Text 204, 82 rader
Skriven 2004-09-19 15:32:47 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 701
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PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 701 September 17, 2004
by Phillip F. Schewe and Ben Stein
        
AN ANTENNA FOR VISIBLE LIGHT, analogous to antennas for radio waves, can be
made with carbon nanotubes.  In a radio antenna, whose size is equal to the
wavelength of the incoming wave or a fair fraction of it, the wave excites
electrons into meaningful currents .  Such a response, amplified and tuned, is
the backbone of radio and TV broadcasting.  At optical wavelengths, where the
wavelength is hundreds of nm, this is harder to do.  Nevertheless, a
rudimentary antenna effect for visible light has now been observed by
scientists at Boston College using an array of carbon nanotubes, in which
infalling light excites miniature electrical currents.  According to Yang Wang
(wangyq@bc.edu,617-552-3436) one would like to measure these electrical
excitations directly, but this requires nano-diodes capable of processing
electrical pulses oscillating at optical frequencies (10^15 Hz), and these are
not yet available.  The next best thing is to observe the secondary radiation
emitted by the faint excitations.  The nanotubes used in the experiment are in
effect little metallic antennas about 50 nm wide and hundreds of nm long (see
figure at www.aip.org/png).  Not only can the nanotubes respond in the manner
of dipole radio antennas to incoming light, but they also exhibit a
polarization effect; when the incoming light is polarized at right angles to
the orientation of the nanotubes, the response disappears.  Possible
applications for visible-light antennas?  Optical television: a TV signal,
superimposed on a laser beam sent down an optical fiber, is demodulated at the
customer end by an array of nanotubes (each functionalized by a fast diode). 
Or efficient solar energy conversion: incoming light is turned into charge
which is stored in a capacitor.  (Wang et al., Applied Physics Letters, 27
September 2004; contact Zhifeng Ren, Boston College, 617-552-2832,
renzh@bc.edu)

CLOCK SYNCHRONIZATION WITH ENTANGLED PHOTONS has been proposed as an idea and
now demonstrated in an experiment. One of the important issues in the theory of
special relativity is the synchronization of clocks. How close can be the time
at one clock, t1, be to the time at a second clock, t2?  Modern clocks have
improved to such a level that the resolution and accuracy of the comparison
techniques have become the limiting factors to determine the degree of
synchronization, t1-t2.  New ideas, exploiting the novel aspects of entangled
photons, say that quantum mechanics can overcome the classical limit in regard
to clock synchronization (see Update 499). Physicists at the University of
Maryland, Baltimore County, have now confirmed the idea by doing an experiment
in which two entangled photons are sent respectively to two detectors some
distance apart. Pairs of entangled photons are produced in a nonlinear crystal
and will retain a special quantum correlation between themselves (belonging, as
they do, to a single quantum state) even if they were to move apart to
distances of trillions of km. The Maryland physicists (contact Alejandra
Valencia, avalen1@umbc.edu) synchronized two distant clocks, each attached to a
photodetector, by building up a statistical sampling of the clock responses,
first sending a photon from one emerging beam to one detector while its mate
went to the other detector, and then switching the entangled pairs to the
opposite detectors.  In this way, two clocks 3 km apart were synchronized
within a picosecond.  Synchronicity is of course critical in many areas of
telecommunications, especially in GPS. (Valencia et al., Applied Physics
Letters, 27 September 2004)

THE EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH, CERN, celebrates its 50th
anniversary on 29 September.  A sort of United Nations of physics, with
numerous European member states and many more non-European affiliates, the
Geneva-based CERN has been the site of several notable achievements and
discoveries in the area of elementary particle physics.  These include the
observation (1973) of neutral-current weak interactions, a type of scattering
event in which two particles interact via the interchange of a heavy neutral
boson force particle; later the production (1983) of that same force particle,
the Z boson, and its charged cousins, the W+ and W- bosons; the creation of the
World Wide Web (1990) as a means of transferring huge amounts of data; hints of
a novel kind of new nuclear matter (perhaps quark-gluon plasma) amid
high-energy, heavy-ion collisions (2000); and creation of slow-moving
anti-hydrogen atoms (2002).  The Large Electron Positron collider (LEP),
recently retired, was the scene of additional high-precision measurements of
the weak nuclear force and other aspects of the standard model.  LEP is lending
its 27-km-round tunnel for the construction of the Large Hadron Collider (LHC),
in which two beams of 7-TeV protons (or heavy ions) will be collided head on. 
Out of the violence of these smash-ups, physicists hope to achieve such
long-sought goals as producing the Higgs boson and various members of a family
of supersymmetric particles (consisting of boson cousins of known fermion
particles and fermi cousins of known boson particles), and maybe even discern
evidence for the existence of extra dimensions.  Completion is expected in the
year 2007. (See http://intranet.cern.ch/Chronological/2004/CERN50/)

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