Laser cooling uses lasers to slow down and cool atoms by causing them to absorb and emit photons in random directions. It selectively targets atoms moving in certain directions and speeds by tuning the light to a specific frequency. The coldest temperatures ever generated were achieved using a combination of laser cooling and evaporative cooling.
Laser cooling is a method of slowing down atoms, and thus cooling them, using lasers. We typically think of lasers as heating elements, and they certainly do on macroscopic scales, but for single atoms or small groups of atoms, they can be used for cooling. The coldest temperatures ever generated, less than half a billionth of a Kelvin (0.5 nanoKelvin), were achieved using a combination of laser cooling and evaporative cooling. These temperatures are achieved with small amounts of diffuse gases.
The primary mechanism by which laser cooling slows down atoms is by causing them to absorb and emit photons in random directions. As long as the speed of the atom is greater than the recoil speed of the photon emission, the overall speed is reduced. If you were floating in a hovercraft, moving at significant speed in one direction and randomly throwing metal balls from the hovercraft, your speed would eventually slow down and your movements would be entirely dictated by the recoil effect of the balls being thrown. This is how laser cooling works.
Laser cooling selectively targets atoms moving in certain directions and at certain speeds within the gas. By tuning the light to a specific frequency, just below the resonant frequency of the substance, the laser trap targets only those atoms moving towards it. This is due to the Doppler effect: as the atom moves towards the laser source, the frequency of the light increases from that atom’s perspective. This is the same reason that the frequency of sound varies when a train passes a stationary observer: the relative speed between the source and the object manipulates the apparent frequency. For atoms not moving at that threshold speed, they are transparent to the laser and therefore unaffected by it.
When the apparent frequency of light relative to certain atoms in the laser heat trap is just right, the atom absorbs incoming photons, becomes temporarily more energetic, then emits a photon. Then atoms moving in a certain direction above a threshold speed are selectively slowed down by the laser cooler. By arranging the lasers in a three-dimensional array, which surrounds the diffuse gas, atomic velocity in all three degrees of freedom can be damped, leading to less atomic motion and thus a lower temperature. The gas must be diffused to ensure that photons are not reabsorbed by adjacent atoms. Slowly manipulating the frequency of the laser can also be beneficial, as it may require several cooling stages to bring the gas down to the desired temperature. Do it carefully, and maybe you’ll get that scholarship you’ve always wanted.
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