This Week in Science

Science  28 Sep 2007:
Vol. 317, Issue 5846, pp. 1829
  1. When Quantum Arithmetic Doesn't Add Up


    If you add an item to your shopping basket and then remove one that is identical, your total bill does not change. If you shop in a quantum supermarket, however, the probabilities and the sequence of adding or subtracting to your shopping basket matter and can change the total. Parigi et al. (p. 1890; see the Perspective by Boyd et al.) demonstrate this counterintuitive behavior for photons in a light field. Adding or subtracting a single photon to or from a light field produces a different result depending on the sequence of the event, with the final state of light field being different from the initial one. This capability of engineering the quantum state of a light may prove useful in areas such as quantum communication.

  2. Fluorinated Drugs on the Rise

    Organofluorine substituents are playing an increasingly important role in synthetic small molecule pharmaceuticals, including such major drugs as Lipitor and Prozac. In a review, Müller et al. (p. 1881) highlight the emerging understanding of how fluorine interacts with proteins during docking events. Elucidation of these intermolecular interactions complements more traditional paradigms of fluorine's electron-withdrawing effects, which influence substrate basicity, and its wide-ranging stereoelectronic impact on substrate conformation. The discussion is supported by analysis gleaned from extensive searches of structural databases.

  3. Quantum Noise Reduction

    A successful quantum computer must overcome decoherence effects caused by interactions with its environment. Such effects could be mitigated if they fully characterized, which in principle can be done by process tomography. However, this process requires resources that grow exponentially with the number of qubits in the system, which renders it impractical. Emerson et al. (p. 1893; see the Perspective by Bacon) describe a theoretical technique through which key features of the decoherence can be efficiently measured, which reduces the number of experiments from exponential to polynomial. The authors present an experimental implementation of the method on quantum-information processors based on liquid-state and solid-state nuclear magnetic resonance methods.

  4. Up in the Air


    Widespread free oxygen is thought to have evolved in Earth's atmosphere only after about 2.4 billion years ago. Candidates for the cause of increase include dynamic effects in the planet's interior, escape of hydrogen, or the evolution of cyanobacteria. Anbar et al. (p. 1903) and Kaufman et al. (p. 1900) examined the geochemistry of a detailed section obtained in a drill core through the Mount McRae Shale, Western Australia, dated to about 2.5 billion years ago. They used trace element chemistry (particularly of molybdenum and rhenium, which respond to oxidative weathering) and the massindependent fractionation of sulfur isotopes (which can indicate the lack of abundant ozone) to trace the presence of free oxygen. The data indicate the first hints of atmospheric oxygen, but still at low levels, at this time.

  5. Seen On Edge

    In a rare alignment, the plane of Uranus' rings appeared edge-on to Earth in August 2007. This unusual configuration reveals the unlit side of the rings, including faint rings that are brightened by scattered light. With the Keck telescope in Hawaii, de Pater et al. (p. 1888, published online 23 August) snapped an infrared picture of the side-on rings. Diffuse dust envelops the entire ring system but is unconnected with any particular ring or feature. The pattern of dust has changed significantly since the rings were first photographed by the Voyager spacecraft in 1986, which indicates that such changes are common in the solar system and occur on much larger scales than had been expected.

  6. Spin, All Together Now

    Manipulation of the spin of an electron in a single quantum dot is a strong contender for quantum- information processing protocols. However, the spin dynamics of each dot depend strongly on environment, and the distribution resulting from dot-to-dot variability presents a formidable problem in addressing a system with many quantum dots. Greilich et al. (p. 1896) report on femtosecond magneto-optical pump-probe experiments on an ensemble of self-assembled semiconductor quantum dots. A sequence of laser pulses can induce all of the electronic spins across the ensemble to precess coherently. As the spin information is stored in the nuclear spin, this process effectively results in a long-term memory for the electronic phase information.

  7. Hawaiian Getaways

    Some Polynesian legends describe voyages from Hawai'i back to other Polynesian islands, but evidence for these return trips has been lacking. Collerson and Weisler (p. 1907; see the Perspective by Finney) have examined the chemistry of 19 basalt adzes collected on a coral atoll in the Tuamotus to trace their origin. The data link many of the adzes to nearby islands, but the isotope and trace element chemistry uniquely links one to Hawai'i, 3400 kilometers away. Thus, the Polynesians had a maritime trade network extending over thousands of kilometers and some repeated contact with Hawai'i.

