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Controlling the Quantum World: AMO Science in the Coming Decade

Atomic, molecular, and optical (AMO) science demonstrates powerfully the ties of fundamental physics to society. Its very name reflects three of 20th century physics’ greatest advances: the establishment of the atom as a building block of matter; the development of quantum mechanics, which made it possible to understand the inner workings of atoms and molecules; and the invention of the laser, which changed everything from the way we think about light to the way we store and communicate information. The field encompasses the study of atoms, molecules, and light, including the discovery of related applications and techniques. This report illustrates how AMO science and technology touches almost every sphere of societal importance—navigation using the latest atomic clocks; surgery with a host of new laser tools; ensuring the nation’s defense using global positioning satellites and secure communication; defending the homeland with screening technologies to detect toxins in the air and hidden weapons in luggage or on persons; improving health care with improved drug design tools and new diagnostic scanners; and underpinning the world’s economies with a global communications network based on high-speed telecommunication by laser light.1

The immense advances in science over the past century have only just begun to explain the mysteries of the universe. One of the primary goals of AMO science is to

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For further detail on the connections between AMO science and society’s needs, see National Research Council, Atoms, Molecules, Light: AMO Science Enabling the Future, Washington, D.C.: The National Academies Press (2002), available at <http://www.nap.edu/catalog/10516.html>, accessed June 2006.



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Controlling the Quantum World: The Science of Atoms, Molecules, and Photons 1 Controlling the Quantum World: AMO Science in the Coming Decade Atomic, molecular, and optical (AMO) science demonstrates powerfully the ties of fundamental physics to society. Its very name reflects three of 20th century physics’ greatest advances: the establishment of the atom as a building block of matter; the development of quantum mechanics, which made it possible to understand the inner workings of atoms and molecules; and the invention of the laser, which changed everything from the way we think about light to the way we store and communicate information. The field encompasses the study of atoms, molecules, and light, including the discovery of related applications and techniques. This report illustrates how AMO science and technology touches almost every sphere of societal importance—navigation using the latest atomic clocks; surgery with a host of new laser tools; ensuring the nation’s defense using global positioning satellites and secure communication; defending the homeland with screening technologies to detect toxins in the air and hidden weapons in luggage or on persons; improving health care with improved drug design tools and new diagnostic scanners; and underpinning the world’s economies with a global communications network based on high-speed telecommunication by laser light.1 The immense advances in science over the past century have only just begun to explain the mysteries of the universe. One of the primary goals of AMO science is to 1 For further detail on the connections between AMO science and society’s needs, see National Research Council, Atoms, Molecules, Light: AMO Science Enabling the Future, Washington, D.C.: The National Academies Press (2002), available at <http://www.nap.edu/catalog/10516.html>, accessed June 2006.

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Controlling the Quantum World: The Science of Atoms, Molecules, and Photons reveal the workings of nature on a fundamental level. In addition, society continues to have many urgent challenges that AMO research seeks to address. The unifying thread between the pure and applied work is quantum mechanics: AMO research develops tools and seeks knowledge on the quantum level, enabling progress in many other fields of science, engineering, and medicine. The overarching emerging theme in AMO science is control of the quantum world. The six broad grand challenges outlined in this report describe key scientific opportunities in the coming decade. They are precision measurements; ultracold matter; ultra-high-intensity and short-wavelength lasers; ultrafast control; nanophotonics; and quantum information science. These challenges will drive important advances in both experiment and theory. Each of these science opportunities is linked closely to new tools that will also help in meeting critical national needs (see Figure 1–1). WHAT IS THE NATURE OF PHYSICAL LAW? What are the undiscovered laws of physics that lie beyond our current understanding of the physical world? What is the nature of space, time, matter, and energy? AMO science provides exquisitely sensitive tools to probe these questions. For example, a force that alters the fundamental forward-backward symmetry of time has been studied extensively by high energy physicists, but another such force beyond the current Standard Model of the universe is now widely expected to exist. This tiny but revolutionary effect could show up first in the next decade in AMO experiments that look for deviations in the nearly perfect spatial symmetry found in atoms. A second question asks whether the laws of physics are constant over time or across the universe. A new generation of ultraprecise clocks will enable laboratory searches for time variations of the fundamental constants of nature. Answers will also come from AMO research that is helping to interpret astrophysical observations of the most exotic and most distant realms in the universe. The advanced technologies developed for such fundamental physics experiments have many other uses. They will improve the accuracy of direct gravity-wave detection and of next-generation global positioning satellites and will produce new medical diagnostics. These advances are described briefly in the next paragraphs and explored more fully in Chapter 2. Since the atomic concept was finally accepted at the beginning of the 20th century, atoms have proven central to the discovery and understanding of the laws of physics. Today remarkably sensitive techniques probe the properties of atoms, molecules, and light over enormous ranges: from submicroscopic to cosmic distances, in both familiar environments and the most exotic realms in the universe. The unprecedented sensitivity with which these fundamental properties can be

