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Part I

Physics Frontiers



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Page 17 Part I Physics Frontiers

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Page 18

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Page 19 1 Quantum Manipulation and New Materials The world of quantum mechanics, although strange and beautiful, seems remote from daily experience. Yet the physicist sees quantum mechanics in action all around us, in everything from the hardness of diamond to the colors of the rainbow. The central role that quantum mechanics plays in modern technology—from the lasers that read music from compact disks to the Global Positioning System (GPS) that guides aircraft through our crowded skies—remains largely unknown to users of these devices. We are now in the early stages of a second “quantum revolution” in which we can see and control tiny clusters of atoms and indeed even individual atoms. This second revolution is bringing together two central strands of the physics endeavor: atomic and condensed matter physics. Atomic physicists have begun to study and control the new quantum properties that emerge in large collections of atoms, a traditional theme in condensed matter physics. At the same time, condensed matter physicists have begun to learn how to shrink the materials they study to sizes in which discrete quantum excitations (the traditional province of atomic physics) play a central role. Their common meeting ground at the intersection of the microscopic quantum and the macroscopic classical worlds is rich in new physics and new technologies. The ability to observe individual atoms and see how they assemble to form larger structures has been made possible by the development of a host of new observational tools. Scanning atomic probe microscopes can place single atoms on surfaces and measure many different physical properties of these atoms. Refinements in electron microscopy have attained single-atom resolution in bulk materials, and major advances have been achieved in the brilliance and coherence of the x-ray and neutron sources used to probe the structures of solids and large biomolecules. A new ability to control charged and neutral atoms has been made possible by the development of tools that use magnetic and laser fields to manipulate the positions and velocities of

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Page 20 atoms in ways that are impossible using material containers. Tools like these have led to the discovery of new quantum states of matter—Bose-Einstein condensates in atomic vapors and fractional quantum Hall states in two-dimensional layers of electrons. They have shepherded in an era of control of spatially extended quantum states that shows promise for extremely precise clocks and raises the hope of using the strange properties of quantum information for radically new forms of cryptography and computation. These tools and the understanding they provide have moved us much further toward the ultimate goal of “designer” structures: objects tailored to have desired optical, mechanical, magnetic, electronic, chemical, and thermal properties. It is astounding how far technology has progressed since the discovery of the electron only a hundred years ago. Electrons inside tiny transistors now tell the bank how much salary should be deposited in our accounts each month, and those coming along wires into our houses bring us pictures and information almost instantaneously from anywhere in the world. The ability to manipulate individual atoms will lead in the years ahead to a new generation of electronic devices that may be just as revolutionary. NEW TOOLS FOR OBSERVATIONIN THE QUANTUM REGIME The idea of atoms is 2000 years old, yet only in the last few decades have individual atoms been seen. The optical microscope approached the limit of its resolution nearly one hundred years ago with highly polished brass and glass machines capable of seeing individual cells about 0.0001 cm in size. The resolution of such microscopes was limited by the actual size of a ray of light, its wavelength. But the revolution in quantum mechanics of the early 20th century began a new era in microscopes: Particles such as electrons can also act like waves but can have far smaller wavelengths than visible light and hence can be used to make more powerful microscopes. Modern electron microscopes have resolution 10,000 times finer than conventional optical microscopes and can see the individual atoms inside a diamond crystal. The last 20 years have also seen the development of new types of high-resolution microscopes that scan surfaces. These instruments, which come in many varieties, use a sharp tip that is scanned across a tiny area of surface. They sense the surface much as a blind person does when reading a page of Braille text and display an image at an atomic scale. This image can be generated in response to some surface property due to, say, the force on the probe exerted by the local surface electron density. Scanning tunnel-

