So I thought we asked, we should make a cat video.

Just a cat is a character from Alice in Wonderland on this cat is famous for the fact that when it grins, the cat disappears, but the grin stays behind.

Well, I've often seen a cat without a grin, thought Alice, but a grin without a cat.

It's the most curious thing I ever saw in all my life.

So this is a paper entitled Observation of a Quantum Cheshire Cat in a Matter Wave Interferometer experiment, Although is a great tagline, it actually fits as well.

They have done this amazing thing.

So just as your Cheshire cat, somehow the smile gets separated from the cat.

They've basically taken some property of an elementary particle, in this case, a neutron, and separated it from the particle.

So it's green is actually it's it's ah, magnetic moment, its magnetism.

So they have somehow managed to take a new drone, which has a magnetic moment, and they've separated the magnetism from the particle neutrons, neutral in the sense that they have no electric charge associated with them.

But they do have a magnetic moment.

In other words, they do behave like tiny little maintenance.

Probably the way to think about it is if you take a neutron and put it in a magnetic field, then the magnetic field and the magnetic moment off the neutron will interact with each other on.

Do you know they'll attract and twist each other around and so on, because the two magnets are repelling or a repulsive each other.

What?

What's this team down?

They have physically separated the particle from its magnetic moment.

Okay.

You told me to show you how they did it.

I love you.

Okay, so it's a bit involved, but I have some pictures.

Okay, so we're in the world of quantum mechanics, which means basically things behaving very strange ways, but it means we have to sort of think in some of the quantum mechanical terms, and in particular, one of the things that's gonna feature in this experiment is this concept of weak measurement, which we also talked about in the past.

It's basically trouble with quantum mechanical systems is when you start measuring them.

The properties of the system change, but this is concept.

That week measurement, which involves measuring things sufficiently gently that actually you don't change the fundamental quantum mechanical properties of the system.

So this is a schematic view of what goes on.

So basically, you've got a beam of neutrons coming in from the side over here on these three blue blocks are what's known as an interferometer.

And then we're just going to detect what comes out the other end.

The other thing you need to know is that neutrons have a spin associated with them.

They act like little rotating bodies.

And in this weird, quantum mechanical world, you can line them with their spins in different directions, typically up and down, or forwards and backwards.

But you can align the spins.

You can play around with the spins of these particles.

Professor, when we talk about spin of a neutron, should I be imagining like a planet spinning on its access?

Or is it different property?

Yes and no is the short answer to that in that it behaves very much like a body spinning on its axis.

It has many other saying properties, but actually you are in this really fundamentally quantum mechanical world where actually the particle you can't think of it as a little spit solid sphere.

If it's an electron, it's completely point like, for example, but you can still have a spin associated with it.

So you shouldn't be thinking about these things that solid spheres you gotta think of.

The battle was fundamentally different sort of entity, but nonetheless, they still have a property which is analogous to the rotation of a little sphere.

So we gotta be with neutrons heading into this apparatus, and they've been oriented so that their spins rule pointing upwards.

Okay, which is just to say, You know, I keep thinking around clockwise or anti clockwise, and you can think of that as the spin, pointing up all the spin pointing downwards, and it's just convenient to show that rotation as an arrow.

This thing is an interferometer which basically splits that beam of neutrons into two.

Now, this is where we get into the weird quantum mechanical world because actually, you can't think of one neutral going this way and another neutron going this way.

In this weird quantum mechanical world, one particle will actually follows.

Both poles were getting into the properties of quantum mechanical things that they behave a bit like waves in that we split our beams are particles go both ways, and then when they re combine coming out the other side, we can actually generate these interference effects.

What?

We can actually think of the neutron as a way of going this way and a wave going this way and depending on exactly how we orient the apparatus.

Sometimes those waves are laid up in a constructive way.

And so we got a big signal.

Other places they'll add up in the destructive way, and we'll get very little signal.

In other words, by messing around with the the Paul difference, we can create one of these interference patterns that sometimes we get lots of neutrons coming, and then we change the operators a little bit, and suddenly we get very few neutrons coming.

So it's a classic interference.

Experiment is one of the classic ways of showing the this fundamental property of quantum mechanics that things that we think all those particles neutrons actually have wavelike properties as well.

So that's the beginning of the setup.

Okay, the next thing we need to do is change the operators of it so The next thing we do is we introduce one of these things called a spin rotator.

It takes a spin and twist it round.

So instead of pointing upward, suddenly this pin point backwards and we put a spin rotator in one beam.

That road takes the things one way.

We put a spin rotator in the other beam, which rotates the part.

It was the other way.

So now we've got our beam of neutrons going these two different parts.

But actually one of them has.

