You have-- In my hand.

DIANNA COWERN: In your hands.

Right here.

DIANNA COWERN: You have another one.

I have another BBO crystal that we can look at.

DIANNA COWERN: I'm super excited.

I'm really excited too.

DIANNA COWERN (SINGING): You go aaaah!

So that's it.

That's-- DAN WALSH: This little guy here.

DIANNA COWERN: It's so small.

DAN WALSH: It's really small and kind of frosty.

Like, it looks like-- I don't know, like sea glass or something.

You know what I mean?

DIANNA COWERN: Yeah, yeah.

Hey, I'm Dianna.

You're watching "Physics Girl."

And there is a reason that I'm really excited about this random crystal-- not this one, the other one.

But this is going to be my metaphor.

The crystal can entangle photons.

I made a recent video about a crazy experiment testing quantum entanglement.

Quantum entanglement is like the choose-a-gift game of quantum physics.

You choose a hand, and you automatically know something about the other hand-- that that's where the gift was.

Except that with real quantum entanglement experiments, it's typically photons that you're entangling.

[CLEARS THROAT] And you don't decide where the gift goes beforehand.

Carry on.

I know this is not the best explanation of quantum entanglement.

But I did go into it more in the other video.

So you can check that out if you want to.

Focus, Dianna.

So how do you make entangled photons?

I don't know!

It turns out, creating pairs of entangled photons is a process that involves nonlinear crystals and splitting photons.

On this optical table, this whole setup, you have a nonlinear crystal.

That's right, yeah, the BBO crystal.

DIANNA COWERN: Where is it?

Oh, it's right inside of that little cylindrical unit that's in between these two green beans here.

DIANNA COWERN: So it's like protected.

It's like really hidden.

DAN WALSH: It's really hidden.

Do you want to go see it?

DIANNA COWERN: Yes!

DAN WALSH: Let's go see it.

DIANNA COWERN: Amazing!

DAN WALSH: Now, it's inside of a cylinder, because you're trying to keep it as clean as possible.

So you'll see, there's actually a tube going in there and just blowing in oxygen-- DIANNA COWERN: Oh, wow.

DAN WALSH: --to keep it, so it doesn't get-- DIANNA COWERN: So that that tube is-- DAN WALSH: --bounced out.

And inside of that little pedestal, as we call it, there is the BBO crystal.

So this is the crystal you need to split photons.

But wait, you can't actually split photons, because photons are elementary particles, my dear Watson, which means that you can't chop them up into smaller bits.

And yet, beta barium borate crystals, or BBO crystals, as Dan was calling them, are the common crystals used to take one photon in and out pops two, because a BBO nonlinear crystal can "split" photons.

The incoming photon is absorbed into the crystal.

And then a process involving vacuum energy fluctuations happens.

And you can get out two photons, but only for every billion or trillion or so photons that you send in.

So there are some cool hints about how these crystals work.

They have to be perfectly grown for each individual experiment.

Plus, there are rules.

So the energy of the incoming photon or photons has to exactly match the total energy of the outgoing photon or photons.

Another rule is that momentum needs to be conserved.

So for a photon coming in straight, the photons coming out need to be going off at different angles, just like with colliding pool balls.

And the photons can kick back the crystal, so you've got to conserve all that momentum.

So the important things for entangling photons are the version of the interactions with the crystal, where in goes one photon and out comes two.

This process is known as parametric down-conversion.

The conservation of energy and momentum and spin is also important, because you know the energy and spin and momentum of the photons going in, so you know the energy and spin and momentum of the photons coming out have to be correlated.

And it's this correlation that's used to entangle photons.

The final process of entanglement involves the direction of polarization of the photons and multiple crystals.

But I'm not going to get into it in detail.

So there's a really great video that does.

And I'll put that in the description.

Now, entanglement is not the end of the story.

Physicists realized they could use nonlinear crystals for all kinds of fun.

Dan's lab is using crystals to make a UV laser beam.

So you're a theorist.

That's true, yes.

DIANNA COWERN: And this is-- I don't belong here.

[LAUGHTER] DIANNA COWERN: But you're doing theory work-- Yes.

DIANNA COWERN: --to work on this experiment.

In Dan's lab, they take in two photons, and they get out one.

They take in two green photons, and they get out a UV photon.

The whole purpose of having the crystal in your optical setup here is to make a UV laser.

DAN WALSH: It sounds ridiculous, right, that we need this whole table just to make ultraviolet light, but-- Like, it sounds a little bit ridiculous.

Remember, it's important to be able to tune the frequency of the ultraviolet light.

DIANNA COWERN: Yeah.

Dan's lab uses UV light and shines it on clouds of plasma for some sweet experiments.

Another interesting use of parametric down-conversion is this-- say you want to send a single photon to do something.

Maybe you want to test if the human eye can sense a single photon.

I talked to a researcher who's working on this exact question.

I'm Rebecca Holmes.

I'm-- well, I was a graduate student at the University of Illinois.

I've just recently graduated.

So my research was focused on using quantum states of light, including single photons, to study the human visual system.

So it turns out, your visual system is actually really sensitive.

You can maybe even see a single photon.

156 00:05:32,630 --> 00:05:36,350 So what I use it for is I wanted to generate single photons.

And a good trick for doing that is to make two photons at once.

So you make a photon pair from spontaneous parametric down-conversion.

And then you use one photon from the pair.

You just take that one and send it straight to a single photon detector and count it.

So when that detector clicks, you know that that photon was there, so its partner is there on the other side.

Cool stuff.

So that's how a mommy photon becomes two baby photons-- with the assistance of nonlinear crystals and parametric down-conversion.

Thank you so much for watching this video and happy physicsing.