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Page 21ing microscopes, for instance, can examine the electrons at each atom on a surface (see sidebar “Scanning Tunneling Microscope”), magnetic force microscopes can show the magnetic bits on a hard disk, near-field scanning optical microscopes probe optical properties, and atom probes can identify SCANNING TUNNELING MICROSCOPE The scanning tunneling microscope (STM) uses an atomically sharp metal tip to scan a surface and image the location of individual atoms. It can also be used to pick up and place single atoms in desired positions. The figure shows a “quantum corral” consisting of an elliptical ring of 34 cobalt atoms placed on a copper surface. Quantum electron waves traveling along the copper surface are reflected by the corral atoms and trapped inside. The elliptical shape was used because waves emitted from one focal point are reflected and concentrated at the other. In a remarkable recent experiment, a magnetic atom was placed at one focus so that its magnetic properties were “projected” to the other focus. The resulting “phantom atom,” also known as a “quantum mirage,” was detected by scanning the STM tip over the other focus. Electrons tunneling from the tip to the surface were used to probe the magnetic structure of the atom, which was faithfully reproduced at the second focus, as illustrated in the top portion of the figure. The large peak at the left is associated with the electronic structure of the atom, and the smaller peak at the right is the mirage. ~ enlarge ~

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Page 22 individual atoms one by one as they are pulled from a surface. Some tips can also move individual atoms around on surfaces to engineer the smallest man-made structures. To investigate the deeper structure of materials, and in particular to examine crystal structures, x rays and neutrons can be used as probes. New high-intensity synchrotron x-ray sources have revolutionized the study of material structure. These developments in physics have important spin-offs in medicine and biology, where protein and drug structures can be analyzed routinely and quickly without the laborious work of preparing large crystals. What took years in the days when Crick and Watson were deciphering the structure of DNA now takes but a few days. Better neutron and x-ray sources that will allow much better and faster imaging are on the horizon. Spallation neutron sources, which will give information much more rapidly than current neutron sources, are coming on line, and the brightness of synchrotron x-ray sources continues to grow with time (see sidebar “Neutrons As Probes”). MANIPULATING ATOMS AND ELECTRONS Atom Cooling and Trapping Electric, magnetic, and laser fields are now used to confine (trap) and cool electrons and atoms, making it possible to reach far lower temperatures than ever before, as low as a billionth of a degree above absolute zero. Atoms cooled to such low temperatures change their behavior dramatically. It becomes highly nonintuitive as quantum physics dominates and new states of matter are formed: ion liquids, ion crystals, and atomic Bose-Einstein condensates. Strange new quantum states can be created, providing arenas in which to test basic quantum mechanics and novel methods of quantum information processing. The most widespread advances have come from using laser radiation to slow, and hence cool, a sample of atoms. Radiation pressure arises from the kick an atom feels when light scatters off it. If a sample of atoms is irradiated with beams from all six directions and the color of the light is carefully adjusted to the proper value, one creates “optical molasses.” Because of the Doppler shift, all the atoms in this molasses preferentially scatter photons that are opposing their motion, and the atoms are quickly slowed. This results in temperatures of less than one thousandth of a degree above absolute zero. A variety of methods have been found for cooling isolated atoms to even lower temperatures. In so-called Sisyphus laser cooling, precooled

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Page 23 NEUTRONS AS PROBES The neutron is an electrically neutral particle existing in the nucleus of atoms along with its positively charged counterpart, the proton. Because it is electrically neutral, the neutron can penetrate into the nucleus of atoms and so is an especially useful experimental probe of the structure of materials. It is very sensitive to hydrogen atoms and can be used to locate them precisely within complex molecules, enabling a more accurate determination of molecular structure, which is important for the design of new therapeutic drugs. Because large biological molecules contain many hydrogen atoms, the best way to see part of a biomolecule is through isotope substitution—replacing hydrogen with heavy hydrogen (deuterium) atoms. Deuterium atoms and hydrogen atoms scatter neutrons differently. Thus, in a technique called contrast variation, scientists can highlight different types of molecules, such as a nucleic acid or a protein in a chromosome, and glean independent information on each component within a large biomolecule. We see below a model of two insulin molecules containing zinc ions (white balls). Insulin molecules pick up zinc ions when crystallized for neutron diffraction studies. Neutrons for use in basic and applied research are produced in nuclear reactors or with particle accelerators. Most of the world's neutron sources were built decades ago, and although the uses and demand for neutrons have increased throughout the years, few new sources have been built. But now the U.S. Congress, through the Department of Energy's Office of Science, has funded the construction of a new, accelerator-based neutron source, the Spallation Neutron Source (SNS) (below) at Oak Ridge National Laboratory, which will provide the most intense pulsed neutron beams in the world for scientific research and industrial development. When complete in 2006, the SNS will be over 10 times more powerful than the best existing pulsed neutron source in the world. Its innovative set of instruments will enable forefront research that will benefit industry and health and increase our understanding of the underlying structure of materials. ~ enlarge ~ ~ enlarge ~

