PHYSICS NEWS UPDATE The American Institute of Physics Bulletin of Physics News Number 759 December 22, 2005 by Phillip F. Schewe, Ben Stein
A SCALABLE QUANTUM COMPUTER CHIP FOR ATOMIC QUBITS has been built for the first time by researchers at the University of Michigan (Christopher Monroe, [EMAIL PROTECTED]), offering hopes for making a practical quantum computer using conventional semiconductor manufacturing technology. Exploiting the strange rules of the atomic world, quantum computers could potentially break top-secret codes and perform certain kinds of searches much more quickly than conventional computers. The building blocks of quantum computers are called "qubits," or quantum bits, made of such objects as atoms or photons. Connecting multiple qubits via an electrostatic (or other suitable) interaction could then result in a quantum computer, similar to how wiring together individual transistors can make a traditional computer. Unlike a conventional computer's bits, which can have values of either 0 or 1, a qubit can possess a value of 0 and 1 simultaneously, analogous to a light switch that's on and off at the same time. For their qubit the Michigan group chose an individual cadmium ion, held in free space by a number of electrodes inside a postage-stamp-sized gallium arsenide semiconductor chip. There additional electric fields are able to manipulate the position of the ion, and laser beams could control the qubit value in the ion. Ions pose an advantage over other potential qubits (such as photons and electron dots) in that they are easier to isolate and shield from external disturbances (noise) that can disrupt their operation. An integrated semiconductor chip is a markedly different environment for ion qubits, which were previously held in hand-made ion traps that could not be easily scaled up or mass produced. The researchers have not yet demonstrated a quantum computer based on this design, as it only consists of a single qubit. Making a quantum computer would require scaling up a single chip so that it contains enough electrodes to trap many ions simultaneously. This has been a busy month for announcements in the ion computing realm: groups at NIST and Innsbruck independently reported entangling up to eight ions (Nature, December 8), while the Michigan group used another setup to perform a simple version of a quantum search known as Grover's algorithm (Brickman et al., Physical Review A, November 2005) FAST X-RAY PICTURES OF SAND JETS. Granular materials---possessing both solid-like and liquid-like characteristics---exhibit much strange emergent behavior even in the simplest of experiments. When, for example, a heavy sphere is dropped into a bed of sand, what happens, if you look carefully enough, can still surprise seasoned researchers. Heinrich Jaeger of the University of Chicago and his colleagues watched the jets kicked up by the sphere: they used high speed video and ordinary light to view the outside of the jets and high-speed radiography (the x rays supplied by the Advanced Photon Source at Argonne) of the jet interior. The impact kicked up a bizarre two-tiered jet structure: a thick shaft at the bottom and, projecting up out of the top, a further and thinner shaft (see figures at http://jfi.uchicago.edu/~jaeger/group/granular.html ). That the jets are so well collimated is a surprise: why doesn't the sand just fly out at all angles? In moving up in a sort of directed beam, with very little lateral motion, it seems to act like an ultracold gas (at least in the sideways direction). Another surprise is the twofold jet structure. The lower, thicker jet is surely sculpted by collisions between sand grains and air molecules since it gets progressively scantier until, at pressures close to vacuum, it goes away altogether, leaving only the thinner spiky jet. The jet interior pictures are unprecedented: taken with an exposure rate of 5000 frames per second, the x ray flux provided the equivalent of a 50-watt halogen lamp illumination---only at x-ray wavelengths. The x-ray pictures proved that air squeezed among the grains was the driving force in forcing up the thick stage of the jet formation, and not as one might have expected a force for dissipating the jet. (Royer et al., Nature Physics, December 2005; by the way, Nature Physics is a new journal that began publication in October 2005.) WHY ARE SOME COLEOPTERA BEETLES BLUE? Because light striking the beetle's external hard parts undergoes destructive interference. Precisely how this happens is now being studied quantitatively by a team of scientists in Namur, Belgium. Electron microscope pictures of the beetle's scaly cuticle (see http://www.aip.org/png/2005/243.htm ) help to explain that each scale is made of alternating layers of pure chitin (high index of refraction) and mixtures of chitin and air (low index of refraction). The resulting structure is a photonic crystal: because of wave interference, light of certain frequencies are excluded. In this case blue light is forbidden from being absorbed by the animal's shell; all blue light is reflected while other frequencies are absorbed in the cuticle, and the creature consequently has a blue appearance. Artificial photonic crystals have been studied for many years. Often featuring a honeycomb structure, these materials are to light waves what semiconductors are to electrons: transmission in certain energy bands is permitted while other bands are forbidden. Lord Rayleigh in 1918 was the first to suggest that the iridescence of some insects arose from interference effects. And by now the photonic-crystal effect is known to occur naturally in many places, such as the opalescence of weevils and the striking colors in the peacock's tail feathers. According to Jean Pol Vigneron at the University of Namur ([EMAIL PROTECTED]), lessons learned from the beetle scale's iridescence might be applied to the manufacture of paint, clothes, paper and in simplifying the kinds of windows and windshields that currently employ interference effects. The beetle's optics might also help in designing micro-fabricated displays in which different colors could be obtained through the clever reflection, rather than by emission, of light. (Vigneron et al., Physical Review E, December 2005) *********** PHYSICS NEWS UPDATE is a digest of physics news items arising from physics meetings, physics journals, newspapers and magazines, and other news sources. 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