Cooling by Heating
Don’t worry if you find this confusing, it somewhat is. Researchers at the University of Copenhagen have successfully cooled a semiconductor membrane down to -269 C, or 4.15 K. This was accomplished by heating the material with a laser. Yes, the researchers heated the material to cool it.
The membrane, a 1 mm x 1 mm x 160 nm piece of gallium arsenide (GaAs) was placed in a vacuum chamber and had a laser aimed at it. The membrane partially absorbed and partially reflected the light. The reflected light struck a mirror and was bounced back to the membrane, creating an optical resonator. When the light is absorbed by the membrane, electrons within it get excited and move around some. When these electrons fall they release energy as heat, which causes the membrane to expand and contract. These fluctuations cause the distance between the membrane and the mirror to change. Though the fluctuations are small, they are enough to cause some interesting interactions. One of the resulting interactions is for the fluctuations themselves to cool. Even though the membrane, in general, is warming, there are specific oscillations of the fluctuations that cause it to cool down to near absolute zero.
Here is a way to visualize it; think of a string vibrating up and down, like on a guitar. At certain points the string stops moving, because it has gone as far as it could. At these points there is less energy, and thus a lower temperature.
The temperatures this technique reached are needed for quantum computers and some sensors. Many things that require being cryogenic cooled could benefit from this.