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Page 24 atoms moving through intersecting laser beams feel as though they are climbing hills, causing them to slow down no matter in which direction they are moving. This process can cool atoms to a few millionths of a degree above absolute zero. The very lowest temperatures (billionths of a degree) are achieved by the nonlaser technique of evaporative cooling of trapped atoms. This is analogous to the way a cup of hot coffee cools, by giving off the most energetic molecules as steam. As the energetic atoms escape from the trap (or coffee cup) the remaining atoms become very cold. As in any refrigerator, to cool an object, whether it be a cloud of atoms or a carton of milk, the object must be insulated from the hotter outside world. Traps based on laser and magnetic fields can now do this for small numbers of isolated atoms in a much more controlled and gentle way than is possible in a normal material container. Magnetic bottles use properly shaped magnetic fields to hold atoms with large magnetic moments. In the dipole force laser trap, atoms are sucked into the center of a focused laser beam. This laser trap also works on much larger objects, such as living cells or large molecules. These optical tweezers, as they are now called, have moved from the atomic physics lab to the biology lab, where they are used routinely to manipulate cells and large biological molecules such as DNA. The workhorse of ultracold neutral atom research is the magneto-optical laser trap, attractive because of its simplicity of construction and its ability to simultaneously cool and confine. The radiation pressure from laser beams converging on a center creates the trap. A weak magnetic field is used in a subtle way to control this pressure such that the atoms are all pushed toward the trap center and cooled. The laser trap provides a simple and inexpensive source of very cold trapped atoms and is seeing widespread use in the research laboratory as well as in novel applications for improving atomic clocks and lithography. Achieving and Using the Quantum Regime Novel cooling and confinement techniques allow atoms to be sufficiently cooled so that they no longer act as a group of indistinguishable particles but rather as waves corresponding to a single coherent quantum state. This produces simple quantum systems that have much larger spatial extents (macroscopic) than were possible in the past and atomic motion that is entirely wavelike. The superior insulation of electromagnetic atom traps also means that the electronic energy levels of the atom are perturbed much less than in conventional containers. This makes it possible to create and

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Page 25 preserve particular, desired quantum states and to measure atomic and electronic properties much more precisely. Experiments on individual trapped electrons have measured their magnetic moment, which is due to their spin, to an accuracy of better than 5 parts in 1012 (1000 billion). The value of the magnetic moment obtained from these measurements agrees with that predicted by the fundamental theory of electrons and light—quantum electrodynamics. This successful comparison of experimental data with theoretical calculations represents the most stringent test of a theory in all of science. Atomic structure and interactions have also been measured much more accurately than before using such cooled and trapped samples. A special case has been the atomic clock, a device using the structure of the atom to define the second precisely (see sidebar “Atomic Clocks”). The world's most precise clocks now use cesium fountains, in which samples of ultracold cesium atoms are launched upward. As the atoms ascend and then fall back to the source, the clock transition is measured with an inaccuracy of only 2 parts in 1015 (7 millionths of a second in a century). The cooling of atoms to temperatures where they behave like quantum mechanical waves has spurred the development of atomic counterparts to optical lenses, mirrors, and diffraction gratings, allowing atomic beams to be reflected, focused, split, and recombined in much the same way as light beams. In these devices it is the quantum waves of the atoms, rather than light waves, that are interfering. Optical interferometer gyroscopes are now widely used in navigational systems, but their atom interferometer counterparts can in principle be far more sensitive. Atom-wave interferometers also have measurement capabilities for which there is no optical analogue, such as the sensitive detection of electric, magnetic, and gravitational fields. Gaseous Bose-Einstein Condensates An exciting outcome of atom cooling and trapping techniques has been the creation of a novel form of matter, the gaseous Bose-Einstein condensate (BEC). Einstein predicted this effect in 1925, and superfluid helium and superconductivity are manifestations of it in liquids and solids. However, the original concept of Einstein, the possibility of condensation in a dilute atomic gas, was not realizable at that time, requiring temperatures much colder than could be attained. BEC in a gas was finally realized in 1995 by cooling a cloud of rubidium atoms to less than 100 billionths of a degree above absolute zero, and it has been widely duplicated since then using a

