers, amorphous silicon and crystalline silicon are of the order of 10 4, 1 and 10 3 cm 2 /Vs respectively. One can see the large gap between properties of polymers and silicon. However, a dramatic lead was done by Frincis Garnier and co-workers. They reported a totally organic transistor. This transistor is known as thin film transistor (TFT) or organic FET. This transistor is a metal insulator semiconductor structure comprising an oxidized silicon substrate and a semiconducting polymer layer. It has grater flexibility and can even function when it is bent (disorder is acceptable). The operating speed is still poor. The problem of low carrier mobility of insulating polymer is under active research. The diodes made of semiconductor with rectification s ratios in excess of 10 3 have been reported in, and light emitting diodes, made in organic semiconductor with external quantum efficiencies in excess of 1% photons per electrons are reported in; and organic photovoltaic cells are reported in. However, within a short period, a rapid progress has been observed on use of semiconductive polymers and oligomers in electronic devices. If this progress is maintained, in near future it could be competitive to silicon. CHAPTER 1

8 8 INTRODUCTION TO QUANTUM COMPUTING The field of optical computation starts with the search of a bi-stable optical switch based on non-linear optical properties of materials. Nonlinearity can be used for device basically by two techniques: frequency conversion and reflective index modulation. The frequency conversion technique, which is due to second order non-linearity, may be used to second harmonic generation frequency mixing and parametric amplification etc. Refractive index modulation particularly Kerr effect which is due to third order non-linearity may be used for optical bi-stable switches and parallel processing. Till date a few optical gates and all optical bi-stable switches have been reported, but the field is still confined in the laboratories. Yet optical computation is a promising field. Optical computing and processing of information are the important application of photonic. The gain of photonics switching speed (of order of femto second ) is many order of magnitudes over that of electronic switching. Optical processing is free from interference from electrical or magnetic sources. Based on the prospect of three dimensional interconnectivity between sources and receptors of light concepts of optical neutral networks that mimic the fuzzy algorithms by which learning takes place in the brain have been proposed and experimentation has begun. Integrated optical circuits, which are counterparts of electrical circuits photons, can provide for various logic, memory, and multiplexing operations. Utilizing non-linear optical (NLO) effects, analogs of transistors or optical bistable devices with which light controls light have also been demonstrated. So far NLO materials are concerned, all materials in forms of gases, liquids or solids, exhibit NLO phenomena. However, broadly we can defined two classes of NLO materials: (1) molecular materials or organic materials which consist of chemically bonded molecular units that interacts in the bulk through weak van der Waals interactions and (2) bulk materials and traditional inorganic materials. Today rapid progress and research in organic NLO materials proved to be attractive. The NLO devices utilize two different techniques: frequency conversion and refractive index modulation. Based on letter effects, the developments of frequency converter and light modulator have been reported elsewhere. However, organic materials are seen to be quite attractive for electro-optic light modulation as their low-frequency dielectric constant is quit low leading to a small RC time constant, thus permitting a higher bandwidth for light modulation compared to that achievable using inorganic materials.

9 HISTORY AND BASIS OF QUANTUM COMPUTING 9 The application of second order non-linearity needs that the crystal must not be centrosymmetric structure. In centrosymmetric structure the non-linearities, which are vectorial, cancel each other to give zero microscopic effect. This is a stumbling block in the progress of application of second order non-linearity. To solve the problem two approaches are being examined: 1. Use of LB films with either alternating layers of a polar molecule or molecules which inherently from polar multi-layers. 2. Inclusion of non-linear optically active molecules in polymer films which are poled with an applied electrical field. In a single way, a materials with a bulk where, its molecules are noncentrosymmetric nature may be defined as an isotropically oriented over volumes measure in cm 3. These conditions are best achieved by growing a crystal. The Langmuir-Blodgett (L-B) technique is a comparable high tech organic fabrication method, appropriate when the implementation of the function requires a high degree of molecular an isotropy in an extremely thin layer of uniform thickness. For OICs, particularly for single processing, L-B technique offers the possibility to orient molecules with in a thin layer of highly precise thickness. It has thus become an attraction. However, films are not the final answers. There are many drawbacks with films namely mechanical softness, limited high temperature range and extremely slow rate of deposition etc. But rapid research is going on L-B film technology and its application in molecular electronics materials both for ME and MSE. CHAPTER Molecular Scale Electronic (MSE) The quest for an ever decreasing size but more complex electronic components with high speed ability gave the birth of MSE. The concept that molecules may be designed to operate as a self-contained device was put forwarded by Carter, and he only proposed some molecular analogous of conventional electronic switches, gates and connections. Accordingly Aviram and Ratner first advanced a molecular P-N junction idea. MSE is a simple interpolation IC scaling. Scaling is an attractive technology. Scaling of FET and MOS transistors is more rigorous and well defined than that of bipolar transistor. But there are problems in scaling of silicon technology. In scaling on the one hand propagation delay should be minimum and packing density should be high; on the other hand, these should not be at the expenses of the power dissipated. With these scaling rules in minds, scaling technology of silicon is to reach a limit. Another thing is that scaling can due to the quantum nature of physics. At this junction molecular scale scaling technology.