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Controlling the Quantum World: The Science of Atoms, Molecules, and Photons FIGURE 1–1 NIST-F1 is a cesium fountain atomic clock in Boulder, Colorado. Along with other international atomic clocks, NIST-F1 helps define the official world time standard. In 2000 the uncertainty in the clock’s accuracy was about 1×10–15, but by the summer of 2005, the uncertainty was reduced to about 5×10–16, which means it would not gain or lose more than a second in more than 60 million years. For more information on this clock, see <http://www.tf.nist.gov/cesium/fountain.htm>. SOURCE: National Institute of Standards and Technology, copyright Geoffrey Wheeler.

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Controlling the Quantum World: The Science of Atoms, Molecules, and Photons measured is not only advancing science but is also yielding new technology for applications as diverse as studying the brain and detecting lung disease, for terrestrial guidance and space navigation, and for mapping local gravitational fields and detecting subsurface features in the Earth. AMO experiments could provide an understanding of a fundamental property of time and physical law. How the laws of physics might change if time went backward is not just a whimsical question from science fiction; it is one of the most vigorously debated questions in the physics of fundamental forces. The measurement of atomic electric dipole moments (EDMs) could provide an answer to this question. The EDM is a tiny separation between the centers of positive and negative charges in an atom, which has been predicted by nearly every class of advanced theory in particle physics, including supersymmetry. EDMs have never been observed and must be very tiny if they even exist; we do, however, possess the technology that could allow their detection in the next decade. They would reveal new physics beyond our current understanding of the subatomic nature of our universe, as described by the so-called Standard Model. While much of our knowledge about the Standard Model of fundamental interactions comes from high-energy particle accelerators, AMO experiments have provided critical complementary information. Unprecedented precision has practical consequences. The techniques developed for these fundamental experiments are now surpassing low-temperature superconducting quantum interference devices (SQUIDs) in the precise measurement of magnetic fields, reaching sensitivities better than 10 parts per trillion of Earth’s magnetic field. Such sensitivity will improve our ability to measure more accurately the weak magnetic fields of the brain and the heart, thereby helping to diagnose epilepsy, cardiac arrhythmias, and other diseases. Similarly, advances in measuring the magnetic properties of the atoms of noble gases are opening up a new field in medical imaging that will allow high-resolution studies of the lung. Such images cannot be obtained using standard MRI techniques. Current devices based on this new diagnostic tool promise enormous improvement in the early diagnosis of lung disease. Extraordinary advances in optical spectroscopy are leading to superb atomic clocks. Ultrashort pulsed laser sources have been exploited to create an “optical comb” spanning the entire visible and near-infrared spectrum. With this revolutionary development (recognized by the Nobel prize in 2005) it is possible to count optical frequencies (about 1015 Hz) literally in cycles per second and to measure the ratio of optical frequencies with unprecedented precision. New ultra-accurate clocks will test whether fundamental “constants” of nature are changing over time. They also have many direct and near-term technological impacts, including enhancement of the performance of high-end analog-to-digital converters in advanced radar, more accurate global positioning satellites, and many other applications.

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Controlling the Quantum World: The Science of Atoms, Molecules, and Photons Optical and atom interferometry will lead to new navigation tools and measurements of gravitation. New AMO devices are enabling ever more precise measurements of motion by detecting tiny changes in the interference not just between beams of light but also between beams of atoms, as discussed below. Interferometers are the cornerstone of gravitational wave observatories on Earth and in space that are expected to provide new insight into the structure of our universe. Ring laser and fiber-optic gyroscopes are now standard sensors that play a broad role in state-of-the-art navigation systems. Matter-wave interferometers promise a huge improvement in navigational systems accuracy. Laser-based gravimeters are being used worldwide to characterize Earth’s gravitational field for the management of oil deposits and other resources. Future systems based on atom-wave interference will enable airborne characterization of gravitational anomalies at unprecedented levels to detect hostile underground structures and tunnels. Atomic data and atomic theory provide critical support in astrophysics exploration. Our universe serves as an extraterrestrial laboratory in which to test the laws of physics under extreme conditions. Satellite observatories can probe the environments near black holes and the surfaces of neutron stars. Studying the universe can provide clues to the nature of fundamental physical laws at times and at energies than cannot be reached with today’s earthbound laboratory experiments. AMO science plays a central role in helping us to understand what the data from radio, optical, and x-ray telescopes are telling us about these extreme astrophysical environments. Collisions of atoms, molecules, electrons, and ions in these extreme regimes yield new spectral features that can be modeled by theorists and used to understand the full range of extraordinary conditions observed in the universe. As the following chapters show, there are many areas in which AMO transcends disciplinary lines and provides techniques and data that improve both our understanding of the universe and our daily lives. For example, AMO data lie at the heart of the development of plasma processing, efficient lighting, and many other hightemperature chemical reactions. Exciting new developments in biology include results from electron-molecule scattering, where it has been recently discovered that resonant dissociative electron capture plays an important role in radiation damage through DNA strand breaking. Some of these kinds of cross-cutting possibilities are discussed in summary form in the NRC report Atoms, Molecules, and Light: AMO Science Enabling the Future.2 2 For further detail on the connections between AMO science and society’s needs, see National Research Council, Atoms, Molecules, and Light: AMO Science Enabling the Future, Washington, D.C.: The National Academies Press (2002), available at <http://www.nap.edu/catalog/10516.html>.