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Page 26 ATOMIC CLOCKS The atomic clock is a wonderful example of the unanticipated benefits from basic physics research. Originally physicists were interested in measuring the separation of atomic energy levels to a higher precision to understand better the physics of atoms. They soon found that their experiments were more accurate than the clocks used to determine the frequency of the microwaves that were used to excite the atoms. This realization led to the atoms themselves becoming the time standard. The second is now defined in terms of the separation of the two lowest energy states in the cesium atom. Atomic clocks provide the basis for precise navigation, including the global positioning system. The GPS is based on a set of orbiting satellites, each carrying a very precise atomic clock. A GPS receiver uses the exact time that it receives a clock signal from each satellite to tell the distance from the satellite. By knowing the distance to several satellites, the position of the receiver can be determined. Although originally implemented for its military uses, the GPS is now in wide use for nonmilitary applications. Hand-held units costing a few hundred dollars can give a precise location anywhere on Earth to less than a hundred feet. ~ enlarge ~ ~ enlarge ~

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Page 27 variety of different atoms. It was accomplished using the techniques of laser cooling and trapping combined with magnetic trapping and evaporative cooling. At these low temperatures, a large fraction of the atoms goes into the lowest-energy quantum state allowed in the atom trap. BEC in a gas is proving to be a fascinating new system because it is a quantum state with very special properties. It is enormously large (about the diameter of a human hair and containing millions of atoms) compared with quantum states in other systems, so it can be observed and manipulated in a way that had not been possible until now. Adding visible light and/or microwaves to the magnetic traps is proving to be a particularly convenient way to manipulate the condensates. A number of such techniques have been found to eject the condensate samples from the trap to obtain bright atomic beams, so-called “atom lasers.” Finally, the behavior of the condensate is amenable to theoretical analysis because the interactions between the atoms involved are very well understood. BEC has opened up opportunities to explore quantum behavior in a novel regime as well as allowed manipulation of atomic samples with a precision limited only by the uncertainty principle of quantum mechanics. Experiments on condensate behavior involving dissipation and coupling between the quantum state and the environment have provided many surprises and challenges to theoretical understanding. This work will lead to a much better understanding of dissipation in quantum mechanics, and it connects to the studies on the fragility of quantum entanglement that are of paramount importance for the future of quantum computing and quantum-limited measurement instruments. Laser Control of Electronic States The quantum engineering of BEC involves primarily the motion of atoms as a whole and not of their electrons, because it is difficult to compete with the binding forces within the atom that control the electron. However, this is beginning to change as a result of recent improvements in the laser technology used in producing extremely intense short pulses of light. Table-top systems can produce pulses of light so intense that the electric fields of the light can not only be as strong as those found inside atoms and molecules, but can also be shorter than the time scale for the motion of the electrons inside the atoms and molecules. An atom or molecule in such a light field is really no longer an atom or a collection of atoms but rather a new regime of matter, with the electrons, atomic nuclei, and light field having equal roles in determining the structure

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Page 28 and behavior. There is still much to be understood about this system. To control the electron dynamics and consequently the interatomic interactions requires the detailed manipulation of the time dependence of the light pulses. Clever techniques for controlling and observing the behavior of light pulses on time scales of 10−15 seconds are being developed to make this possible. This control is still very limited, and much of the basic physics of these systems is yet to be understood. The ultimate goal will be to use laser light to control the chemical processes at the atomic scale—to enable chemists to, say, produce efficiently a desired molecule or a new chemical compound. Another practical use of the novel behavior of atoms in intense laser fields has been the creation of brighter sources of light in the far-infrared, ultraviolet, and x-ray spectral regions with shorter pulses. Pulsed x rays produced in this interaction between atoms and intense light will make it possible to study problems in surface science, such as catalysis, chemistry, atomic physics, and biological imaging, with unprecedented temporal and spatial resolution. Techniques using these infrared pulses are being developed for monitoring the water content of leaves and food, for distance-ranging applications, and for medicine. The use of these novel laser sources in future particle accelerators is also being pursued. NEW MATERIALS The ability to create new materials and structures is inextricably linked to advances in the understanding of fundamental phenomena in materials physics. These advances, along with improvements in synthesis and processing, have led to an astonishing array of new materials with unexpected properties and to dramatic improvements in the properties of established materials. Some of these developments have provided fertile ground for the exploration of novel fundamental phenomena; others show promise for finding applications quickly. Some even have the potential to change our lives. Entirely new and unexpected phenomena often appear in a new material. Layered cuprate high-temperature superconductors are a new class of materials that has kept experimentalists and theorists searching to understand the physical basis of high-temperature superconductivity (see sidebar “High-Temperature Superconductivity”). This basis appears to be very different from that of conventional superconductors. New materials allow entirely new device concepts to be realized or lead to a dramatic change in scale, such as single-molecule wires made of carbon nanotubes. Semiconductor nanoclusters, which emit light whose wavelength depends on cluster

