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Post Info TOPIC: Light


Posts: 131433
RE: Light

It's the ultimate trick with light. Scientists have succeeded for the first time in extinguishing light in one spot and then making it reappear somewhere else.
This unprecedented control over light could pave the way for superfast computers and totally secure communication systems.
University of Sydney physicist Stephen Bartlett said the research by an American team was an important step towards the development of light-based technologies.
Nothing travels faster than light, which travels from the sun to Earth in only eight minutes, but scientists want to slow it down so they can better manipulate it.
Researchers led by Lene Hau at Harvard University had their first success in 1999, when they reduced the speed of light by 20 million fold, to 60kmh by passing it through a cloud of very cold gas (just above minus 273C).
In 2001 her team brought light to a dead stop in a similar cloud, known as a Bose-Einstein condensate.
For the new study, published overnight in the journal Nature, Professor Hau created two condensates a tiny distance apart. When a laser light pulse was beamed into one it slowed to a halt, as expected. But the light was then completely converted to matter, which travelled over to the second condensate at a leisurely pace of 200 metres an hour.
The original light pulse was reincarnated in that condensate, and went on its way. Professor Hau said the fact that the light was momentarily present as matter in the tiny gap between the condensates was very important, because matter was easy to manipulate, unlike light.

 "It looks rather like black magic, but it is just quantum mechanics" - Michael Fleischhauer, of the Technical University of Kaiserslautern in Germany.

Dr Bartlett said the light could be extinguished in one condensate and revived in the other because, at the quantum level, atoms of the same kind are indistinguishable, no matter how far apart they are.

"You have to think of the two condensates as one condensate in two different places"

Superfast quantum computers based on light would be able to handle huge amounts of information, for example to better model climate change.




Posts: 131433
RE: Excitons

When light hits a semiconductor material and is absorbed, its photons can become "excitons," sometimes referred to as "heavy photons" because they carry energy, like photons, but have mass, like electrons.

typically exist for only a short time - trillionths of a second--and travel only a few microns before turning back into photons, which are then emitted from the material.

In the June 10 issue of the journal Physical Review Letters, scientists from the University of Pittsburgh and Bell Labs, the R&D arm of Lucent Technologies, report that they have designed and demonstrated a two-dimensional semiconductor structure in which excitons exist longer and travel farther than previously recorded.

In their paper, titled "Long-Distance Diffusion of Excitons in Double Quantum Well Structures," David Snoke, senior author and associate professor of physics and astronomy at Pitt, and his colleagues report a system in which excitons move freely over distances of hundreds of microns. Their findings open up the possibility of new applications, such as excitonic circuits.

The researchers "stretched out" the excitons by pulling them apart with an electrical field. This extended the excitons' lifetimes by a million (up to 30 microseconds) and expanded the distances the excitons travelled (up to a millimetre). They were able to "see" the excitons by observing the emitted photons.
The semiconductor structures designed in the experiment are of "world-record quality," said Snoke.

The ability to control excitons over long distances could lead to excitonic circuits in which photons are converted directly into excitons, which are then steered around a chip and converted back into photons again at a different location, such as an optical memory device.
"It's another tool in our optics toolbox," - David Snoke.
"We're doing this with semiconductor circuits now designed for moving electrons. It's a completely new type of control over the system." - David Snoke.

Other authors of the paper are Zoltan Voros and Ryan Balili, graduate students in Pitt's Department of Physics and Astronomy, and Loren Pfeiffer and Kenneth West of Bell Labs.




Posts: 131433

Light may arise from tiny relativity violations, according to a new theory.

Speaking most recently at last month's American Physical Society meeting of the Division of Atomic, Molecular, and Optical Physics in Nebraska, Alan Kostelecky of Indiana University described how light might exist as a result of breaking an assumption of relativity theory known as Lorentz symmetry.

In Lorentz symmetry, the laws of physics stay the same even when you change the orientation of a physical system or alter its velocity.
According to special relativity, the speed of light is the same in every
direction, a notion that current experiments verify to a few parts in 10^16.

However, if physicists find variations in the speed of light with direction, this would provide evidence for broken Lorentz symmetry, which would radically revise notions of the universe.
Broken Lorentz symmetry would give space-time a preferred direction.

In its simplest form, broken Lorentz symmetry could be visualized as a field of vectors existing everywhere in the universe.
In such a picture, objects might behave slightly differently depending upon their orientation with respect to the vectors.

In a recent paper, published in Physical Review D (Bluhm and Kostelecky,
Physical Review D, 71, 065008, published 22 March 2005
), the authors propose that the very existence of light is made possible through a vector field arising from broken Lorentz symmetry.
In this picture, light is a shimmering of the vector field analogous to a wave blowing through a field of grain.
The researchers have shown that this picture would hold in empty (flat) space as well as in the presence of gravity (curved space-time) which is often ignored in conventional theories of light.

This theory is in contrast to the conventional view of light, which arises in a space without a preferred direction and as a result of underlying symmetries in particles and force fields.
The new theory can be tested by looking for minute changes in the way light interacts with matter as the earth rotates (and changes its orientation with respect to the putative vector field).
In addition, neutrino oscillations might arise from interactions between neutrinos and the background vector field, as opposed to the conventional explanation, which invokes neutrino mass as the explanation for the oscillations.
Experimentalist Ron Walsworth of Harvard-Smithsonian comments that the nice thing about Kostelecky's work is that he proposes detailed experiments to test his theories; and that the results of such experiments, no matter how they turn out, promise to deepen our understanding of physics.
Read More (PDF)

-- Edited by Blobrana at 18:52, 2005-06-15

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