This Week in Science

Science  01 Jun 2007:
Vol. 316, Issue 5829, pp. 1249
  1. Quantum Storage and Repetition


    Quantum registers allow the basic units of information, the qubits, to be transferred along channels and processed locally. Dutt et al. (p. 1312) demonstrate the coherent control and coupling of an electronic qubit and nuclear qubit stored in a single nitrogen vacancy (NV) center in diamond. Because the spin degree of freedom of the NV center is largely decoupled from the vibrational (and other kinetic) degrees of freedom, these two qubits can form a room-temperature quantum register where the electronic qubit could enable optical communication with other registers through state-dependent fluorescence, whereas the nuclear qubit could be used to store information. These registers can be used as a basis for scalable, optically coupled quantum information systems. The use of quantum processing in applications such as encryption requires the distribution of entangled quantum states over networks. These states have finite lifetimes, and networks will likely need to use quantum repeaters, which would reliably pass on the information without actually destroying it. Chou et al. (p. 1316, published online 5 April) distribute an entangled state over a distance of 3 meters using two pairs of cold atomic gas clouds, thereby demonstrating the potential for long-distance quantum communication.

  2. West African Monsoon Paleohydrology

    Monsoons account for much of the rainfall at low latitudes and are an important control of the moisture and heat budgets of the atmosphere. The West African monsoon affects a broad region of that continent, but it is unclear how it is connected to other large-scale climate processes, such as the El Nio-Southern Oscillation or nearby changes in sea surface temperatures (SSTs). Weldeab et al. (p. 1303; see the Perspective by Barker) analyzed the Mg-Ca and Ba-Ca ratios, as well as oxygen isotopic compositions of planktonic foraminifera from a marine sediment core recovered from the Gulf of Guinea, in order to document changes in SSTs and river runoff there during the past 155,000 years. The variability of freshwater input more closely resembled that of high northern latitude temperature than local SSTs for much of the record, which implies that high northern latitude climate exerted the dominant control on precipitation. Local SSTs were more closely related to low-latitude insolation.

  3. Disobeying Transport Rules

    The empirical Wiedemann-Franz law, which states that the ratio of thermal and electronic conductivities of metals is directly proportional to temperature, is one of the oldest relations in condensed matter physics. It has also been thought to be a robust property of metals because it is theoretically underpinned by quantum mechanics and the standard model of metals, Fermi-liquid theory. Tanatar et al. (p. 1320; see the Perspective by Coleman) present measurements of thermal and charge transport in CeCoIn5 and report that this law is obeyed in one direction in the crystal lattice but violated in another. They discuss the consequences of this anisotropic violation in terms of how Fermi-liquid theory should or could be modified to describe the behavior of such strongly correlated systems.

  4. Soft Iron Center

    The core of Mars is widely thought to be similar to that of Earthmostly iron with a smattering of light elements. In high-pressure experiments that mimic the conditions of the martian core, Stewart et al. (p. 1323) show that Mars' core is presently completely liquid and that its future crystallization behavior will differ from that of the Earth. Mars will not form an outwardly crystallizing iron-rich inner core as does the Earth. Instead, planetary cooling will lead to core crystallization following either a snowing core model, in which iron-rich solids nucleate in the outer portions of the core and sink toward the center, or a sulfide inner-core model in which an iron-sulfide phase crystallizes to form a solid inner core.

  5. Preservation and Decay

    Competition between degradation and preservation of organic matter in the sea and on the sea floor plays a central role in controlling how much oxygen is in the atmosphere, as well as how carbon, nitrogen, phosphorus, and other elements are cycled through the biogeosphere. Because the quantity of carbon in marine sediments is huge, small differences in the balance between these processes can have large impacts, which has made this problem challenging to model. Rothman and Forney (p. 1325; see the Perspective by Middelburg and Meysman) now present a consistent model in which the intrinsic reactivity of organic material is constant and that the rate of decay depends on bacterial abundance. This explanation is fundamentally different from chemical models in which organic-matter degradation rates depend on intrinsic reactivity.

  6. Not Like Peas in a Pod


    Legumes, such as peas and beans, have residing within nodules on their roots symbiotic bacteria that fix atmospheric nitrogen. The rhizobial symbiont produces signaling molecules, the so-called Nod factors, that are recognized by the plant. Giraud et al. (p. 1307; see the Perspective by Downie) report the complete genomic sequences of two photosynthetic Bradyrhizobium strains that produce nodules in their host plant roots but lack the nodABC genes and Nod factors. Instead, during this atypical symbiosis, a purine derivative may act as a signal molecule to trigger root nodule organogenesis.

  7. Walking Tall with Hand Holds

    The predominant bipedal locomotion of humans, which helps distinguish us from great apes, has generally been thought to have arisen since or with the last common ancestor between humans and chimps. Thorpe et al. (p. 1328, see the cover and see the Perspective by O'Higgins and Elton) show that orangutans, a more distant relative, move across thin branches in trees in a bipedal fashion, much like running humans. These observations suggest that this more ancient ability was retained in the great apes and increasingly used by ancestral humans as they moved out of the trees, whereas chimps and gorillas essentially lost this ability.

  8. Spaced Out

    Methodological approaches for geospatial models of infectious disease encompass analyses of the effects of patches, distance transmission, multigroups, and networks. Within this framework, Riley (p. 1298) has reviewed key results on infections such as measles, foot-and-mouth, influenza, and smallpox, illustrating the significance of the epidemiological insights generated by these models.

  9. A Growing List of Diabetes Genes


    Type 2 diabetes, the most common form of diabetes, affects more than 170 million people worldwide, and its prevalence is increasing rapidly. An individual's propensity to develop the disorder is determined by a combination of life-style and hereditary factors. Three independent international consortiaScott et al. (p. 1341, published online 26 April), Zeggini et al. (p.1336, published online 26 April), and the Diabetes Genetics Initiative (p. 1331, published online 26 April)have conducted comprehensive surveys of the human genome to identify genetic variants that affect type 2 diabetes risk and then shared their data to increase the statistical power of their analyses. In addition to validating several sequence variants previously implicated in the disorder, the authors identified several previously unknown susceptibility variants. At least 10 genetic loci have been now reliably linked to type 2 diabetes, each exerting a modest effect on risk.

  10. Let the RNA Do Its Business

    Viruses need to ensure that infected cells survive long enough to allow the production of progeny. Reeves et al. (p. 1345) now show that human cytomegalovirus uses an unusual strategy to this endan abundant noncoding RNA specifically interferes with an apoptotic trigger in the mitochondrion. In cells in which apoptosis has been triggered, the RNA binds to an enzyme in the mitochondrion and helps to maintain functional mitochondria, which prolongs the life of the infected cells. This mechanism obviates the need for the virus to translate a protein product to perform this function and may thus exploit the infected cell's resources more effectively.

  11. Designer Proteasome

    In the immune system, T cells respond to fragments of antigenic proteins presented at the cell surface by molecules encoded by genes of the major histocompatibility complex. This process is vital not only for recognition of antigens carried by pathogens and tumors, but also in the selection of T cells as they develop in the thymus. These peptides are generated by a large multisubunit complex called the proteasome, which exists in a variety of forms containing different catalytic subunits. Murata et al. (p. 1349; see the Perspective by Bevan) now identify a proteasome subunit, 5t, that was found exclusively in the thymic epithelial cells and directs positive selection of T cells. Indeed, mice lacking 5t showed significant disruption of T cell development.