May 2010 Archives

Robert Eagle of Caltech and his collaborators have shown that they can determine the body temperature of living and long-dead vertebrates by measuring the abundance of a molecule made of isotopes—an isotopologue—in bones, scales, and teeth. The isotopologue is a heavy version of the carbonate ion CO₃²⁻. In a typical piece of bone or other biomineral, all but 1.8% of the CO₃²⁻ ions are made of the lightest carbon and oxygen isotopes, ¹²C and ¹⁶O. At around 45 ppm, ¹³C¹⁸O¹⁶O₂²⁻ is barely present, but its scarcity is made up for by a useful property: the isotopologue’s precise abundance depends on the ambient temperature when the biomineral first crystallized. The temperature dependence arises because lower temperatures boost the propensity of a heavy isotope to form a bond with another heavy isotope rather than with a light isotope. Five years ago, Prosenjit Ghosh, who is now at the Indian Institute of Science, and his colleagues extracted CO₂ gas from carbonate crystals they’d made in the lab. From their measurements they derived a robust formula relating the abundance of ¹³C¹⁸O carbonate to its formation temperature. By applying the formula to tooth samples, Eagle could accurately predict the body temperature of five vertebrates, including the white rhino (37 °C) and the sand tiger shark (23 °C). From fossilized samples he could also predict the body temperature of the woolly mammoth (38 °C). Applying the paleothermometer to samples of other extinct vertebrates could reveal when vertebrates first became warmblooded. (R. A. Eagle et al., Proc. Natl. Acad. Sci. USA, in press, doi:10.1073/pnas.0911115107.)—Charles Day

One hallmark of Albert Einstein’s genius is his 1905 theory that the kinetic energy of pollen grains, dust, and other similarly sized objects in thermal equilibrium depends solely on temperature—the classic definition of Brownian motion. Einstein then concluded that the instantaneous velocity of such particles would be impossible to physically measure, and for more than a century, it seemed that he was right. But now, Mark Raizen and his colleagues at the University of Texas at Austin have used optical tweezers in a vacuum chamber to trap a 3-μm-diameter silica bead, observe its ballistic (inertial) motion at short time scales, and determine its instantaneous velocity. The bead is held at the focal point of two noninterfering laser beams, similar to the setup in the image. When the bead makes a random move, it deflects the beams, which allows its position to be traced and the instantaneous velocity to be measured. From those measurements, the researchers calculated root mean square velocities; even when taken at varying air pressures, the results agreed with each other and with the theoretically predicted value, proving that in the ballistic regime, the bead’s mean velocity is solely dependent on temperature and not on pressure or the inertial effects of the surrounding air molecules. Raizen says they will next attempt to cool the particle’s motion to the quantum ground state and confirm that the kinetic energy will be nonzero even at 0 K. (T. Li et al., Science, in press, doi:10.1126/science.1189403.)—Jermey N. A. Matthews

As famously predicted by Hendrik Casimir in 1948, parallel conductors in a vacuum will attract each other because the conductors impose boundary conditions that affect the vacuum energy of the electromagnetic field (see the article by Steve Lamoreaux in Physics Today, February 2007, page 40). In general, the Casimir force depends on the shape of the conductors and its value is notoriously difficult to calculate, but research groups worldwide have been developing increasingly applicable computational techniques. Now a team at MIT has shown how tabletop measurements might provide the key information needed for the general calculation. The Casimir force may be expressed as an integral over frequency (ω) of correlation functions that involve electric and magnetic fields. In principle, those frequency-dependent correlations can be obtained in a suitably scaled tabletop experiment from measurements of how an antenna at one point responds to a current generated at a distant point. In practice, such measurements won’t work because the integrand oscillates wildly with ω. The integrand becomes well behaved—it decays and doesn’t oscillate—if the integration is performed in the complex plane, but real antennas respond to real frequencies. The key observation made by the MIT team is that their mathematical expressions always involve ω in the combination εω², where ε is the permittivity. Thus, the researchers predict, a force integral with real vacuum permittivity and complex contour can be calculated from a tractable number of antenna measurements made at real ω in a medium of complex permittivity—for example, salt water. (A. W. Rodriguez et al., Proc. Natl. Acad. Sci. USA, in press, doi:10.1073/pnas.1003894107.)—Steven K. Blau

A central tenet of quantum information processing asserts that an unknown qubit cannot be cloned (see Physics Today, February 2009, page 76). But the unknown state of one qubit can be transferred to another qubit in a process termed quantum teleportation. The first experimental demonstrations succeeded in teleporting a qubit state a meter or so (see Physics Today, February 1998, page 18). Subsequent experiments with photons, whose polarizations form a convenient basis for quantum information, have used fiber optics to achieve teleportation over hundreds of meters. But practical quantum communication will require teleportation over much greater distances. Jian-Wei Pan, Cheng-Zhi Peng, and coworkers at the University of Science and Technology of China and Tsinghua University have now transferred a qubit state through free space over a distance of 16 km, from "Alice" in the Beijing suburb of Badaling, across towns and roads, to "Bob" in Huailai, on the other side of Guanting Reservoir. The experiment employed a standard teleportation protocol: Alice and Bob each receive one of a pair of entangled photons; Alice measures hers in combination with an unknown qubit and sends the result, by classical means, to Bob; armed with that result, Bob projects his photon onto the state of the unknown qubit. The new work, though, adds many refinements, including novel telescope designs for open-air transmission, active feedback control for increased stability, and synchronized real-time information transfer. The resulting teleportation fidelity was nearly 90%. Such high-fidelity transmission, say the researchers, could help enable quantum teleportation to orbiting satellites. (X.-M. Jin et al., Nat. Photon., in press, doi:10.1038/nphoton.2010.87.)—Richard J. Fitzgerald

