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Ultra-Simple Desktop Device Slows Light to a Crawl

The speed of light is of great interest to RF engineers. It determines how long a wavelength is at a given frequency and appears as a constant in many equations used in RF design. However, a scientist at the University of Rochester has found a simple way to dramatically slow light, according to a News Release from the University.

The speed of light does slow down when it passes through water or glass, but the difference is not dramatic. Scientists have been able to slow the speed of light down to 38 miles per hour using a Bose-Einstein condensate (BEC). A BEC is not easy to create--it is a state of matter that exists at 459 degrees below zero Fahrenheit "where all atoms act in unison, like a single giant atom." Robert Boyd, Professor of Optics at the University, came up with a much easier way to slow light to 127 miles per hour.

Boyd, working with students Matthew Bigelow and Nick Lepeshkin, used two lasers and a ruby crystal to slow light. One of the lasers pumped an intense green light beam into the ruby, which partially saturated the chromium ions that give the ruby its red color. A second beam from a "probe laser" at a slightly different frequency than the other laser was also aimed at the ruby. According to the news release, "the probe beam has a frequency slightly different than the first laser, and these offset frequencies interact with each other, causing variations the same way two ripples encountering each other on a pond might create waves higher and lower than either one had alone. The chromium ions respond to this new frequency of rhythmic highs and lows by oscillating in sympathy. One consequence of this oscillation is that it allows the probe laser to pass through the ruby, even though the laser is green, but it only allows it to pass 5.3 million times more slowly than light would otherwise travel."

This quantum quirk is called "coherent population oscillations." The technique is expected to find use in the telecommunications industry where optical signals from two fibers need to be merged. Boyd compared it to slowing cars down on a freeway to allow others to merge. However, more research is needed to make it practical. Boyd's experiment used long pulses of light, so the space between the pulses increased only slightly in proportion to the length of the pulse. Different material may be needed to obtain slowed light that can transmit the shorter pulses used for telecommunications.

A much more detailed explanation of the experiment, including equations and diagrams, is available in the publication "Observation of Slow Light in Ruby" by Bigelow, Lepeshkin and Boyd.