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Controlling the Quantum World: The Science of Atoms, Molecules, and Photons WHAT HAPPENS AT THE LOWEST TEMPERATURES IN THE UNIVERSE? The Bose-Einstein condensates (BECs) developed in the laboratories of AMO physicists in the last decade are the coldest objects that have ever existed anywhere in the universe. These remarkable clouds of trapped atoms are about a billionth of a degree above absolute zero, much colder than the dark, frigid, furthest reaches of intergalactic space. Furthermore, BECs and their close cousins, ultracold degenerate Fermi-Dirac gases, are not just cold; these quantum condensates are proving to be very special states of matter (see Figure 1–2). Scientists have discovered that FIGURE 1–2 A three-dimensional rendering of an image of a matter-wave soliton train. Each peak in the train is a Bose-Einstein condensate of lithium atoms, held together by the atoms’ self-attraction. The spikes are called matter-wave solitons. Solitons are localized bundles of waves constrained to move in only one dimension and that propagate without spreading. These atomic lithium solitons occur because the ultracold atoms bunch together due to the quantum forces they exert on each other. SOURCE: K.E.Strecker and R.G.Hulet, Rice University.

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Controlling the Quantum World: The Science of Atoms, Molecules, and Photons they have strange and wonderful properties, and in the next decade we can expect a rich harvest of interesting new physics ideas and applications—from technological breakthroughs such as clocks and inertial sensors of unprecedented accuracy, to insights into the physics of ordinary matter as well as matter under extreme conditions. More information on cold quantum gases is contained in Chapter 3 and summarized below. When breakthrough science happens, it defines a new frontier. Today, AMO science is camped on one of the most exotic frontiers in science—the push toward ever lower temperatures obtained in atomic physics labs. In the last decade, six physicists have won the Nobel prize for their work at the frontier of ultracold atomic gases. The record low temperature stands, as of early 2006, at about a billionth of a degree above absolute zero. By contrast, intergalactic space is a relatively hot 2.7 degrees above absolute zero owing to the existence of the cosmic microwave background. An ultra-low-temperature gas is a fruitful frontier to explore for two reasons: The atoms and molecules are nearly free from thermal fluctuations, and the quantum (or de Broglie) wavelength of the particles becomes extremely large. As a result, the field of ultracold atoms has become a remarkable meeting place for scientists of many different professional specialties, all of whom have come to realize how much can be learned from this new discipline, and all of whom bring their own expertise to the mix. When a gas of atoms is cooled to “degeneracy”—that is, it becomes so cold that the quantum wave of one atom starts to overlap with that of its nearest neighbor—and the atoms in question are bosons, the result is a BEC. In such a gas, all of a macroscopic fraction of the particles can simultaneously occupy the lowest-energy “wave” in the box in which the atoms are kept. Ultracold gases offer the intriguing possibility of building completely controllable models for matter. They can be confined in a variety of geometries, including situations that are effectively one- or two-dimensional rather than three-dimensional. It is also possible to confine them in ways that mimic the periodic structure in solid crystals. These uses of ultracold atoms as quantum simulators are among the most exciting recent developments in AMO science. They have a considerable impact on precision measurements, as discussed in Chapter 2, and on quantum information science, as discussed in Chapter 7. Also, these applications occur at the intersection of AMO physics and condensed matter and indeed AMO physics and other fields of physics. These strengthening links between AMO and other parts of physics promise exciting new discoveries in AMO science in particular and in physics more broadly.

