From: LARRY KLAES (ljk4_at_msn.com)
Date: Thu Dec 11 2003 - 10:25:38 PST
----- Original Message -----
From: physnews_at_aip.org
Sent: Thursday, December 11, 2003 11:58 AM
To: ljk4_at_MSN.COM
Subject: Physics News Update 665
PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 665 December 10, 2003 by Phillip F. Schewe, Ben Stein, and
James Riordon
LIGHT FROZEN IN A HALL OF ATOMIC MIRRORS. In a new experiment a
pulse of light has been stopped without losing its optical energy.
A few years ago, two different Harvard groups succeeded in slowing
and then storing a pulse of light in atomic vapor. In that work the
propagation of light pulses was halted by vesting the properties of
incoming photons into the spin orientations of the atoms in the
vapor. Thus light pulses had been stopped by ceasing to exist in
the form of electromagnetic energy while ceding all of its signal
qualities to the atomic vapor. Later they could be reconstituted
into propagating light beams
(http://www.aip.org/enews/physnews/2001/split/521-1.html). Now, a
new experiment, also conducted at Harvard, brings light to a halt
but leaves the pulse intact as an optical entity. Mikhail Lukin and
his colleagues begin as before by converting the incoming light
pulse into a corresponding ensemble of spins in a vapor. But then
something else is added: a pair of counter-propagating laser beams
ease the pulse back into existence. But the control beams also
serve to herd the atoms in just such a way as to cause them to act
like a stack of mirrors. In this hall of atomic mirrors, the
original pulse still exists as electromagnetic radiation, but it
cannot move---it persists within a fixed stationary envelope. Thus
the light pulse containing optical photons is literally frozen in
space. It can be held and released into motion again on command.
The present experimental work follows a theoretical proposal
published last year in Physical Review Letters (89, 143602, 2003).
Researchers believe that the new phenomenon that they demonstrated
may be used to controllably localize, shape and guide stationary
photonic pulses in three spatial dimensions. This can create ideal
conditions for different light beams to interact or "talk" to each
other since localized light electromagnetic energy can be held in
one place for a relatively long time. Such techniques may enable
nonlinear interactions between faint laser pulses that could be
useful for processing light signals. For example, this process might serve in
optical computing, where calculations are carried out not with
electrons but with photons. Another ambitious goal would be to
perform logic operations between individual photons in future
quantum computers. But the researchers say that much further work
is still needed to determine if the present work can aid of any of
these applications.
For now, its just another step toward ultimate control of light.
(Bajcsy, Zibrov, and Lukin, Nature, 11 December 2003.)
DO QUANTUM MEASUREMENTS CHANGE IF THE DETECTOR MOVES? For example,
could a count of the number of photons in a burst of light depend on
the location of the detector in an extreme gravitational field?
These ideas, long pondered by physicists, might be verifiable in the
lab, according to a new theory in which a Bose Einstein condensate
(BEC) of cold atoms acts as a stand-in for the universal vacuum.
The related notion that potential energy residing in the vacuum can
influence the geometry of spacetime and thus the expansion of the
cosmos could also be testable in a tabletop experiment here in
Earth.
The pertinent phenomenon that would facilitate this line of research
is called the Unruh-Davies effect, which suggests that a detector
accelerating (not just moving at a constant speed but actually
moving ever faster) through a vacuum will effectively encounter
photons coming out of the vacuum. (A related phenomenon is the
Gibbons-Hawking effect, in which photons, "Hawking radiation," can
be detected in the gravitationally intense region of a black hole).
In the Unruh effect the energy needed to turn virtual photons into
real photons would be supplied by the accelerating detector itself.
The detector would see the vacuum not as an empty space but as a
thermal bath of photons. The same effect can disrupt quantum
teleportation (see the Update from a few weeks
ago---http://www.aip.org/enews/physnews/2003/split/660-2.html ). The
"temperature" of this bath would be proportional to the detector's
acceleration. Actually observing such a thermal bath (equivalent to
an effective temperature of something like 10^-15 K for a detector
acceleration one hundred thousand times more than that felt by us on
the surface of the Earth) with any foreseeable manmade detector is
close to impossible, but two physicists at the
Leopold-Franzens-Universitaet in Innsbruck, Petr Fedichev
(peter.fedichev_at_uibk.ac.at) and Uwe Fischer
(uwe.fischer_at_uni-tuebingen.de), believe the effect could be probed
by studying how sound waves ripple through BECs in the lab. The
superfluid condensate of atoms would correspond to the vacuum and
phonons would be analogous to photons moving through a curved
space-time. Before the experiment can be performed, larger BECs
than used so far will be needed, as well as sharper optical
manipulation of atoms in the BEC. (Physical Review Letters, 12
December 2003)
***********
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