From: LARRY KLAES (ljk4_at_msn.com)
Date: Fri Sep 26 2003 - 09:07:31 PDT
----- Original Message -----
From: physnews_at_aip.org
Sent: Friday, September 26, 2003 11:59 AM
To: ljk4_at_MSN.COM
Subject: Physics News Update 655
PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 655 September 26, 2003 by Phillip F. Schewe, Ben Stein, and
James Riordon
AN ULTRABRIGHT TUNABLE PHOTON-PAIR SOURCE created at MIT is the best
generator so far of entangled photon pairs, a development which
should help quantum communications systems to do their job more
smoothly. Entangled photons possess a special correlation unlike
anything in classical physics: if, say, we measure the spin
(polarization) of one photon, then we automatically know the
polarization of the other photon, even though it might be on the
other side of the galaxy and even if, until the moment of
measurement, the spins of both photons had been indeterminate. This
weird property of quantum reality, it is hoped, will be a boon to
encryption (perhaps in a "quantum teleportation" scheme---see
Physics News Update 350,
http://www.aip.org/enews/physnews/1997/split/pnu350-1.htm ) and
future quantum computers. Indeed, for some time now quantum effects
have been an important factor in communications engineering
applications, especially insofar as quantum fluctuations
(uncertainty in our knowledge of where an electron is or the value
of its energy) can produce levels of electrical noise that can
limit the effectiveness of practical devices. The use of entangled
photons might be able to mitigate this problem. Quantum limitations
are already a problem in such devices as optical amplifiers (whose
amplified spontaneous emission noise limits communication
performance) or soliton pulses (supposedly non-dispersing light
pulses that are subject to quantum-induced timing jitter
accumulation) used in fiber-optic communications. MIT's Research
Laboratory of Electronics is a place where quantum aspects of
electrical engineering are taken very seriously. The head of the
lab, Jeffrey H. Shapiro (jhs_at_mit.edu, 617-253-4179), will report on
progress in a program aimed at developing a system for
long-distance, high-fidelity teleportation of photon states at the
upcoming Frontiers in Optics meeting of the Optical Society of
America. As part of this work the MIT team has developed a source
of entangled photons some ten times brighter than previous sources.
The correlated photons are engendered by shooting a laser beam into
a nonlinear optical crystal, where incoming photons are, in effect,
split into two related photons of half the wavelength. This
"down-conversion" process is even tunable over a certain wavelength
range. Up to 12,000 photon pairs per second per milliwatt of input
power have been produced. (Paper MI3, OSA meeting 5-9 October in
Tucson, AZ; meeting website at http://www.osa.org/meetings/annual/ )
THE RELATIVITY OF TIME, as set forth in Einstein's theory, has been
affirmed once again, with new higher precision. Time dilation is
the name for the notion that elapsed time as recorded by two
observers with identical clocks will differ if one of the observers
is traveling at a velocity v with respect to the other. The amount
of dilation will become more noticeable as v becomes a larger
fraction of the speed of light. In an experiment performed by
Gerald Gwinner, Dirk Schwalm and their colleagues at the Max Planck
Institute for Nuclear Physics in Heidelberg the clocks are lithium
ions. The ions are struck by laser light from in front and from the
back, putting them temporarily into an excited state and inducing
fluorescence. By comparing the resonant laser wavelengths with the
transition wavelength of
the stationary ion, and by taking into account the Doppler effect
(the apparent wavelength of a wave emitted from a traveling source
will always be different from a stationary source owing to bunching
or thinning of the wave crests---but this has nothing to do with
relativity) the researchers can arrive at a value for time dilation.
In the Heidelberg experiment, the lithium ions moved with a speed of
19,000 km/sec, or about 6.4 % of the speed of light (and
corresponding to an energy of 13.3 MeV, the largest energy
obtainable at the local heavy-ion storage ring). The precision of
the new time dilation measurement, an
uncertainty of 2.2 x 10^-7, is about a factor of four better than
the best previous value. (Saathoff et al., Physical Review Letters,
upcoming article; contact Guido Saathoff,
guido.saathoff@mpi-hd.mpg.de49-6221-516-547; website at
http://www.mpi-hd.mpg.de/ato/rel/)
MALLEABILITY OF SPACETIME, as set forth in Einstein's general
relativity theory, has been affirmed, once again, by watching radio
waves from the Cassini spacecraft, on its way toward Saturn, be
deflected by the sun. Einstein said that a massive object would
distort the fabric of spacetime in its vicinity, and that this
distortion would slightly redirect the trajectory of light waves
passing the object. Scientists from three Italian universities
(those of Pavia, Rome, and Bologna) have carefully scrutinized
Cassini's radio report and found that the observed light deflection
is in accordance with the conventional form of relativity.
Furthermore, the sensitivity of their measurements is at a level
where some alternative gravity models can be probed for veracity.
(Bertotti et al., Nature, 25 November 2003.)
***********
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