Question: Whats quantum entanglement?

Daniel Scully answered on 21 Nov 2011:

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One of the strange things in Quantum Mechanics is that particles don’t decide what state they’re in until someone tries to measure that state…

For example, when you flip a coin it can be in one of two “states”: heads or tails. There’s a 50% chance of one state (heads) and 50% chance of the other state (tails).

Let’s think about an experiment with a normal coin:

Close your eyes while you flip the coin.
The coin lands and it’s either heads or tails, but you don’t know because your eyes are closed.
If you want to know what state the coin is in… you need to measure it, so open your eyes and see whether it’s heads or tails.

Great. That all makes sense for big objects like coins.
But suppose the coin behaved like a quantum particle:

You close your eyes and flip the coin.
You don’t know whether the coin is heads or tails, because you haven’t measured that yet, you haven’t opened your eyes.
But if this coin is behaving like a quantum particle… it hasn’t decided either!
It knows there’s a 50% chance it’s heads, and a 50% chance it’s tails, but it hasn’t decided which yet.
It only decides, when you open your eyes and measure whether it’s heads or tails, then it makes a 50:50 decision and chooses which it is.

Weird? Yes. But that’s how quantum particles work.

Now, Quantum Entanglement…

Sometimes in physics we can make two quantum particles so that their properties have to obey some rules, so the properties of those two particles are “entangled”.

Let’s carry on with the coin analogy…

Suppose we had two coins and we flipped them in such a way that they each have to give a different answer…
So if coin 1 is heads, coin 2 must be tails.
And if coin 1 is tails, then coin 2 must be heads.

But remember that quantum particles don’t decide which state they’re in until it’s measured.
So, you flip the coins with your eyes closed.
You don’t know which is heads and which is tails because your eyes are closed, you haven’t measured that yet.
But the coins haven’t decided either. They know there’s a 50% chance of being heads or tails, and they know they must be different, but they haven’t yet decided which one is heads and which is tails.

That is until you open your eyes… the quantum coins suddenly decide heads or tails and they always choose so that one is heads and the other is tails.

Now that is definitely weird!

Don’t worry though, it gets weirder…

Let’s suppose you and a friend both have your eyes closed when you flip the entangled quantum coins.
But instead of opening your eyes to measure which is heads and which is tails, you each take one and travel to opposite sides of town… all the time not looking/measuring what state the coin is in.

Now. The coins are opposite sides of town. No one has looked at them yet, so they still haven’t decided whether they are heads or tails. So while you have coin 1, there’s a 50% chance it’s head and a 50% chance it’s tails but it won’t be decided until you look at the coin.

Furthermore, the coins have to be different. So coin 2 on the other side of town is in the same double state – 50% heads and 50% tails.

Now, both you and your friend open your eyes and “measure” your coin’s state at 12:00. Suddenly, at the moment you measure the coin’s state, it has to decide whether it’s head or tails, and it has to be in agreement with the coin on the other side of town… even if you’re both looking at the coins at exactly the same time – so there’s no time for the coins to communicate their decision to each other during the measurement- they always end up with one heads and one tails.

That’s how quantum entanglement works. Obviously it doesn’t really happen with coins, but it does happen with quantum particles like electrons and photons, and instead of heads and tails, they entangle properties like their spin.

It’s a very strange process and no one actually knows how it works… quantum mechanics tells us WHAT we will measure, but it doesn’t tell us HOW the universe made that happen.

So if it’s confusing don’t worry… no one knows how this happens… only that it does!