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Controlling the Quantum World: The Science of Atoms, Molecules, and Photons WHAT HAPPENS AT THE HIGHEST TEMPERATURES IN THE UNIVERSE? Lasers in the next decade will reach peak powers of a million billion watts concentrated in a single beam of light for a little more than a millionth of a billionth of a second. This power exceeds, for an instant, all the electrical power production on Earth. The huge electric fields in these focused beams overwhelm the forces that bind electrons in atoms and molecules, leading to exotic states of matter usually found only in neutron stars, the early universe, hydrogen bombs, and particle accelerators. These lasers will help unravel the violent forces we see in the universe around us. High-powered optical lasers have applications to many other important technological problems as well, ranging from the prospect of controlling nuclear fusion as a source of clean, abundant energy to creation of next-generation compact x-ray microscopes with unprecedented resolution. Advanced lasers open new scientific and technological frontiers that benefit from two closely related technical advances: the development of optical-wavelength laser beams with unprecedented power and a new generation of lasers that produce coherent x rays. Both high-powered lasers and x-ray lasers will expand our knowledge as they extend our use of electromagnetic radiation. Bright, ultrafast sources of x rays will revolutionize the study of matter in the next decade. Synchrotron x-ray light sources have been important tools for determining structure at the atomic scale. Today there are dozens of accelerator storage rings around the world, heavily used and devoted to research in materials science and chemistry as well as biology and medicine. Another revolutionary tool will become available in the next decade: the x-ray free-electron laser. These x-ray lasers will be more than one billion times brighter than the brightest synchrotrons, with pulses more than one thousand times shorter. This means that they will be capable of concentrating unprecedented energy on the atomic scale in chemicals, materials, and biological molecules. An important challenge of the next decade is to find ways to take full advantage of this new capability to advance chemistry, biology, and medicine. One particularly important application, biomolecular imaging, is discussed in Chapter 5. Other applications are covered in Chapter 4. New scientific and technological frontiers will be explored using ultraintense visible lasers, that can reach peak powers in excess of 1 million billion watts. Focused beams from the highest-powered lasers can concentrate the equivalent power of the entire electrical grid of the United States onto a spot only a tenth of a human hair in diameter. The enormous electric fields in these focused laser beams dwarf the forces that bind electrons in atoms and molecules, literally tearing them apart in an instant. Such energetic states of matter are usually found only in the most exotic places in the universe—in the center of stars or in the explosion of a nuclear

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Controlling the Quantum World: The Science of Atoms, Molecules, and Photons weapon. Scientists are learning how to harness the electric fields generated by intense lasers to create directed beams of electrons, positrons, or neutrons for medical and materials diagnostics. New ultraintense laser sources can accelerate electrons to high energies in shorter distances than any other method yet devised, opening up the possibility of building powerful particle accelerators in quite small spaces (see Figure 1–3). The accelerated beams have been employed to make radioisotopes for medical use. These lasers have also generated plasmas capable of nuclear fusion, and their light has been converted to x rays for research on dynamics in laser-excited solids. These applications are discussed in more detail in Chapter 4. Any one of these demonstration experiments could become the basis for expanded research and technology in the coming decade. Controlled thermonuclear fusion is one particularly important challenge from the past decade that will continue to be important in the next. Laser-heated plasmas FIGURE 1–3 A plasma channel shown in blue and white, about a millimeter long and denser toward the edges, guides the laser and allows it to form high-quality electron beams. As the laser pulse travels from left to right, it traps and accelerates bunches of electrons to high energies in very short distances. SOURCE: W.Leemans, Lawrence Berkeley National Laboratory.

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Controlling the Quantum World: The Science of Atoms, Molecules, and Photons FIGURE 1–4 Workers stand within the target chamber of the National Ignition Facility (NIF), which measures 30 feet in diameter and will contain 192 high-power laser beams, all directed at one small target. NIF can thus deposit enormous amounts of energy in very small volumes over very short times. SOURCE: University of California, Lawrence Livermore National Laboratory. for fusion energy have been under development in the United States and Europe for decades, but recent progress in ultrafast and high-field lasers holds particular promise for rapid advances toward a device that will produce more fusion energy than it consumes in heating the plasma (the so-called breakeven point). Advanced high-intensity lasers may be a key technology to achieve breakeven. This will be tested in the next decade (see Figure 1–4). CAN WE CONTROL THE INNER WORKINGS OF A MOLECULE? In the next decade we will begin to observe the processes of nature as they play out over times shorter than a millionth of a billionth of a second (less than 1 femtosecond—that is, in the attosecond regime). This remarkable new capability is enabled by advances in ultrafast laser- and accelerator-based x-ray strobes, which can detect the motion of electrons in atoms and molecules. Scientists anticipate

