- [Narrator] Arguably the most important change in quantum physics in recent decades is a deeper understanding of a special kind of shared state called quantum entanglement.

Imagine a machine that spits out pairs of coins, which on the surface look like ordinary coins.

If you flip one, it comes up heads or tails about 50% of the time.

Nothing strange there.

But using a pair of coins fresh out of the machine, you flip one, it comes up heads, and then the other, it also comes up heads.

That could just be luck.

So then you do the same thing with another fresh pair.

This time, the first coin is tails, and so is the second.

Agreement again.

So you flip another pair and then another and another, and another.

Pair after pair, the two coins always agree on the first flip.

What's going on?

Maybe the first flipped coin, once it comes up heads or tails, is somehow telling the other coin how to behave.

To make sure that can't happen, you separate the coins by flying one to the moon and flip them at the same time, so no message could possibly travel between them.

Still, they come up in agreement.

It all sounds too strange to be true, but particles really can behave like those coins.

In quantum physics, it's called entanglement.

- Entanglement is really just a stubborn, stubborn, exciting and/or frustrating fact that takes a long time to try to get our heads around.

- Entanglement is certainly the most interesting and the most confusing aspect of quantum.

- It's one of these things we don't see that are naively in the world around us, but it is taking place deep in the materials that exist around us every day.

- [Narrator] And while you probably won't come across a coin entangler anytime soon, in the lab, scientists routinely generate pairs of entangled particles that share a quantum state so fully they can be thought of as one quantum object.

- You simply can't differentiate between them.

It's just one pure state.

- It's as though you have a single entity that's spatially separated without a physical connection.

- [Narrator] Entangled particles remain connected even when they're separated by hundreds of miles, likely far more.

- So does that mean it can go between here and Andromeda?

Probably, the equations give us no reason to think it wouldn't.

- [Narrator] Entanglement sounds bizarre.

Einstein derided the idea as spooky action at a distance, but since the 1970s, experiment after experiment has confirmed entanglement is a real quantum phenomenon.

(dramatic music)