One of the coolest things in physics is used as a common tool to make things really cold. Lasers are used to cool a bunch of atoms to extremely low temperatures, temperatures in the micro-Kelvin range. Extremely successful, laser cooling is surely one of the hottest topics in Physics.
Firstly, let’s note few important points:
- The ‘temperature’ of a substance is the average kinetic energy of the constituent atoms/ molecules. This is the definition. And, yes, this is exactly what we measure with a thermometer. Thus, in short, the faster the atoms in a substance move, the higher its temperature.
- There is a well known effect called the Doppler Effect. If you move towards a light source (or equivalently, if the source moves closer to you) the frequency of light you see goes up (“blueshift”). This is, obviously, a velocity dependent phenomenon. The faster you move, the more the shift in observed frequency. Similarly, if you move away from the source, the frequency goes down (“redshift”). So if a stationary observer observes green light, an observer moving towards it might see a blue light, while the observer moving away may see a red light.
- A substance absorbs energy only at discrete and specific frequencies of radiation. For example, if a substance absorbs at a frequency of red light, radiating it with a light of slightly lower wavelength will not cause it to undergo any transition. This light will not be absorbed. This is due to the quantum nature of the energy levels of the atoms.
- Light is made up of tiny packets of energy called photons. Each of these photons carries only one value of energy, dependent on the frequency of the radiation. The higher the frequency, the higher the energy. Also, photons carry momentum. Momentum for a photon is equal to its energy.
Armed with these points, we can finish off the explanation for laser cooling in a few lines. Take an atom and irradiate it with light with a frequency slightly lower than the one it would absorb. The light would just bounce off without any absorption (point 3). Now say that the atom is moving towards the laser. The frequency it will see is greater than the static laser frequency (Point 2). This higher frequency is now good enough for absorption.
Imagine two lasers irradiating the atom from opposite directions. One laser forces the atom to bounce off towards the other laser. This induces absorption.
Now this excited atom can release energy as a photon. The momentum it loses due to the emission of this photon lowers the kinetic energy. Thus, the kinetic energy decreases. But this is basically a lowering of temperature (point 1). Lo and behold, we have cooling.