Physicists¹ from the University of Bonn have developed a completely new source of light, a so-called Bose-Einstein condensate（冷凝物，浓缩物） consisting of photons. Until recently, expert had thought this impossible. This method may potentially be suitable for designing novel light sources resembling lasers that work in the x-ray range. Among other applications, they might allow building more powerful computer chips. The scientists are reporting on their discovery in the upcoming issue of the journal Nature. By cooling Rubidium atoms deeply and concentrating a sufficient number of them in a compact space, they suddenly become indistinguishable. They behave like a single huge "super particle." Physicists call this a Bose-Einstein condensate.

For "light particles," or photons, this should also work. Unfortunately, this idea faces a fundamental problem. When photons are "cooled down," they disappear. Until a few months ago, it seemed impossible to cool light while concentrating it at the same time. The Bonn physicists Jan Klärs, Julian Schmitt, Dr. Frank Vewinger, and Professor Dr. Martin Weitz have, however, succeeded in doing this – a minor² sensation.

How warm is light?

When the tungsten filament³（钨丝） of a light bulb is heated, it starts glowing – first red, then yellow, and finally bluish（带蓝色的） . Thus, each color of the light can be assigned a "formation temperature." Blue light is warmer than red light, but tungsten glows differently than iron, for example. This is why physicists calibrate⁴（校正，调整） color temperature based on a theoretical model object, a so-called black body. If this body were heated to a temperature of 5,500 centigrade, it would have about the same color as sunlight at noon. In other words: noon light has a temperature of 5,500 degrees Celsius⁵ or not quite 5,800 Kelvin (the Kelvin scale does not know any negative values; instead, it starts at absolute zero or -273 centigrade; consequently, Kelvin values are always 273 degrees higher than the corresponding Celsius values).

When a black body is cooled down, it will at some point radiate no longer in the visible range; instead, it will only give off invisible infrared⁶ photons. At the same time, its radiation intensity⁷ will decrease. The number of photons becomes smaller as the temperature falls. This is what makes it so difficult to get the quantity of cool photons that is required for Bose-Einstein condensation⁸（冷凝，压缩） to occur.

And yet, the Bonn researchers succeeded by using two highly reflective mirrors between which they kept bouncing a light beam back and forth⁹. Between the reflective surfaces there were dissolved（溶解的，溶化的） pigment¹⁰ molecules¹¹ with which the photons collided periodically. In these collisions, the molecules 'swallowed' the photons and then 'spit' them out again. "During this process, the photons assumed the temperature of the fluid," explained Professor Weitz. "They cooled each other off to room temperature this way, and they did it without getting lost in the process."