Topic Name: Theoritical solution of supercomputers problem
Category: Computer science & technology
Research persons: Physicists Dietrich Leibfried and David Wineland
Location: NIST, 100 Bureau Drive, Stop 1070, Gaithersburg, MD 20899-1070, United States
GAITHERSBURG, MD—Physicists at the Commerce Department’s National Institute of Standards and Technology (NIST) have induced thousands of atoms trapped by laser beams to swap “spins” with partners simultaneously. The repeated exchanges, like a quantum version of swinging your partner in a square dance but lasting a total of just 10 milliseconds, might someday carry out logic operations in quantum computers, which theoretically could quickly solve certain problems that today's best supercomputers could not solve in years.
The atomic dance, described in the July 26 issue of Nature,* advances prospects for the use of neutral atoms as quantum bits (qubits) for storing and processing data in quantum computers. Thanks to the peculiarities of quantum mechanics, nature's rule book for the smallest particles of matter and light, quantum computers might provide extraordinary power for applications such as breaking today's most widely used encryption codes. Neutral atoms are among about a dozen systems being evaluated around the world as qubits; their weak interactions with the environment may help to reduce computing errors.
Led by Nobel Laureate William Phillips, the NIST group demonstrated the essential part of a so-called swap operation, in which atom partners exchange their internal spin states. (Spin can be visualized as a rotating top pointing up or down.) In the binary language of computers, the atoms swap values from 1 (“spin up”) to 0 (“spin down”), or vice versa. Unlike classical bits, which would either swap or not, quantum bits can be simultaneously in an unusual state of having swapped and not swapped at the same time. Under these conditions, spin swapping has the effect of “entangling” the pairs, a quantum phenomenon that links the atoms' properties even when they are physically separated. Entanglement is one of the features that make quantum computers potentially so powerful.
“This is the first time these spin-entangling interactions have been demonstrated between pairs of atoms in an optical lattice,” says Trey Porto, one of the authors. “Other research groups have entangled atoms in lattices as extended clusters. By isolating pairs, we can focus on the simplest units for quantum logic.”
The swapping process is a way of creating logical connections among data, crucial in any computer. A logic operation is the equivalent of an “if/then” statement, such as: If two qubits have opposite states, then they should exchange values. The logical connections in quantum computers are created using entanglement, which in effect allows for multiple simultaneous, correlated possibilities.
The NIST experiment was performed with about 60,000 rubidium atoms in a Bose-Einstein condensate (BEC), a special state of matter in which all atoms are in the same quantum state. They were trapped within a three-dimensional grid of light formed by three pairs of infrared laser beams. The lasers were arranged to create two horizontal lattices overlapping like two mesh screens, one twice as fine as the other in one dimension. This created many pairs of energy “wells” for trapping atoms.
The scientists attempted to place a single atom in each well, with one atom spin up (or 1) and the other down (or 0). Then, they merged all double wells to force each pair of atoms into the same well, where they could interact with each other. When two such identical atoms are forced into the same physical location, quantum mechanics imposes a specific type of symmetry (only two of four seemingly possible combinations of quantum states are allowed). Due to this restriction, the merged atoms oscillate between the condition in which one atom is 1 and the other is 0, to the opposite condition. This behavior is unique to identical particles.
As they swap spins, the atoms pass in and out of entanglement. At the “half-swap” points the spin of each atom is uncertain and, if measured, might turn out to be either up or down. But whatever the result, a measurement on the other atom, equally uncertain before the measurement, would be sure to be the opposite. This entanglement is the key feature that enables quantum computation. According to Porto, the work reported in Nature is the first time that quantum mechanical symmetry (“exchange symmetry”) has been used to perform such an entangling operation with atoms.
The current set-up is not directly scalable to an arbitrary computer architecture, Porto says, since it performs the same spin-swap in parallel for all pairs of atoms. Researchers are developing ways to address and manipulate any pair of atoms in the lattice, which should allow for scalable architectures. Furthermore, not all atoms participated in the swap process, primarily because of imperfect initial loading of the atoms in the lattice. (Some double-wells contained only one atom and had no partner to exchange with.) The scientists estimate that the swap worked for at least 65 percent of the double wells.
