Text 355, 79 rader
Skriven 2005-03-06 19:18:36 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 722
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PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 722 March 3, 2005
by Phillip F. Schewe, Ben Stein
        
240 ELECTRONS SET IN MOTION.  A soccerball-shaped carbon-60 molecule,
possessing a mobile team of up to about 240 valence electrons holding the
structure together, is sort of halfway between being a molecule and a solid.  
To explore how all those electrons can move as an ensemble, a team of
scientists working at the Advanced Light Source synchrotron radiation lab in
Berkeley, turned the C-60 molecules into a beam (by first ionizing them) and
then shot ultraviolet photons at them.  When a photon absorbed, the energy can
be converted into a collective movement of the electrons referred to as a
plasmon.  Previously a 20-electron-volt "surface plasmon" was observed: the
absorption of the UV energy resulted in a systematic oscillation of the
ensemble of electrons visualized as a thin sphere of electric charge.  Now a
new experiment has found evidence of a second resonance at an energy of 40 eV. 
This second type of collective excitation is considered a "volume plasmon"
since the shape of the collective electron ensemble is thought to be
oscillating with respect to the center of the molecule (see figure at
http://www.aip.org/png/2005/230.htm).  The collaboration consists of physicists
from the University of Nevada, Reno (Ronald Phaneuf, 775-784-6818, phaneuf@unr.
edu), Lawrence Berkeley National Lab, Justus-Liebig-University (Giessen,
Germany), and the Max Planck Institute (Dresden).  (Scully et al., Physical
Review Letters, 18 February 2005)

FIRST EVIDENCE FOR ENTANGLEMENT OF THREE MACROSCOPIC OBJECTS has been seen in a
superconducting circuit built at the University of Maryland.  By examining an
electrical circuit operating at temperatures near absolute zero, the
researchers have found new evidence that the laws of quantum mechanics apply
not just to microscopic particles such as atoms and electrons, but also to
large electronic devices called superconducting quantum bits (qubits). While
researchers have previously created superconducting qubits, and other groups
have entangled two macroscopic objects (Update 558), this research is the first
to observe the quantum interaction of three macroscopic components: a niobium
inductor-capacitor (LC) circuit plus a pair of Josephson junctions, each a
sandwich of two superconductors separated by an insulator.  Remarkably, all
three macroscopic devices seem to act, when cold enough, like huge atoms.  The
LC circuit coupled the Josephson junctions in such a way as to transfer
quantized oscillations of current in one junction to the other junction.  The
LC circuit was more than a simple connector; its condition depended upon the
two Josephson junctions in a way defined by the laws of quantum mechanics.  The
researchers obtained evidence of the entanglement indirectly, through the use
of microwave pulses that probed the Josephson junctions; future experiments
will seek to directly control the junctions and obtain evidence more directly.
Superconducting circuits such as this one provide a promising route towards a
practical quantum computer, which would require the entanglement of many
qubits. Scaling up superconducting devices to many-qubit systems should be
possible once single superconducting qubits are perfected, according to team
member Frederick Strauch, (now at NIST, 301-975-5159,
Frederick.Strauch@nist.gov). The challenge will be to fabricate sufficiently
high-quality circuits so that the superconducting qubits achieve the very low
noise levels necessary for quantum computing. (Xu et al., Physical Review
Letters, 21 January 2005)
                        
X-RAY THUNDERBOLT.  Scientists have long suspected that lightning might
generate x rays.  However, until recently the observation of such x-rays has
remained elusive, largely owing to the unpredictable nature of lightning.  In
the last few years a series of experiments by Joseph Dwyer and his colleagues
at the Florida Institute of Technology and the University of Florida has shown
that lightning indeed emits large bursts of x rays with energies up to about
250 keV (about twice that of a chest x ray).  These x rays are mostly produced
not by the bright return strokes, but by the leaders that precede the stroke,
as they propagate from the cloud to the ground. Now, Dwyer and his colleagues
have discovered that these bursts of x rays are produced at the precise moment
that the lightning steps forward along its jagged path.  For unknown reasons,
lightning does not travel to the ground in a continuous manner, but instead
traverses the distance in a series of discrete steps.  It is this stepping
process that gives lightning its jagged, sometimes forked, appearance, and
Dwyer has now shown that this same stepping process also makes x rays.  The x
rays are likely produced by strong electric fields that accelerate electrons to
close to the speed of light.  These so-called runaway electrons collide with
air producing bremsstrahlung ("braking radiation" in German) x-rays. Dwyer says
that higher energy gamma rays are also emitted sometimes, but that these seem
to come from the thunderstorm cloud itself and not from the lightning stroke. 
(Dwyer et al., Geophysical Review Letters, 16 January 2005.)

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