Number 701, September 17, 2004
by Phil Schewe and Ben Stein
An Antenna for Visible Light
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 (1015 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).
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
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
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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