Text 679, 107 rader
Skriven 2005-12-22 15:18:22 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 759
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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, crmonroe@umich.edu), 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 (jean-pol.vigneron@fundp.ac.be), 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)

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 * Origin: Big Bang (1:106/2000.7)