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Researchers at the National Institute of Standards and Technology(NIST) have developed a novel way of producing light pulses thatare "superluminal"-in some sense they travel faster than the speedof light. The technique, called four-wave mixing, reshapes parts of lightpulses and advances them ahead of where they would have been hadthey been left to travel unaltered through a vacuum. The new method could be used to improve the timing ofcommunications signals and to investigate the propagation ofquantum correlations. According to Einstein's special theory of relativity, lighttraveling in a vacuum is the universal speed limit. No informationcan travel faster than light. But there's kind of a loophole. A short burst of light arrives as asort of (usually) symmetric curve like a bell curve in statistics.The leading edge of that curve can't exceed the speed of light, butthe main hump, the peak of the pulse, can be skewed forward orbackward, arriving sooner or later than it normally would. Recent experiments have generated "uninformed" faster-than-lightpulses by amplifying the leading edge of the pulse and attenuating,or cutting off, the back end. The method introduces a great deal of noise with no great increasein the apparent speed. Four-wave mixing produces cleaner, lessnoisy pulses with a greater increase in speed by "re-phasing" orrearranging the light waves that make up the pulse. In four-wave mixing, researchers send 200-nanosecond-long "seed"pulses of laser light into a heated cell containing atomic rubidiumvapor along with a separate "pump" beam at a different frequencyfrom the seed pulses. The vapor amplifies the seed pulse and shiftsits peak forward so that it becomes superluminal. At the same time, photons from the inserted beams interact with thevapor to generate a second pulse, called the "conjugate" because ofits mathematical relationship to the seed. Its peak, too, cantravel faster or slower depending on how the laser is tuned and theconditions inside the laser. In the experiment, the pulses' peaks arrived 50 nanoseconds fasterthan light traveling through a vacuum. One immediate application that the group would like to explore forthis system is quantum discord. Quantum discord mathematicallydefines the quantum information shared between two correlatedsystems-in this case, the seed and conjugate pulses. By performing measurements of quantum discord between fast beamsand reference beams, the group hopes to determine how useful thisfast light could be for the transmission and processing of quantuminformation. R. Glasser, U. Vogl and P. Lett. Stimulated generation ofsuperluminal light pulses via four-wave mixing. Physical ReviewLetters, published online April 26, 2012. I am an expert from cellphone-replacement-parts.com, while we provides the quality product, such as China IPod Video LCD Replacement , Apple IPad Spare Parts Manufacturer, Apple IPad Spare Parts,and more.
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