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Controlling the Quantum World: The Science of Atoms, Molecules, and Photons the possibility of capturing images of motion inside a molecule or taking a snapshot of a protein or virus (see Figure 1–5). We will also be able to control physical phenomena on all of the timescales relevant to atomic and molecular physics, chemistry, biology, and materials science. These previously unavailable tools of quantum control could help tailor new molecules for applications in health care, energy, and security. A frontier of AMO science is to observe the basic processes of chemistry and biology on the scale of a single molecule. A key ingredient in taking slow-motion pictures is the ability to freeze the action by recording images with a shutter speed much faster than the motion of the object of interest. In atomic and molecular motion, as elsewhere in high-speed photography, the mechanical shutter has been replaced by a short pulse of light, which acts as a stroboscope. The picosecond (or faster) processes within molecules require very short pulses that can only be produced by a laser. The rotational motion of the molecules in a gas cell can be captured by illuminating the gas with laser pulses that are a fraction of a picosecond in duration, while freezing the vibrational motion of molecules requires pulses of a few femtoseconds. Freezing the motion of electrons as they move about the molecule requires subfemtosecond, or attosecond, laser pulses. FIGURE 1–5 An x-ray free-electron laser (XFEL) can image single biomolecules. Left: a simulated model of the anthrax lethal factor protein. Right: A simulation of the diffraction pattern of the molecule on the left, which can be processed by a computer to generate an image of the molecular structure with atomic resolution. SOURCE: Lawrence Livermore National Laboratory.

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Controlling the Quantum World: The Science of Atoms, Molecules, and Photons Capturing the motion of atoms in the attosecond regime is now possible. Rapid molecular vibrations can be observed with a sequence of two subpicosecond pulses in which the first pulse excites the molecule and the second pulse probes the resulting molecular response as a function of the time interval between the pulses. The science breakthroughs brought about by this advance were recognized by the 1999 Nobel prize in chemistry. At the beginning of 2006 strobes as short as one hundred attoseconds have even captured the motion of the electrons within an atom. This motion is the fundamental physical basis for chemistry. Why do some atoms bind, and others not? Why do reactions take the time they do, and why do molecules bend one way but not another? Watching the steps in the dance of electrons will provide a wealth of new insight into the mechanisms of chemistry. Ultrafast pulses of x rays show great promise for investigating the structure of complex molecules. X rays are very short wavelength light rays with two important differences from ordinary light: They can penetrate ordinary matter and reveal the interiors of solid objects and they can resolve very small objects, down to single atoms. X rays were used to reveal the double helix shape of the DNA molecule, which encodes the genetic information for all living things. Currently the brightest x-ray sources can reveal the atomic details of complex molecules and solids in a static or resting state but are not bright enough to capture changes such as chemical catalysis or absorption of sunlight in photosynthesis. The new x-ray laser tools of 2010 and beyond will help scientists understand, manipulate, and exploit the molecular universe. What makes the molecules in all living organisms so efficient at carrying out particular tasks? Can we design other molecules to be as efficient as the ones that nature has optimized? What makes certain materials effective catalysts in chemical reactions or gives them the remarkable ability to capture sunlight efficiently and turn it into chemical energy? New 21st-century tools also place us on the verge of the new discipline of quantum control. This development is enabled by key advances in laser technology, which let us generate light pulses whose shape, intensity, and color can be programmed with unprecedented flexibility. Our ability to control the positions, velocities, and relative spatial orientations of individual atoms and molecules has led to a broad array of precision measurement technologies and devices, leading to a wide range of experiments and discussed throughout this report, that reveal qualitatively new phenomena. A new capability to manipulate the inner workings of molecules is emerging: Lasers can now be used to control the outcome of selected chemical reactions. This control technology may ultimately lead to powerful tools for creating new molecules and materials tailored for applications in health care, nanoscience, environmental science, energy, and national security.

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Controlling the Quantum World: The Science of Atoms, Molecules, and Photons HOW WILL WE CONTROL AND EXPLOIT THE NANOWORLD? The nanoworld lies in between the familiar classical world of macroscopic objects and the quantum world of atoms and molecules. Nanostructures can have counterintuitive but useful optical properties that arise because they are smaller than the wavelength of light used to observe them. Scientists see unique opportunities to tailor material properties for efficient optical switches, light sources, and photoelectric power generators. Nanomaterials promise the development of singlephoton sources and detectors, photonic crystals, environmental sensors, biomedical optics, and novel cancer therapies involving localized optical absorption. These opportunities are described in Chapter 6 and briefly summarized below. A myriad of powerful new tools are now available to create, visualize, and control structures on the nanoscale. Nanoscience includes fundamental research on the unique phenomena and processes that occur at the nanometer scale (see Figure 1–6). Opportunities that lie in this region, between the quantum scale and FIGURE 1–6 Spin-polarized scanning tunneling microscope image of magnetic cobalt islands on a Cu(111) surface at T=4 K. Single atoms of iron have been deposited over the surface (yellow bumps) to study the exchange coupling between single-atom spin centers and magnetic nanostructures. Size of image is 36×36 nm. SOURCE: M.Crommie, University of California at Berkeley.

