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{at}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{at}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{at}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/)

---