How does laser cooling work?

Lasers make things hot.

Just think of all the lasers in sci-fi-- laser guns, lasers shooting out of your eyes, laser cutting, burning tumors with lasers.

And lasers can be used to burn your hair off.

Hey, Derek from "Veritasium."

Hey, Dianna from "Physics Girl."

So there's one more interesting use of lasers.

What's that?

It's very different.

Uh-huh?

It's laser cooling.

Are you going to keep going?

Or do you want me to say something like, what's laser cooling?

So I first learned about laser cooling in college.

And it was one of those moments where I just couldn't believe that the world works this way.

It's really counter-intuitive.

This is how it works.

OK, but before we explain how it works, we need to know what it means to be cold.

True that.

Cooling something down means taking something like this and being like, cool down, man, chill out.

You turn it into something like this.

Right.

So what Dianna is trying to say is that the molecules in this boiling hot cup of water are moving much faster than in the ice.

And so we're going to try using lasers to slow molecules down.

But that seems very strange because it seems like you're adding energy to try to decrease the energy of the system.

That would be like trying to blow out a candle with a flame thrower.

Just doesn't seem like it's going to work.

Right.

It sounds counter-intuitive.

But the thing about atoms and molecules is that they're constantly in motion in this random motion.

And in order to slow them down, you have to push them in the opposite direction of that motion.

Right.

But you can't really grab them with your big macro fingers and slow them down.

So what you need to do is get down to their level using light.

See, light has momentum in addition to energy.

Which is a weird thing, right?

Everyone knows light as energy, but it also carries momentum even without mass.

That means if there is an atom moving in one direction, and it absorbs a photon moving in the other direction, that momentum gets transferred to the atom, and that will slow it down.

And that's the key to laser cooling.

But you need a specific wavelength of light because atoms and molecules only absorb specific wavelengths of light.

Just like this green balloon won't absorb light from a green laser.

But this red balloon will.

And it will-- [pop] Pop.

Every time, that scares me so bad.

But once the atom has slowed down, isn't it just going to keep getting hit by more photons and speed up in the opposite direction?

Well, this is the real genius of laser cooling.

You want to make sure that your wavelength, the laser wavelength, is tuned to just longer than the absorption wavelength of a still particle, if the particle's not moving.

So that way, when the particle moves toward the source, it sees that wavelength as blue-shifted or squashed.

And then it will only absorb the photon when it's moving toward the source.

Right.

And then once it stops, the photon will just pass straight through.

It's not going to interact at all.

Exactly.

So that will slow down atoms moving in one direction, but what if they're moving in all random directions, as they normally are?

Well, you add more lasers in the other direction.

So if you've got one laser this way slowing down particles that are moving in this direction, well, you add another laser in this direction, and another one, top and bottom, front and back.

So you've got six directions of lasers slowing down the particles that might be moving in any combination of those six directions.

Do you need six lasers to make laser cooling actually work?

You need six lasers.

Or three lasers and mirrors on the other side.

Right.

So in any direction they're moving, they will get hit with photons that slow them down.

But once they're stopped, they no longer interact with the photons.

That really is genius.

How cold can you get these atoms with laser cooling?

Well, scientists have gotten down to microkelvins that is millionths of a degree above zero.

That's pretty close to absolute zero.

But of course, we're never going to quite get to absolute zero because of Heisenberg's uncertainty principle, which states you can never know precisely the position and momentum of a particle at the same time.

And so you can never really bring them to a full stop.

They're going to always have a little bit of zero-point motion.

And for these really, really cold temperatures that we've achieved with laser cooling, like the microkelvins, that's for really small objects.

Didn't MIT get a gram of a substance down to a very cold temperature?

MIT got a gram down to 0.8 Kelvin, which is amazing.

That is pretty good.

That's a macro scale object, which is incredible.

Right.

And what would you use this stuff for?

Why do you want to get it so cold?

If you want to look at a macro scale object and observe quantum behaviors in it, you have to get it extremely cold, and that's what laser cooling could be used for.

The other thing would be, I would think atomic clocks, like the ones in satellites.

They have to be laser cooled to be really, really accurate.

Mm-hmm.

Otherwise, your GPS in your phone would not work.

And so I think that's pretty cool that our ability to locate ourselves on our smartphones depends fundamentally on this mechanism, laser cooling, working in a satellite that is orbiting the earth.

That's applicable directly to your everyday life.

There you go.

Laser cooling, one of the coolest uses for lasers.

Happy physicsing, and thanks for watching.

Thank you so much to physicist Peter Kissin, who helped me out with the balloons.

We popped the balloons, right?

Mm-hmm.

It was good.

Super fun.

And you are here at UCSD?

Yep.

I work in an optics lab at UC San Diego.

Awesome.

Thanks.

It's rolling.

Stop.

I'mma get it.

I'mma get it.

I'mma get it.

I'mma get it.

I'mma get it.

The mome-- the mome-- [laugh] The m-- no!

It happened to me!

I'm not going to hit you in the face.

Let's go again.

This is fun.