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Controlling the Quantum World: The Science of Atoms, Molecules, and Photons the classical scale, involve AMO science in a number of ways. Nanostructures can be constructed from the bottom up using chemical and optical techniques, or from the top down using techniques such as optical lithography. The structures often have novel optical features, including special absorption properties and negative refractive indices. Nanomaterials with negative indices of refraction could dramatically improve optical microscopes or reduce the feature size in chip fabrication. A new field is growing up to take advantage of these opportunities: nanophotonics. Size is everything in the nanoworld. The physical, chemical, and biological properties of nanostructured materials can vary substantially at the nanoscale. This dimensional dependence means that physical properties can be controlled by varying the size of the nanoparticles. Until recently, our ability to view or to control the nanoworld was so limited that harnessing it was impossible. However, owing to recent technological developments—many of which are coming from AMO science—efficient and practical nanoscale synthesis and assembly methods will be developed in the coming decade. Nanofabrication promises to exploit the properties of lasers and optics to improve the production of nanomaterials. Laser ablation is one of the easiest and most widely understood methods for producing nanoparticles from solids. A new twist on this old method is the use of shaped ultrafast pulses to control the size and other characteristics of the nanoparticles. “Atomtronics” is a very new technology that employs trapped ultracold atoms above the surface of a microchip, one aim of which might be to create a single-atom transistor. Nanoscale engineering will allow the creation of new nanostructured media with exceptionally large optical nonlinearities, allowing efficient detection of infrared light. It will also allow the realization of optical nanoparticles, whose size and shape determine their absorption, transmission, and reflection properties. In addition, nanoscale engineering will also allow new optical fibers that can carry the shortest ultrafast pulses without distortion and new lenses that can focus light far more tightly than allowed by the conventional rules of physical optics, and it will allow the construction of new optical displays with unprecedented ruggedness and low cost. WHAT LIES BEYOND MOORE’S LAW? Today’s computers are doubling in performance every year or two. This will end when the ever-shrinking size of electronic components approaches the level of individual molecules and atoms. While it is still uncertain whether a working large-scale “quantum computer” (as we understand the word computer today) will ever be built, it is clear that quantum mechanics offers a radically different approach to information processing, in which single atoms and photons would

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Controlling the Quantum World: The Science of Atoms, Molecules, and Photons be the new hardware. This could lead to computers capable of solving problems that are intractable on any imaginable extension of today’s computers but that are important in areas ranging from basic science to national security. Quantum communication might provide security against interception beyond anything possible in today’s cyber infrastructure. These applications are based on the strangest and least intuitive concepts of quantum physics, such as Einstein’s “spooky action at a distance,” which allows “teleportation,” or the transfer of information (as opposed to actual physical objects) between remote quantum systems without any physical contact between the quantum hardware during the communication. The possibility of quantum computing is forcing us to explore both theoretical and experimental quantum mechanics at their deepest levels. Should quantum computers be realized, they would be more different from today’s high-speed digital computers than those machines are from the ancient abacus. These opportunities are described in Chapter 7 and summarized briefly below. Quantum mechanics and information theory were two of the scientific cornerstones of the 20th century. One describes physics at very small scales, from molecules and atoms to electrons and photons; the other is a mathematical description of data communication and storage. With the last decades having witnessed the remarkable shrinking of electronic components that carry and process information to near-atomic scales, these two disciplines are naturally beginning to merge. Moore’s law of exponentially shrinking computer chip components will soon slow as individual electronic transistors approach the atomic scale, where there is no more room for packing additional components. However, the revolutionary principles of quantum mechanics could offer a way out. Quantum information science may have profound and far-reaching relevance to economic growth, secure communication, and specialized number-crunching. The quantum hardware now found in AMO systems is a key to realizing future quantum devices and will be crucial to the understanding and development of quantum hardware in complex condensed matter systems. Quantum mechanics contains radical features not found in any other physical theory. The quantum mechanical concept of superposition, where objects can exist in many states simultaneously, is at center stage. When multiple systems are prepared in certain types of “entangled” superpositions, there is a linkage between the systems that does not involve any apparent physical interaction. Einstein called this “spooky action at a distance,” and it is the key to the information processing power of quantum information science. The binary digits or bits from conventional information theory now take the form of quantum bits (“qubits”), which can store and process superpositions of numbers in a way that is impossible in any conceivable conventional information processor.

