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As a theorist without much experimental physics background, I am reading some reviews to understand how these systems work. I've learned that one needs to use Doppler cooling before trapping the atoms optically. What is the purpose of the cooling?

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We generally think about temperature as how hot or cold something is, but in physics it's really a measure of the average speed of the atoms that make up an object, with hotter things have atoms moving around faster. For example, at room temperature, air molecules have an average speed around 500m/s. Intuitively, trying to catch atoms moving at the speed of a bullet is nearly impossible, so we first need to cool them. Specifically, to trap an atom we need it to be moving slowly enough that its kinetic energy is less than potential energy of the trap, otherwise it will be able to escape.

Now if we're thinking about creating a neutral atom quantum computing device, our end-goal is to have a bunch of atoms trapped in an array so we can individually address them with lasers and microwaves to measure and perform gates on them. Trapping neutral atoms is tough because there is no charge to exploit. Thankfully some smart people have figured out how to use tightly confined laser beams (called optical tweezers) in combination with magnetic fields to build magneto-optical traps (MOTs) capable of confining neutral atoms as we'd like for quantum applications. However, these traps are a bit weak, only having trapping potentials on the order of a few milli-Kelvin, so we need our atoms to be substantially cooler (micro-Kelvin) to be trap them effectively.

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