  8. Viral Pesticide Resistance

    Although insect resistance to chemical agents is well documented, there is little information on virus insecticides. Generally, these have been considered safe reagents, with little prospect of resistance developing in the host; however, resistance to the codling moth granulovirus has been observed in field populations. Asser-Kaiser et al. (p. 1916) now show that the resistance factor is a Z sex-linked feature. After selection, a population was isolated that was 10,000- to 100,000-fold more resistant to the virus challenge, which is considerably higher than the naturally resistant field strains.

  9. Giving the Brain a Magnetic Massage


    Transcranial magnetic stimulation (TMS) is an increasingly common technique used to selectively modify neural processing. Although TMS reportedly alters neural and hemodynamic activity, basic neurophysiological evidence for these effects is largely unexplored. Allen et al. (p. 1918; see the news story by Miller) applied TMS to anesthetized cats while measuring neural and hemodynamic activity simultaneously in a co-localized region of the neocortex, and provide quantitative data on the neural effects of TMS and how they relate to standard neuroimaging techniques. These results also provide insight into the mechanisms of brain plasticity that are thought to underlie long-lasting therapeutic effects of TMS.

  10. Parasitic Evolutionary Oddity

    Giardia is a common intestinal protozoan parasite and an important human disease agent. Morrison et al. (p. 1921; see the Perspective by Keeling) offer a genome analysis of Giardia that reveals a wealth of unusual attributes, including an extremely simplified metabolic capacity relating to its parasitic life-style; little DNA heterozygosity in a cell thought to lack a sexual cycle; functionally enigmatic amino acid insertions in otherwise conserved regions of proteins; an unusual actin cytoskeleton that lacks conventional myosin; and simplified DNA replication and RNA processing machinery.

  11. Mammoth Mitochondrial Sequencing Effort

    Ancient DNA survives well in hair, is found in copious quantities in cold environments, and can be decontaminated easily. Gilbert et al. (p. 1927) used these advantages to completely sequence the mitochondrial genomes of 13 Siberian woolly mammoths. One of the samples came from the Adams mammoth, which was found in 1799 and has been stored at ambient temperatures for the last 200 years. This finding will facilitate analysis of samples of organisms that can only be found in museums.

  12. Sulfated Tyrosine and HIV Entry

    In order for human immunodeficiency virus type 1 (HIV-1) to enter host cells, its envelope glycoprotein gp120 must bind to the host-cell surface receptor CD4 and to a co-receptor. An unusual posttranslational modification, tyrosine sulfation, is important to the co-receptor interaction. Huang et al. (p. 1930) have investigated HIV-1 gp120 interactions with a sulfated N-terminal peptide from the co-receptor CCR5 and determined the crystal structure of a tyrosine-sulfated antibody in complex with gp120 and CD4. A conserved site in gp120 recognizes sulfo-tyrosine and might be a target for design of therapeutics.

  13. Modeling Developmental Segmentation

    One of the classic problems in embryology is the segmentation of the body plan. In vertebrates, this process is underpinned by a multicellular oscillatory genetic network. Riedel-Kruse et al. (p. 1911, published online 16 August) model this complex “segmentation clock” as a population of simple, coupled oscillators in the presence of noise and compared their modeling with experimentally induced segment defects after quantitative inhibition of Notch signaling in the zebrafish embryo. The model predicts the position of posterior defects, a synchronized transition, and the existence of resynchronization, and allows the estimation of coupling strength, noise, and robustness to perturbation.

  14. Neuronal Pathfinding

    During development, the growing tips of neurons must locate and identify their preferred targets. Neurons follow attractive and repulsive guidance signals to build a network to connect muscle activity to brain signals, sensory inputs to behavioral responses, and so on. Fujisawa et al. (p. 1934) have now discovered another component, EVA-1, in the guidance system for neurons that fine-tunes the guidance signals sent and received by different neurons in the model organism Caenorhabditis elegans.

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