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Page 29 HIGH-TEMPERATURE SUPERCONDUCTIVITY High-temperature superconductors, first discovered in the 1980s, are materials that can conduct electrical current without resistance at temperatures far above those possible with any superconductors known up to that time. This group of materials, known as cuprates, contain planes that are networks of copper and oxygen as well as other elements that control the number of electrons in each copper-oxygen plane. Understanding the behavior of cuprates continues to be one of the most formidable challenges in modern science. Ever more sophisticated experimental probes and ever more refined materials preparation yield overwhelming evidence that novel concepts are needed to describe these materials. Cuprate superconductors have unusual symmetry properties. Electrons in all superconductors form pairs, and in most superconductors the distribution of electrons in the pair is completely symmetric. In cuprates, however, the electron distribution is less symmetric, as illustrated by the fourfold symmetry in the figure below. Today's high-temperature superconductors are finding growing use in filters for the wireless communications industry, as magnetic field sensors in medical scanning applications, and in electrical transmission cables for high-power, high-current applications. ~ enlarge ~

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Page 30 size, allow the tailoring of material properties to suit a particular need. Even mature techniques, such as those for crystal growth, demand continuous improvements in process control to produce the material required for technological applications and fundamental studies. Many of the new materials and structures are far more complex than those that were studied previously, requiring advances in processing to allow control of the increased complexity. Some involve the synthesis of an entirely new compound or material with unexpected properties. In other cases, advances in processing have allowed the fabrication of new or modified materials or structures whose properties were suspected before the material was actually made. This may allow a well-known compound to be remade in a new form with different properties. Finally, well-known materials sometimes exhibit new (and in some cases unexpected) properties that appear when the ability to process them is improved. Many of the materials advances listed in Table 1.1 addressed a technological need, such as information storage and transfer. Others were driven TABLE 1.1 Some New Materials of the Past 15 Years Advance Driver Nature of Advance New compounds/materials High-temperature superconductors Science Unexpected Organic superconductors Science Unexpected Rare-earth optical amplifiers Technology Evolutionary High-field magnets Technology Evolutionary Organic electronic materials Technology Evolutionary Magneto-optical recording materials Technology Evolutionary Amorphous metals Technology Evolutionary New structures of known materials Quasicrystals Science Unexpected Buckyballs and related structures Science Unexpected Nanoclusters Science Evolutionary Metallic hydrogen Science Evolutionary Bose-Einstein condensates Science Evolutionary Giant magnetoresistance materials Technology Unexpected Diamond films Technology Evolutionary Quantum dots Technology Evolutionary Foams/gels Technology Evolutionary New properties of known materials Gallium nitride Technology Unexpected Silicon-germanium Technology Evolutionary