Over the past 10 years, researchers have designed aberration correctors that improve the resolution of transmission electron microscopes (TEMs). The improvements make the biggest difference for low-energy electrons, which experimenters must use to perform nondestructive studies of samples comprising light atoms, such as carbon nanotubes or organic molecules. Some researchers hope to use the new generation of higher-resolution TEMs to determine the chemical composition of a molecule lying on a thin surface. Now, scientists have taken a step closer to realizing that dream by using a scanning TEM to identify the individual atoms in a single layer of boron nitride having carbon and oxygen impurities. As a tightly focused electron beam raster scanned the surface, an annular dark-field detector measured the extent of wide-angle electron scattering from each atom. The intensity at each plotted bright spot in the image is a function of the atomic number of the atom located there. The superposed colored dots label each atom. Boron (red) and nitrogen (green) atoms constitute the regular BN lattice, while carbon (yellow) and oxygen (blue) atoms are substitutional defects. The experiment was performed by Ondrej Krivanek of Nion Co in Washington and colleagues from Oak Ridge National Laboratory, the University of Oxford, and Vanderbilt University. (O. L. Krivanek et al., Nature 464, 571, 2010.)—Barbara Goss Levi

Earth is awash in vibrations—literally. Interference between ocean waves near a coastline excites faint ambient noise throughout the planet. Thanks to a recent innovation in seismic imaging, long time sequences of the ambient noise can be used to reveal details about Earth’s interior hundreds of miles inland. The technique, ambient noise tomography, involves looking for correlations between the diffuse seismic surface waves recorded at pairs of closely spaced seismometers and then extracting the group and phase velocities from the correlated signals. By tapping high-frequency wave components, which are typically lost to scattering and attenuation in signals from distant earthquakes but strong in ambient noise, seismologists can probe the roughly 30-km-thick continental crust, whose rheology differs from that of the deeper mantle. Using the technique, Michael Ritzwoller and his colleagues at the University of Colorado, Boulder, have now found evidence for widespread crustal deformation in the western US, a region long thought to have suffered strain from extension ever since the Cenozoic era began 65 million years ago. Ritzwoller’s team processed waveforms captured between 2004 and 2007 by the USArray Transportable Array, which comprises some 400 broadband seismometers on a 70-km² grid, and extracted the radial anisotropy—the differences in speed of vertically and horizontally polarized surface waves, plotted here. The anisotropy, a proxy for strain in crustal rock, is consistent with the lattice orientation of minerals found in surface outcrops and constrains models of how continents are built. (M. P. Moschetti et al., Nature 464, 885, 2010.)—R. Mark Wilson

Researchers in Russia and the US have collaborated to synthesize a new element with atomic number Z = 117. They’ve done it using the same technique they used over the past dozen years to make elements 113–116 and 118: bombarding an actinide target—berkelium in this case—with a beam of calcium-48 ions energetic enough to fuse with the actinide nuclei. Six atoms of element 117, five with 176 neutrons and one with 177, became lodged in position-sensitive detectors, where each underwent a series of alpha decays followed by a spontaneous fission. The timing and energy pattern of the observed decays allowed the researchers to identify new atoms. Along the alpha-decay chains were new isotopes of elements 115 and 113, roentgenium (Z = 111), meitnerium (109), bohrium (107), and dubnium (105), all with more neutrons than the previously known isotopes of those elements. The greater neutron numbers N brought with them greater stability and longer half-lives, in agreement with the theory that predicts an island of stability somewhere around Z = 114–126 and N = 184. In particular, the new isotopes of element 113 each lived for several seconds, long enough that the researchers hope to be able to probe the element’s electronic—that is, chemical—properties. With electrons moving at relativistic speeds, element 113 might not behave at all like thallium, the element just above it in the periodic table. (Y. T. Oganessian et al., Phys. Rev. Lett. 104, 142502, 2010.)—Johanna Miller

In living organisms, the directional-transport capabilities of cell membranes ensure the concentration and confinement of DNA and the ingredients needed for its replication. But how was that accomplished at the dawn of life on Earth, when there were no such membranes to constrain indiscriminate diffusion and the attendant entropy growth? At the University of Munich, Dieter Braun’s biophysics group has addressed those questions with a suggestive demonstration of efficient replication and accumulation of DNA driven only by a quasistatic thermal gradient in a fluid-filled glass capillary stocked with nucleotides, a polymerizing enzyme, and a small initial template charge of the DNA to be replicated. The 0.1-mm-wide capillary is meant to mimic pores at hydrothermal vents in early seas. The thermal gradient is provided by an IR laser. The gradient drives DNA molecules away from hot spots and sustains a convective flow that subjects the DNA to thermal cycles that create new DNA and concentrate it at the sealed ends of the capillary. The convective cycles unzip the 143-base-pair double helices at 86 °C and then let them replicate at 60 °C. The DNA population doubles every minute or so, until exhaustion of the nucleotide stock eventually puts an end to that exponential growth. The figure shows the growing DNA concentration at one of the capillary ends over a span of 14 minutes. With continual nucleotide replenishment, such a system might replicate DNA at the rate of 1700 doublings per day. So aside from its possible relevance to paleobiology, the result is of biotechnological interest. (C. B. Mast, D. Braun, Phys. Rev. Lett., in press.)—Bertram Schwarzschild