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If you want to make atoms THIS cold, you can’t just stick them in the freezer…you’ll need to take advantage of quantum mechanics!

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The coldest parts of space have a temperature of about 1 Kelvin, or -272 degrees Celsius. That’s one degree above the coldest temperature possible: absolute zero.

But the coldest places in the whole universe are made by scientists right here on Earth, at one billionth of a Kelvin — otherwise known as a nanokelvin. Temperatures on that scale are used in what’s known as ultracold physics. The colder particles are, the less they move, and these temperatures allow researchers to look at the properties of individual atoms and molecules and test some very fundamental physics.

You can also use ultracold temperatures to create something called a Bose-Einstein condensate: a weird state of matter where atoms move in perfect unison, like some kind of quantum flash mob. Maybe you’ve spotted the challenge here: Most of the time when we cool something down, it’s by using something colder to transfer heat away from it. But if you want to get atoms down to a billionth of a Kelvin, you can’t just stick them in a freezer.

Instead, you need to get quantum mechanical and fire up some lasers. There are two main techniques scientists use to get to nanokelvin temps. The most common type of laser cooling is called.

Doppler cooling, and it’s a real workhorse of ultracold physics. It was first demonstrated in the late 1970s, and even though we now have more sophisticated techniques that can reach lower temperatures,. Doppler cooling is still used in a lot of physics labs worldwide.

It’s based on the idea that, thanks to quantum mechanics, atoms can absorb and emit particles of light, or photons, but only when those photons have just the right energy. Atoms are very fussy about that sort of thing. And when an atom is hit by a photon, the photon can actually take momentum from the atom, or even give it momentum.

It’s like when two billiard balls bounce off of each other — depending on how they collide, sometimes one of them slows down while the other picks up momentum. Which means that just by shining a light on an atom, you can change the atom’s kinetic energy, or the energy associated with its speed. And that’s what ‘temperature’ really is when you break it down: a measure of how much kinetic energy is in atoms or molecules.

More heat means higher temperature and faster-moving particles, so to cool atoms down all you need to do is take some of their kinetic energy away. But if you just shine a laser on a bunch of atoms, their temperature won’t decrease, because they’ll be colliding randomly. And like with the billiard balls, whether an atom speeds up or slows down depends on how the photon hits it.

So you need to be a bit more clever, and this is where the ‘Doppler’ part comes in. You know how the siren of an ambulance that drives past you sounds different depending on if it’s moving towards you or away from you? That’s the Doppler effect for sound.

The pitch, or frequency, that you hear is affected by the siren’s velocity relative to you. The same thing can happen with light, too. You see light at a slightly different frequency, or color, if you’re moving very fast.

And the frequency of light is directly connected to its energy: the higher a photon’s frequency, the more energy it has. So here’s the trick: you hit your atom with a laser tuned to just below the frequency the photons need to be at for the atoms to absorb them, and let the Doppler effect make up the difference. That means only atoms moving quickly towards the photons will see them at the correct frequency and actually interact, and when they do, it’ll be a head-on collision that will slow the atom down.

Cooling down an atom like this involves hitting it with a lot of photons. It’s kind of like slowing a car by hitting it with pool noodles. But, whatever works!

The method does have its limits. It can get your atoms down to a few millionths of a Kelvin before other quantum effects lead to something called the Doppler limit. To go even colder, you need other techniques.

One of these methods is called Sisyphus cooling, and it was invented in the late 1980s. In Greek mythology, Sisyphus was a mortal who was punished for angering the gods. Side note: the number one lesson from Greek myths is … don’t anger the gods.

Sisyphus’s punishment in the Underworld was to eternally roll a boulder uphill even though it kept falling back down. Sisyphus /cooling/ works on the same principle: pushing an atom up a hill to take kinetic energy away from it. But while Sisyphus was trying to fight the force of gravity with his boulder, the atoms of Sisyphus cooling are climbing a different kind of hill: an electromagnetic one.

Like with all light, you can think of the light coming from a laser as photon particles, or as electromagnetic waves. Together, these waves create an electromagnetic field, which is kind of like a landscape, with peaks and valleys. Picture a rollercoaster going over hills: at the top, it’s going slowly, so it doesn’t have much kinetic energy, but it does have lots of potential energy from gravity.

As it slides down along the track, it turns that potential energy into momentum that takes it up to the top of the next hill. Something similar happens to the atom. As it gets to the peaks in the electromagnetic field, it loses kinetic energy and gains potential energy.

When it goes down again, the reverse happens. To cool the atom down, you would need to make it spend more time going uphill than downhill. That way it loses more kinetic energy than it gains.

The trick to doing that is to pump the atom to an even higher energy level whenever it reaches the top of a hill. This is one of those weird quantum mechanical things that’s hard to picture based on how physics works at the scales we’re used to in our everyday lives. But if you set things up in exactly the right way, the atom will emit a photon when it’s at that higher level, losing energy in the process and jumping back down to the bottom of the hill, ready to restart the cycle.

Each time, it will lose a bit more energy, which also lowers its temperature. Sisyphus cooling can be used to overcome the Doppler limit to allow scientists to reach those nanokelvin temperatures, and the development of the technique won its inventors the. Nobel Prize in 1997.

Researchers today are working on ways to adapt Doppler cooling, Sisyphus cooling, and other methods like them to cool down bigger and bigger molecules. Besides for teaching us more about quantum physics, this type of precision control could help us make more accurate atomic clocks or lead to new developments in things like quantum computers. In some ways, the future of physics is very, very cold.

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