eli5 How does laser cooling work
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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.
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