Quantum entanglement on a big scale

It's one of the oddest phenomena in physics - and now it's been measured in the macroscopic world...
11 May 2021

Interview with 

Fran Chadha-Day, Durham University


Lines of colour joined together.


When physics deals with things at the very small scale, like atoms or molecules, the familiar rules that explain physics are left behind and things behave in strange ways. This is the weird world of quantum mechanics. And one of the oddest quantum phenomena is called “entanglement” - where two separate entities act as if they’re one, even when they are separated by vast distances. As an extreme example, if you took two entangled atoms and moved them to opposite sides of the universe and did something to one of them, the other would instantly know about it! But now it’s got even weirder, because a team of American physicists have leapt over the boundaries of this quantum world, and made the same thing happen to much larger entities than just atoms. Durham physicist Fran Chadha-Day wasn’t involved in the research but, as Phil Sansom found out, she was very excited by the results…

Fran - As far as the quantum world goes, it's huge. So this really shows that quantum physics can and does apply to larger objects.

Phil - This is a quantum effect - normally on the scale of tiny particles - but they've managed to make it on the scale of many, many, many particles?

Fran - Yeah, that's exactly right. We normally think of quantum entanglement as something that affects atoms and molecules, but these researchers have achieved quantum entanglement with two objects that are about 10 trillion times bigger than an atom.

Phil - What actually is quantum entanglement then?

Fran - Quantum entanglement is an effect where, when two objects interact, they can no longer be considered as two separate objects. They must be considered as one object. And this is true even if after interacting, you've moved them really far apart.

Phil - That's absolutely wild, though. It sounds like you might have two halves of a locket - something like that - that are still behaving as if they're one locket.

Fran - Yeah. That is what it's like. And it's freaked people out for a long time because... people have called it 'spooky action at a distance' because these things do seem to happen instantaneously. You can affect one object by doing something to the other.

Phil - It's incredibly spooky!

Fran - Yeah. Which is why it's kind of... when it was only happening with atoms and molecules, I think people were a bit less freaked out because they're so small and they're so far removed from us. But now that it is happening with bigger and bigger objects, it's even spookier!

Phil - What are the objects that these researchers have managed to entangle?

Fran - It's two drum heads which are made out of thin film aluminium.

Phil - Drum heads - as in the top of a drum?

Fran - Yeah. It's like the top of a very, very small drum.

Phil - Why have they gone for these objects, do you know?

Fran - It's because of the method they use to entangle them. They placed them in a cavity, and by sending in two pulses of light they can entangle these drum heads. The first pulse has the effect of creating an entangled photon with the motion of the first drum head, and the second pulse has the effect of exchanging that photon with the vibration of the second drum head.

Phil - Is it like they've managed to link these two drum heads together somehow via this interaction with a tiny particle of light - a photon - and that linking together has set up this quantum entanglement state?

Fran - Yep. That's exactly right.

Phil - How do they know that they've actually done it?

Fran - There are correlations between the position and the momentum of both drum heads. Correlations can also happen with classical physics, so they have to do some maths on the positions and the momenta in order to prove that the only way the particular correlations they observe could arise is because of quantum entanglement.

Phil - Right. So classical says one thing, quantum says the other...

Fran - Yeah, that's right.

Phil - ...and they found that it was the quantum one and not the classical one.

Fran - Yeah. So this is a really huge result.

Phil - Could you take these two drums really far apart, and they'd still be entangled and have this correlation?

Fran - Yes. In principle you could. You'd have to be a bit careful that as you moved them far apart, they didn't interact with other things that might destroy the entanglement, but in principle you could.

Phil - Could they in theory make the drums bigger, or add more drums, or do that kind of thing?

Fran - Yeah. They do say in the paper that they hope that this will be a stepping stone. So I think we should be expecting to see this research progressing even further to bigger or more objects.

Phil - Do you think it'll progress to the stage where my example of the locket - that maybe I'm sharing with some long lost love - actually is entangled, and you can actually achieve that?

Fran - Probably not. There's going to be so many interactions with the air, with other things in the environment, that that would destroy entanglement.

Phil - Is this useful for anything, or is this just a cool bit of physics that's never been seen at this scale before?

Fran - It could be useful! There's all kinds of technologies in development like quantum computing and quantum sensors that really rely on having this precise control and measurement of larger quantum systems.


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