With the help of a ruby ​​stone and two lasers, the speed of light was slowed down to 200 km/h

Slowing down the light in the new method may make it easier to deal with signals arriving at the same time from several fibers

Uriel Brizon

optic fibers
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Scientists from the University of Rochester in the USA managed to slow down the progress of a beam of light to a speed of about 200 km/h by very simple means. This breakthrough is another layer in the ability of researchers to control the movement of light and it may pave the way for "slow light", as it is called, from research in the laboratory to application in the field of communication through optical fibers.

The experimental technique, developed at the University of Rochester, makes it possible to slow down the speed of a light beam five million times. The experimental setup the group used is much simpler than the cumbersome devices they used in the past to slow down the movement of light; The size of these facilities was the size of a room. The capabilities of the new facility do not fall short of those of its predecessors, and it was described by its developers as "ridiculously simple to implement".

The speed at which light rays propagate in space is 300 kilometers per second, or about 1.1 billion kilometers per hour, and a beam of light can circle the earth more than seven times in one second. The speed of light is the upper limit for the movement of all objects in the universe. Albert Einstein stated, in formulating the theory of relativity, that no material factor can be accelerated to the speed of light. But as science has known for about 300 years, it is certainly possible to slow down the progress of light itself.
Light slows down when it passes through a transparent medium, such as water or glass. This fact is familiar to us from everyday life as the phenomenon of refraction of light: when we look at a glass of water with a straw in it, the straw seems broken to us. The reason for this is that when the light passes from the water in the glass, through the glass from which it is made, to the air - the angle of its progress changes. The change of angle makes us see the continuation of the straw in a different place than its real position, and as a result it appears broken. The change in the angle of light is a result of the change in its speed when it passes from the water to the air. The speed of light in glass and water is about one and a half times slower than the speed of its movement in air.

The questions that the scientists want to test are to what extent it is possible to slow down the speed of light, what can be learned from this about light and its behavior, and what industrial uses can be derived from "slow light", meaning light that has an extremely low speed. The transmission of light through a medium composed of different materials does indeed cause a slowdown, but it is not particularly great, and in order to significantly slow down the light, more advanced methods are required. Scientists have discovered that using the effects of quantum mechanics makes it possible to build facilities that can slow light down to a speed lower than that of a car.

Until now, in attempts to create slow light, facilities based on an effect known as Bose-Einstein condensation have been used. This is a unique state of aggregation that exists at very low temperatures, where gas atoms act in coordination, as if they were one big atom. A laser beam that was passed through a gas that cooled to a state of condensation was slowed down to only 60 km/h, but the need to keep the gas at a very low temperature (about 270 degrees Celsius below zero) meant that the device required to perform the deceleration was enormous in size and cumbersome to operate. An average room was able to produce an amount of condensed gas only the size of a pinhead.

"If the old way was the most difficult way to slow down a beam of light, the one we discovered now is the simplest," said Prof. Robert Boyd, whose research group developed the new facility. "Now we can slow down light to the same extent using a device the size of which does not exceed a home computer."

The researchers from the University of Rochester used a quantum effect known as "Coherent Population Oscillations". At the heart of the facility he built is a ruby ​​crystal. Two laser beams are aimed at the crystal, whose optical frequencies (that is, their colors) are slightly different from each other. The reason for the ruby ​​stones' red color is that they absorb most of the visible light passing through them, in the frequency ranges corresponding to blue and green colors. To create the slowing down effect, the first laser illuminates the ruby ​​stone with a strong green light, which causes a partial excitation of the chromium ions present in it. As a result of this strong excitation, the ability of the chromium ions, which give the stone its color, to absorb the light is temporarily reduced.

In the next step, the second laser is activated, which as mentioned has a slightly different frequency. The small difference between the frequencies of the lasers produces a pulse effect inside the ruby ​​stone - these are the "coordinated population fluctuations". This is similar to the meeting of two waves on the surface of a lake - at certain points the two waves amplify each other and a stronger disturbance is created. The chromium ions inside the ruby ​​stone react to these amplified vibrations and oscillate accordingly, and one of the interesting results of the situation created is that the second laser light now manages to pass through the ruby ​​stone despite its green color, but the speed of its movement is significantly slowed down: 5.3 million times.

In practice, the researchers point out, a kind of "hole" was created as a result of the strong excitation in the absorption spectrum of the ruby ​​stone. According to physicists, the effect can also be understood as a temporary "imprisonment" of the light inside the crystal and the preservation of its energy in the coordinated population fluctuations.

Prof. Boyd believes that the new effect may lead to progress in the field of telecommunications. In modern communication, optical fibers are widely used. The fibers allow the transmission of information by beams of light and are much more efficient and faster than the old electronic copper cables. One of the problems in the field of optical communication is how to deal with signals arriving at the same time from several fibers to the routing center. It is possible that a device based on the new effect will give these communication routers the ability to slow down some of the light beams for a short time, and thus make it possible to handle all the data that reaches them in an orderly manner. To illustrate, you can think of two roads merging into one highway, and to make it easier for the cars to merge, their speed is regulated in one of the lanes.

However, the new method has some limitations. The most prominent of them is the fact that the device allows the slowing down of light applied in relatively long pulses, unlike the previous methods in which light was slowed down in short pulses, more similar to those used in optical fiber technology. Prof. Boyd expressed hope that improvements that will be introduced in the facility built by his group will make it possible to overcome this limitation.

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