# eli5 How does laser cooling work

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eli5 How does laser cooling work

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Things to know/remember beforehand:

* Temperature is the random motion/bouncing around of molecules and atoms

* Light gets absorbed by atoms when the particles of light (photons) matches very specific kinetic energies that depend on the atom they’re possibly getting absorbed by. This energy is directly related to the lights color (which is also it’s frequency and wavelength). Otherwise the light just passed through without doing anything

* If you are traveling towards the source of the light, you will see the light shift more “blue” (higher energy and frequency, shorter wavelength), opposite is true if you’re moving away. This is called Doppler shift

* When a photon gets absorbed by an atom, the momentum and energy of the photon gets transferred into the atom (conservation laws)

So, when scientists want to cool individual atoms with lasers, their goal is to slow down that random motion of the atom. What they do is set up a bunch of lasers that are tuned to emit light *just below* those absorption energies. This way, the light only gets absorbed when they’re moving towards each other (like a head on collision) because the light gets shifted into those energies to absorb. The energy is conserved by making the electrons jump to higher orbits around the atom, while the atom itself slows down to conserve momentum. This causes the atom to cool to as low as a few millionths of a degree above absolute zero

You have a box full of basketballs that bounce around inside without slowing down. You cannot touch these balls with your puny human hands, you cant open the box, you cant pop these balls either, so how can you slow them down?

We can just throw baseballs through small holes (only big enough for baseballs but not basketballs) of the box at the basketballs and if they are hit at the right time (when the basketball is moving towards the baseball) the basketballs will slow down. Unfortunately we can also speed up the basketballs (if they are moving away from the baseball right before impact). So we need some kind of condition, that the baseball is to hit the basketball ONLY when the basketball is moving towards the baseball, and then with patience the basketballs in the box will settle to a close halt.

On the quantum scale interactions between bodies require specific amounts of energy, if its less than that it is either ignored or something else happens (and same thing if its more). You can design a system around this property to fulfill the condition we need in the above analogy (i.e make baseball=photon hit basketball=atom, but only if atom is moving toward photon, because then we have enough energy for the desired interavtion) and then all the basketballs i.e atoms will come to a close halt. Temperature is the average speed of atoms, so if they all have speeds close to 0 then we have really, really cold temperatures.

You can cool things with lasers?? 🤯🤯