The NIST group is continuing to work on improving the reliability of each step and on completing the logic operation by separating atoms after they interact. The research was funded in part by the Disruptive Technology Office, the Office of Naval Research and the National Aeronautics and Space Administration. The authors are affiliated with the Joint Quantum Institute, a collaboration of NIST and the University of Maryland.
About Quantum Information Research at NIST: Goals and Vision
America’s future prosperity and security may rely in part on the exotic properties of some of the smallest articles in nature. Research on quantum information (QI) seeks to control and exploit these properties for scientific and societal benefits. This remarkable field combines physics, information science, and mathematics in an effort to design nanotechnologies that may accomplish feats considered impossible with today’s technology. QI researchers are already generating “unbreakable” codes for ultra-secure encryption. They may someday build quantum computers that can solve problems in seconds that today’s best supercomputers could not solve in years. QI has the potential to expand and strengthen the U.S. economy and security in the 21st century just as transistors and lasers did in the 20th century.
Nations around the world are investing heavily in QI research in recognition of the economic and security implications. A significant part of the U.S. effort is based at the National Institute of Standards and Technology (NIST), which has the largest internal QI research program of any federal agency.
NIST laboratories routinely develop the measurement and standards infrastructure needed to promote innovation in emerging fields that may transform the future. Few fields need this support as much as QI, which involves entirely new concepts of information processing as well as complex hardware for precision control of individual atoms, very small quantities of light, and electrical currents 1 billion times weaker than those in light bulbs. As the nation’s measurement experts, NIST researchers long have had world-class capabilities in precision measurement and control of atoms, light, and other quantum systems. NIST, therefore, has the world-class skills and facilities needed to advance QI through technology demonstrations, development of new methods and tests for evaluating QI system components, and related scientific discoveries.
NIST first became involved in quantum information science in the early 1990s when physicist David Wineland and colleagues realized that engineering of exotic quantum states could lead to a significantly more precise atomic clock. A few years later, Wineland demonstrated the first quantum logic operation, a pioneering step toward a future quantum computer using ions (electrically charged atoms) to process information. In 1999, the NIST Physics Laboratory launched a broader Quantum Information Program, joined shortly thereafter by NIST’s Information Technology Laboratory and Electronics and Electrical Engineering Laboratory.
This interdisciplinary program, featuring strong collaborations among physicists, electrical engineers, mathematicians, and computer scientists, has established NIST as one of the premier QI programs in the world. Participants include Wineland, a NIST Fellow and Presidential Rank Award winner; physicist William D. Phillips, a 1997 Nobel Prize winner in physics; mathematician Emanuel Knill, a leading QI theorist; and physicist Sae Woo Nam, winner of a Presidential Early Career Award for Scientists and Engineers. A total of nine technical divisions within three different laboratories at NIST’s Gaithersburg and Boulder campuses are involved.
NIST’s work in ion-trap quantum computing is widely recognized as one of the most advanced QI efforts in the world. Scientists building the NIST quantum communications testbed set a record in 2004 for the fastest system for distributing quantum cryptographic “keys,” codes for encrypting messages that, due to the peculiarities of quantum physics, cannot be intercepted without detection. Other NIST research with single photon sources and detectors, and computing with neutral atoms and “artificial atoms,” are also among the leading efforts worldwide. For instance, prospects for practical quantum communications have been improved by NIST’s recent demonstration of a device that detects single photons with 88 percent efficiency, a QI record.
There is strong synergy between NIST’s core mission work on measurement and standards and the QI research program. For instance, NIST scientists gained much of their expertise in quantum systems from decades of work developing atomic clocks. NIST’s ultra-precise atomic fountain clock—the world’s most accurate device for measuring time—is based on the precise manipulation and measurement of two quantum energy levels in the cesium atom. This clock would neither gain nor lose one second in 60 million years (as of March 2005), an accuracy level that is continually being improved. NIST quantum computing research is producing new techniques that may lead to even more accurate atomic clocks in the future.
Ultimately, NIST measurements, tests, and technologies for quantum information science are helping U.S. industry develop new information technologies in an effort to ensure U.S. technological leadership and strengthen national security. The United States may have the lead in this field for now—based in part on NIST’s contributions—but competition from Europe, Japan, Australia, and developing countries such as China is strong and growing
Phone: (301) 975-NIST (6478) or TTY (301) 975-8295
In The Image
Dietrich Leibfried and David Wineland