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Controlling the Quantum World: The Science of Atoms, Molecules, and Photons Quantum information theory is a young and rapidly developing field, spanning many areas of science and engineering. Conventional techniques such as logic gate families and error-correction are being adapted to the quantum realm. The landscape of possible quantum applications is still evolving. The best known application is Shor’s quantum factoring algorithm, which uses a quantum computer to factor a large number exponentially faster than any known classical algorithm. This has far-reaching implications in the world of cryptography, where most public-key current encryption standards are based on the inability to factor large numbers efficiently. The availability of a quantum factoring machine would render obsolete most of today’s encryption standards. Quantum mechanics offers a remarkable new method for secure data transmission. Quantum cryptography exploits the fundamental tenets of quantum mechanics to allow the secure transmission of information with no physical possibility of undetected eavesdropping. Quantum cryptographic instruments are already available commercially, featuring the use of small numbers of photons traversing a length of optical fiber. There is a rich array of other quantum communication protocols that allow the movement and networking of data in ways that are more efficient than corresponding classical procedures. AMO physics is concerned with the control and manipulation of atoms, molecules, and photons and is therefore well placed for the development of quantum hardware. Individual atoms confined with electromagnetic fields can be laser-cooled to be nearly motionless and to act as ideal qubit carriers of quantum information (see Figure 1–7). These atoms can be linked by implementing quantum logic gates through direct atom-atom interactions or through individual photons that couple atoms. In this way, large-scale entangled superpositions can be prepared. The use of atomic ion traps, optical lattices, and photons confined between closely spaced mirrors are but a few of the systems that are just starting to show promise for use as future quantum devices. The grand challenge of quantum information science is the scaling of these AMO systems to the quantum control of even more complex systems. In the realm of condensed matter systems, both superconducting devices that support quantized levels of currents or charges and spin-based devices are now being developed to show rudimentary quantum operations akin to their AMO cousins. While the development of quantum computing and communications hardware currently focuses on AMO science, the future of quantum information science will involve an exciting confluence of scientists and engineers of all stripes.

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Controlling the Quantum World: The Science of Atoms, Molecules, and Photons FIGURE 1–7 At NIST, an ultraviolet laser beam is used to manipulate ions in a high-vacuum apparatus containing an ion trap. These devices have been used to demonstrate the basic operations required for a quantum computer and for teleportation of the quantum state of one atom onto another. SOURCE: National Institute of Standards and Technology, copyright Geoffrey Wheeler. AMO SCIENCE AND NATIONAL POLICIES: CONCLUSIONS AND RECOMMENDATIONS The key future opportunities for AMO science contained in this report are based on rapid and astounding developments in the field that are a result of investments made by the federal government’s R&D agencies in the work of AMO researchers. In summary, the research field of AMO science and technology is

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Controlling the Quantum World: The Science of Atoms, Molecules, and Photons thriving, and the committee offers the following conclusions on the status of the science: Revolutionary new methods to measure space and time have emerged within the last decade from a convergence of technologies in coherent control of ultrafast lasers and ultracold atoms. This new capability creates unprecedented new research opportunities. Ultracold AMO physics was the most spectacularly successful new AMO research area of the past decade and led to the development of coherent quantum gases. This new field is poised to contribute significantly to the resolution of important fundamental problems in condensed matter science and in plasma physics, bringing with it new interdisciplinary opportunities. High-intensity and short-wavelength sources such as new x-ray freeelectron lasers promise significant advances in AMO science, condensed matter physics and materials research, chemistry, medicine, and defenserelated science. Ultrafast quantum control will unveil the internal motion of atoms within molecules, and of electrons within atoms, to a degree thought impossible only a decade ago. This capability is sparking a revolution in the imaging and coherent control of quantum processes and will be among the most fruitful new areas of AMO science in the next 10 years. Quantum engineering on the nanoscale of tens to hundreds of atomic diameters has led to new opportunities for atom-by-atom control of quantum structures using the techniques of AMO science. Compelling opportunities in both molecular science and photon science are expected to have far-reaching societal applications. Quantum information is a rapidly growing research area in AMO science and one that faces special challenges owing to its potential application for data security and encryption. Multiple approaches to quantum computing and communication are likely to be fruitful in the coming decade, and open international exchange of people and information is critical in order to realize the maximum benefit. The compelling research challenges embodied in these conclusions are discussed in more detail in the following chapters, which also highlight the broad impact of AMO science on other branches of science and technology and its strong coupling to national priorities in health care, economic development, the environment, national defense, and homeland security. The linkages to national R&D goals are clear. The White House set forth the

