What is quantum entanglement?

Dan wanted to know what quantum entanglement was, so Chris Smith put the question to physicist Francesca Day...

Fran - Quantum mechanics describes the physics of how very small things work. And it turns out that the physics of how very small things work is super weird, and nothing like the physics of how everyday objects work. And one important point is that in quantum mechanics nothing is decided until we measure it.

Chris - Can I just clarify, when you say really small stuff, how small is really small?

Fran - The size of an atom or an electron, or a proton which are the particles in atoms.

Chris - Does that mean then if I could with a microscope start with something big and watch how it behaves, and get smaller and smaller and smaller and see progressively smaller things I would suddenly, at some point, see the behaviour switching and going into this quantum realm where things behave totally differently than bigger objects?

Fran - That’s a very interesting question whether you could do it gradually. No-one quite knows how the transition from quantum to classical physics works. It’s one of the really open mysteries because there isn’t a microscope that can do that whole range.

Chris - Ah. So quantum mechanics has thought of that and caught us out?

Fran - Yes. Quantum mechanics is very good at doing that kind of little trick.

Chris - Because Niels Bohr said if you’re not baffled by it you didn’t understand it, didn’t he? He’s saying look, you know I understand how it works, I understand the results I get but I don’t understand why I get?

Fran - That’s absolutely right. Quantum mechanics is a set of rules that’s very very good at predicting the results of experiments. So you have to sort of go with it but…

Chris - And just abandon all hope of actually understanding why? You just know it works?

Fran - We haven’t abandoned all hope. There’s a lot of scientists working on it.

Chris - Okay. So coming back to this idea of entanglement, what’s that?

Fran - As I said, we don’t decide things in quantum mechanics until we measure them. So, for example, photons which are particles of light have a property called polarisation and that describes the direction their electric field points in. And before we measure the polarisation of a photon, even the photon itself doesn’t know its polarisation.

Now it’s possible in an experiment to have say an atom emit two photons at once such that they have to have their polarisations pointing in opposite directions. When we do this, and say the photons travel to opposite ends of the world and then we measure one of them and it’s pointing in a particular direction. Then we measure the other photon and its polarisation will always be pointing in the opposite direction because those two photons are then entangled, and this happens instantaneously. So it’s very mysterious how the information can travel from our measurement of one photon instantaneously to our measurement of the other photon.

Chris - Now is it that the entanglement, the decision as to what polarity you’re going to have, that decision is made at the moment the two photons leave the atom or is that decision made only at the time when you measure one of them? Do we know?

Fran - It’s made only at the time when you measure one of them.

Chris - And how do you know that?

Fran - From the experiment I’ve described there isn’t a way of knowing that. So you might think well why isn’t it just decided at the source? But there are actually more complicated experiments you can do involving measuring things at different angles, and doing lots of different measurements that show that it has to be decided only at the point of measurement. They’re called Bell Inequalities if anyone wants to google further.

Chris - Over what sorts of scales can this operate? Or is it infinite as in if I had a particle which was generated at the time of the big bang, and another particle its pair, they’re now on opposite sides of the universe. As far as we know does the same rule apply?

Fran - Yes. As far as we know it is infinite. However, when I say a measurement, something like the photon interacting with a few other particles, interacting with its environment can destroy the entanglement. So you’d have to really isolate your photon somehow as it travelled across the universe.

Chris - But that also means that there is, in some way, information travelling over vast distances in zero time for this to hold. So how on earth is that happening?

Fran - We don’t know. It seems like it’s in conflict with Einstein’s theories which say that things can only travel at the speed of light, not infinitely fast. It’s actually possible to prove that it’s not possible to communicate this way in a way that violates Einstein’s theories. But it’s still very mysterious and people spend a long time worrying about it.

Chris - It certainly is. But it could hold the key, well it does hold the key to information protection, doesn’t it? Because basically, it’s a failsafe way of knowing if information has been tampered with if you entangle some information in this way, and you’ve read one bit of information it will change the other so you can tell? And that’s how online security works, isn’t it?

Fran - Yeah. So people are very interested in this for security purposes. It’s a lot better than anything you can do classically because it’s literally tamper proof from the very laws of physics themselves.