10 10 INTRODUCTION TO QUANTUM COMPUTING Dr. Barker reported in that change, spin, conformation, color, reactivity and lock-and-key recognition are just a few examples of molecular properties, which might be useful for representing and transforming logical information. To be useful, molecular scale logic will have to function close to the information theoretical limit of one bit on one carrier. Experimental practicalities suggest that it will be too easiest to construct regular molecular arrays, preferable by chemical and physical self-organization. This suggests that the natural logic architectures should be cellular automata: regular arrays of locally connected finite state machines where the state of each molecule might be represented by color or by conformation. Schemes such as spectral hole burning already exist for storing and retrieving information in molecular arrays using light. The general problem of interfacing to a molecular system remains problematic. Molecular structures may be the first to take practical advantages of novel logic concepts such as emergent computation and floating architecture in which computation is viewed as a self-organizing process in a fluidlike medium. MSE spans several disciplines and requires a coordination of scientists of different group if the subject is to grow and prosper based on cross fertilization of ideas of different subjects. But problem is how can the properties of individual molecules and/ or small aggregates be studied? Fortunately day-by-day, we are evolving new techniques and methods to tackle this problem. At present we are having technologies like STM (scanning tunneling microscope) AFM (atomic force microscope) and NFOM (near field optical microscope) etc. In addition, sub-micron lithography, L-B films and adsorption/reaction in 2D/3D are also there. L-B technique is particularly important because it provides one of the few ways of marketing separate electrical connection to two ends of a molecule. A very good illustration of molecular electronics logic and architecture can be seen elsewhere Bio/Chemical Computer A new radical information processing system is being thought of where organic cells or bacteria are to act as the basic element. Living organisms are made of organic compounds. As such thinking function can be easily realized in such system. As scaling will be at biological level, very highdensity circuit can be at biological level, very high density circuit can be achieved. Our average brain comprises neurons ranging in size from 0.2 μm linear dimension to about 100 μm, each with an average connectivity of 10 4 giving a crude bit-count of to An equivalent artificial

11 HISTORY AND BASIS OF QUANTUM COMPUTING 11 brain may therefore be of such dense circuit. Enzymes and proteins are being studied. We should not forget that an example of a natural molecular device. 1s the bacterial photo-reaction center. Recent research to produce analogous have been successful through the synthesis of single and complex molecules, which release charge on photo-excitation. This subject of molecular electronics has moved from conjuncture to experimental study and scientific development. With the rapid growth of research and development of few liquid crystals, polymers, L-B films and NLO materials; molecular electronics is now with us. With advances in Physics, Chemistry, Materials Science, Biology and Engineering as our understanding of molecular materials both at microscopic and microscopic level with grow; the field of molecular electronics will prosper. The better understanding of natural system and processes and living organisms, will enhances the capability and potentiality of molecular electronics particularly in terms of its application in radical new computational machines and engineering. Much more work remains to be done. It needs scientific, intellectual and technological challenges on one hand; and Government and Industrial supports on the other hand. The progress of all these will determine actually whether molecular electronics if so, when. But research in molecular electronics and device technology it, will emerge as exciting and frontier fields of science and technology in the current century. The molecular electronics is a revolutionary idea. To attain maximum miniaturization, it is proposed that instead of using transistor s states, namely ON and OFF to implement 1s and 0s, the characteristics of electrons may be used for the same. For example, the positive and the negative spin be respectively used to implement 1s and 0s. The idea is new. It will take lots of time to mature and to develop the technology. This will be the last resort of miniaturization. The molecular electronics is believed to be based on new organic material technology that may lead to bio/chemical computer. A new radical information processing system is being thought of where organic cells or bacteria are to act as the basic element. Living organisms are made of organic compounds. As such thinking function can be easily realized in such system. As scaling will be at biological level, very high density circuit can be achieved. Our average brain comprises neurons ranging in size from 0.2 μm linear dimension to about 100 μm, each with an average connectivity of 10 4 giving a crude bit-count of to An equivalent artificial brain may therefore be of such dense CHAPTER 1

12 12 INTRODUCTION TO QUANTUM COMPUTING circuit. Enzymes and proteins are being studied. We should not forget that an example of a natural molecular device is the bacterial photoreaction center. Recent research to produce analogous have been successful through the synthesis of single and complex molecules, which release charge on photo-excitation. However, while the above new technologies aim to attain miniaturization going in line and/or beyond Moore s law, the quantum computing technology aims in applying quantum properties in information processing.