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Controlling the Quantum World: The Science of Atoms, Molecules, and Photons country’s R&D priorities in the July 8, 2005, memorandum of the science advisor to the President and the director of the Office of Management and Budget. These priorities were reiterated and strengthened in the President’s State of the Union Address on January 31, 2006, and in the President’s Budget Request for FY2007. AMO scientists contribute to these national priorities in several key areas: Advancing fundamental scientific discovery to improve the quality of life. Providing critical knowledge and tools to address national security and homeland defense issues and to achieve and maintain energy independence. Enabling technological innovations that spur economic competitiveness and job growth. Contributing to the development of therapies and diagnostic systems that enhance the health of the nation’s people. Educating in science, mathematics, and engineering to ensure a scientifically literate population and qualified technical personnel who can meet national needs. Enhancing our ability to understand and respond to global environmental issues. Participating in international partnerships that foster the advancement of scientific frontiers and accelerate the progress of science across borders. Contributing to the mission goals of federal agencies. An essential part of maintaining the country’s leadership in AMO science, and one of the White House’s R&D priorities, is to train and to equip the next generation of American scientists. The committee has compiled data on funding, demographics, and program emphasis from the federal agencies to help it assess the current state of AMO science in the United States. In summary, the committee offers 10 more conclusions, this time on government support for AMO science: Given the budget and programmatic constraints, the federal agencies questioned in this study have generally managed the research profile of their programs well in response to the opportunities in AMO science. In doing so, the agencies have developed a combination of modalities (large groups; centers and facilities; and expanded single-investigator programs). Much of the funding increase that has taken place at DOE, NIST, and NSF has been to benefit activities at research centers. The overall balance of the modalities for support of the field has led to outstanding scientific payoffs. The breadth of AMO science and of the agencies that support it is very

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Controlling the Quantum World: The Science of Atoms, Molecules, and Photons important to future progress in the field and has been a key factor in its success so far. Since all of the agencies report that they receive many more proposals of excellent quality than they are able to fund, it is clear that AMO science remains rich with promise for outstanding future progress. AMO science will continue to make exceptional advances in science and in technology for many years to come. In view of its tremendous importance to the national well-being broadly defined—that is, to our nation’s economic strength, health care, defense, education, and domestic security—an enhanced investment program in research and education in physical science is critical, and such a program will improve the country’s ability to capture the benefits of AMO science. Historically, support for basic research has been a vital component of the nation’s defense strategy, making the recent decline in funding for basic research at the defense-related agencies particularly troubling. The extremely rapid increase in technical capabilities and the associated increase in the cost of scientific instrumentation have led to very significant added pressures (over and above the usual Consumer Price Index inflationary pressures) on research group budgets. In addition, not only has the cost of instrumentation increased, but also the complexity and challenge of the science make investigation much more expensive. This “science inflator” effect means that while it is now possible to imagine research that was unimaginable in the past, finding the resources to pursue that research is becoming increasingly difficult. In any scientific field where progress is extremely rapid, it is important not to lose sight of the essential role played by theoretical research. Programs at the federal agencies that support AMO theory have been and remain of critical importance. NSF plays a critical and leading role in this area, but its support of AMO theoretical physics is insufficient. AMO science is an enabling component of astrophysics and plasma physics but is not adequately supported by the funding agencies charged with responsibility for those areas. The number of American students choosing physical science as a career is dangerously low. Without remediation, this problem is likely to create an unacceptable “expertise gap” between the United States and other countries. Scientists and students in the United States benefit greatly from close contact with the scientists and students of other nations. Vital interactions include the training of foreign graduate students, international

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Controlling the Quantum World: The Science of Atoms, Molecules, and Photons collaborations, exchange visits, and meetings and conferences. These interactions promote excellent science, improve international understanding, and support the economic, educational, and national security needs of the United States. Finally, the committee offers the following six recommendations, which as a whole form a strategy to fully realize the potential at the frontiers of AMO science. Recommendation. In view of the critical importance of the physical sciences to national economic strength, health care, defense, and domestic security, the federal government should embark on a substantially increased investment program to improve education in the physical sciences and mathematics at all levels and to strengthen significantly the research effort. Recommendation. AMO science will continue to make exceptional contributions to many areas of science and technology. The federal government should therefore support programs in AMO science across disciplinary boundaries and through a multiplicity of agencies. Recommendation. Basic research is a vital component of the nation’s defense strategy. The Department of Defense, therefore, should reverse recent declines in support for 6.1 research at its agencies. Recommendation. The extremely rapid increase in the technical capability of scientific instrumentation and its cost has significantly increased pressures (over and above the usual Consumer Price Index inflationary pressures) on research budgets. The federal government should recognize this fact and plan budgets accordingly. Recommendation. Given the critical role of theoretical research in AMO science, the funding agencies should reexamine their portfolios in this area to ensure that the effort is at proper strength in workforce and funding levels. Recommendation. The federal government should implement incentives to encourage more U.S. students, especially women and minorities, to study the physical sciences and take up careers in the field. It should continue to attract foreign students to study physical sciences and strongly encourage them to pursue their scientific careers in the United States.