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Page 31 by scientific curiosity. Many discoveries that result from pure scientific curiosity ultimately find their way into products. Low-temperature superconductors are now used in magnets for magnetic resonance imaging. Other discoveries, though originally motivated by a technological need, give rise to very beautiful and fundamental insights. The fractional quantum Hall effect was first observed in high-mobility semiconductor structures now used in high-frequency applications. Many of the advances listed in Table 1.1, such as the development of giant magnetoresistance materials, have driven the technology of the information age. Several of these are described in Chapter 10. ARTIFICIAL NANOSCALE STRUCTURES While atomic physicists have learned how to hold small numbers of atoms or electrons in vacuum, solid-state physicists have begun to contain atoms and electrons in tiny volumes within solids. These structures have many practical applications. Through them we control the movement of electrons that is the basis of electronics. As the structures being explored for new electronic devices become smaller than the wavelength of light, one enters the realm of nanoscale physics, a field that is rapidly evolving and expanding as the size of structures decreases and the quality and complexity increase. Fabrication New tools for materials fabrication have now emerged that permit cutting and pasting almost on the atomic scale. Molecular-beam epitaxy (MBE) is the gentlest spray-painting technique known to man. With a beam of atoms streaming against a surface in vacuum, layers as thin as one atom can be placed on top of one another. The method—developed in the last few decades because of the need for very high speed transistors, such as those used to send and receive radio waves from cellular phones—has had the additional benefit of opening up new areas in basic physics. The developers of MBE could not have imagined that this tool would lead to the discovery of a fascinating new phenomenon that led to the Nobel Prize in physics in 1998—the fractional quantum Hall effect. Because of the thin layers and high perfection of the MBE-created samples, a pancake-like gas of electrons can travel almost without collisions inside a solid. When cooled in a high magnetic field, these electrons act collectively like particles whose charge is a fraction of the charge on a single electron.

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Page 32 The nanofabrication challenge for the future is to achieve control in all three dimensions, rivaling the layer precision of MBE. This is like controlling not just the thickness of the layer of paint but also the width of the brush, requiring it to be atom-sized. Technology here has made great strides. In a silicon chip, such as a state-of-the-art microprocessor, the smallest features are now about 200 nm, one thousandth the width of a human hair. Extensions of the tools developed to make these chips allow structures as small as 10 nm to be fabricated. The real challenge is to control these structures with even greater precision. Also, ways must be found to reduce the damage induced by the microscopic cutting tools used to make nanostructures, because the damaged surfaces can perturb the desired properties by disrupting the movement of the electrons. As the individual atomic scale is reached, a new approach—perhaps using naturally stable structures or relying on self-assembly—will be needed. Self-assembly Many structures in nature are well organized on the nanoscale. For example, a seashell has a complex interleaved structure with exceptional strength yet low mass. Opals are perfectly arranged microspheres of silica. Materials physicists have increasingly come to view sophisticated forms of self-assembly as unique tools to control nanoscale materials. So-called diblock copolymers are a beautiful example from chemistry: polymer blends that give perfectly organized and highly controlled structures on the nanoscale. When atoms are deposited by MBE on some surfaces, they do not form a uniform film but instead ball up to form islands. This effect, seen for example when germanium is deposited on silicon, has only recently begun to be understood. The stress from the differences in atom size causes the germanium islands to repel one another and to self-organize. This will have very important applications—for example, in semiconductor lasers—because of the unique quantum effects when the lasing regions are confined on the nanoscale. A challenge for self-assembly is to reach the perfection often required for specific physical properties. This is the primary reason basic research is needed to gain a better understanding of these complex phenomena. The nanoscale perfection found in biology encourages the belief that this is achievable. Biology and physics will overlap even more in the future as physicists imitate nature for improved nanostructural control and as artificial nanostructures are used in biomedical applications.

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Page 33 Nanoscale and Molecular Electronics The study of nanoscale electronic devices began to blossom in the last decade and a half. It is now possible to fabricate devices that are so tiny that the charging energy needed to add or remove a single electron becomes easily observable. In some cases even the spacing of individual electron energy levels is large enough to be discernible, making these devices analogous to artificial atoms. These technological advances have allowed physicists to address such fundamental issues as how small a superconducting grain can be and still remain a superconductor. They have also permitted the construction of practical devices such as precision charge pumps that use a bucket-brigade technique to pump exactly one electron through the device for each cycle of an applied voltage. A remarkable new invention, the radio frequency single-electron transistor, uses transport through a nanoelectronic device to measure the presence of nearby tiny electrical charges with unprecedented speed and sensitivity. A related and important new direction for the coming decade can be called “molecular electronics.” Using methods made possible by advances in lithographic fabrication and scanning tunneling microscopy, physicists are beginning to study the transport of electrons through a single molecule. In the future it may be possible to identify DNA sequences from their electrical properties. A novel problem of great interest is the transport of electrons through carbon nanotubes. These “molecular wires” are essentially narrow strips of graphite rolled into a single tube a billionth of a meter in diameter, forming a highly elongated molecule of pure carbon. It is now possible to make electrical connections to these tubes and inject electrons into them. Small changes in the way the graphite strip is rolled up cause large changes in the current flow, allowing the fabrication of tubes that are metallic, insulating, or semiconducting. This suggests the possibility of ultraminiaturized transistor-like devices based on these novel molecules. QUANTUM INFORMATION AND THE ENGINEERING OF ENTANGLED STATES Fundamentally, quantum mechanics is a theory of information. Quantum information is radically different from its classical counterpart, which forms the basis of the current information age. Recent progress in the understanding and manipulation of quantum information has brought together physicists from several diverse areas in a quest to advance the frontiers of communication and computation. The exciting but still speculative hope is that their work will lead to important new information technologies.

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Page 34 The smallest unit of classical information, the bit, consists of the answer to a single yes-no (or true-false) question. In the current digital age, bits are represented by tiny electrical switches that are either open or closed (on or off). Even the most advanced computer is nothing more than a series of such switches wired together so that the position of one switch controls the position of others. The positions of a set of input switches ultimately determine the positions of a set of output switches that represent the result of the computation. By contrast, a quantum bit of information (“qubit”) is encoded in a quantum switch whose position is not fully knowable. That is, the switch can be in a coherent superposition of the two possibilities, open and closed, and measurement of its position destroys the quantum information. From a classical information point of view, it would seem that a precise output is not possible. However, a surprising discovery of the last decade was the existence of quantum algorithms that can provide imprecise but good-enough solutions far faster than their classical counterparts. This advantage comes because the quantum computer operates on all the different possible switch positions at the same time rather than having to consider them each sequentially. Quantum algorithms have received a great deal of attention since the discovery of an algorithm to factor large numbers, the basis of much of modern encryption (see sidebar “Quantum Cryptography”). A realistic quantum computer must be based on switches built out of electrons and nuclei of individual atoms, or nanoscopic condensed matter systems. The last decade has seen great advances in the ability to manipulate such systems and to prepare the special quantum states required. These are so-called entangled quantum systems, in which two or more quite different and apparently unrelated properties of an object (usually an atom) are linked. The motion of the object might be entangled with its color, in which case measuring the motion of the object affects the color that one would subsequently measure, or vice versa. The widespread creation and study of entangled quantum states have only recently been made possible by the delicate manipulation of special systems, such as trapped atoms and tiny superconducting metal grains. Using combinations of laser light and radio waves to manipulate the trapped atoms, rudimentary computational operations have been performed on qubits. Quantum engineering methods can be extended from a single atom to entangled states of many atoms using a suitable coupling mechanism. The strong electrical interaction between trapped charged atoms provides such a mechanism, and four-atom entangled states (four qubits) have recently been demonstrated. Physicists are still exploring the fundamental science in

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Page 35 QUANTUM CRYPTOGRAPHY In the modern information age, there has been an almost complete convergence between the technologies for computation and communication. Similarly, recent advances in the understanding of quantum computation have gone hand in hand with great strides in the transmission of quantum information. One application of quantum information that is already becoming practical is the secure transmission of encryption codes (“quantum cryptography”). The biggest obstacle to quantum computing is that any interaction with the environment disrupts the quantum information. For quantum cryptography this fragility is a virtue, because it means that it is impossible for an eavesdropper to intercept the code signal without disrupting its quantum information so much that the interception is easily apparent. Thus, quantum cryptography uses the laws of physics to ensure that there is no undetected interception of a coded message. This has now been demonstrated using entangled states of photons to securely transmit cryptographic keys over distances of several kilometers. Demonstrations in Switzerland have sent the appropriately prepared photons of light down an optical fiber, and in the United States the signals were sent through open air between two locations near Los Alamos National Laboratory. ~ enlarge ~ these technologies to learn what may or may not be feasible for practical applications. Although practical quantum computers are not likely to exist in the near future, more modest uses for multi-atom entangled states, such as improvements in atomic spectroscopy and atomic clocks, appear to be within reach.

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Page 36 We are in the midst of an exciting revolution in the ability to observe and manipulate material at the quantum level. The next few decades are certain to lead to new insights into the strange world of quantum physics and to dramatic advances in technology, as the field of quantum engineering is developed.