It's been aligned in one direction and the other in completely the opposite direction.

But again, remember, these are actually still the same neutrals, more or less that one neutron could be going along both paws and can't have.

It's been simultaneously twisted one way and twisted the other way.

Yes, it's getting very, very weird.

I'm afraid it'll get worse before it gets better.

Okay, now, when we re combine these two beams, you get no interference.

Effect it'll and that's because one of the properties of these particles is there what's known as in or for Colonel State, which means that basically, they don't interact with each other when they re combine and so we don't get these interference effects just cause of the way we've set them up with completely spins pointing in completely opposite directions.

So what do we get coming out?

Just just get good beam of neutrons coming out and we can't mess around with the polls all we want.

And we're never going to see some places where things add up for some places where they don't add up because they don't need to fear anymore because of what we've done with the spins.

All right.

Whoa.

Yeah, it's getting worse, All right, I got three of these, Another one to come after this.

All right, here's page two.

So the same apparatus is before.

We just introduced one more element here, which is this little box which just basically says, is the spin aligned to that?

Pointing in that direction is that's been a line to the left spin.

Identify.

Yes, if it's been is a line to the left, it lets it through.

If it spins a line to the right, it doesn't let it through.

It doesn't make any difference.

We still get the same kind of thing coming out the other side will only get a few rather fewer neutrons because someone will be rejected by it.

But basically we don't see any interference.

So now we can ask.

Okay, so which path The neutrons actually get through here?

Which part did they follow?

And actually, in a non quantum mechanical world, the answer that it's kind of obvious, right?

Causal?

The ones over here were pointed to the left.

All the ones over here were pointing to the right.

This one only lets things that point to the left through and therefore, clearly anything.

The only thing that's gonna come out the other side is something that follow this path here.

That's obvious in the in the rational, normal world.

But in the world of quantum mechanics, where these two things have kind of tangled up together, it's nowhere near is obvious.

But you can do an experiment to figure out which path the neutrons actually followed.

And this is where we have to get into this week measurement thing.

We can't do anything very kind of robust because we'll mess up the quantum mechanical system.

But what you can do is you can put a little attenuated in on what in one beam or the other, So I found something.

We just blocks out a few of the neutrons, but it's only a few percent.

It's not like, you know, half of them or anything like a crocodile just pulling in the exactly.

And, you know, most of the most of the herd gets by, but it just stops one or two on.

What you find is when you put it in these upper beam here.

Indeed, the signal level drops.

You get out when you put the same attenuate or in the lower beam, nothing happens on.

So that immediately tells you that the neutrons that we're seeing that I've made it through clearly followed this up a path here.

Well, that exactly as I say, that's kind of the common sense.

But right now we get to the non common sense.

Paul, I told you, what we're gonna try and show is that the particle and its magnetic moment have been separated from each other.

On one way you can do that is by let's get to the last picture here.

We can put a magnet in, okay.

And then what will happen if you think about what happens here in the outside, Put a magnet here in the upper beam, which is where we know the neutrons that we're actually detecting the going We got neutrons going through the lower being.

Nothing happens to them at all when we put a magnet in that actually starts pulling this spin around because we got them now the magnetic moment of the neutral and interacts with the magnetic field in that twists that spin around, which means that the particle, instead of being in set up in this state where it's purely kind of pointing that way, it'll be mostly pointing that way, but a little bit pointing that way and again is one of these weird quantum superposition things.

But at least there's some probability associated with these neutrons that they might actually have this being aligned in the same direction as the lobby.

But no, again, it has to be a week measurement, so it's a very weak magnetic field, which just means a few percent of them will may have you know, this a few percent probability that is actually in that state rather than that.

So what should happen now is that some fraction of the neutrons, now aligned in the same direction they're spins, are aligned in the same direction on top and bottom, which was the condition we needed for interference to start again.

So we should start seeing fringes again, start seeing the interference effects.

Just little weak little weekend.

Yeah, because we haven't done much, but we should start saying there's a little bit of interference.

Okay, when you put the magnet in the upper beam, nothing happens.

You get no interference coming out the other side.

So for whatever reason, these neutrons have not in any way being affected.

Nothing has happened, too.

But it gets weirder because if we put the magnet in the lower beam instead of the upper beam, then we start seeing interference again.

So it looks like these particles in the upper beam, their magnetic moments are going the lower route, and we can mess with their magnetic moments in the lower route.

And then when we combine the whole thing together, then suddenly we start seeing interference again.

Their magnetic moments ago along the bottom, the pollock was going on top.

We showed we've shown just from the previous experiment with the attenuate er we've shown that the particles were actually detecting here have all gone through the upper room.

But what we've also shown from this experiment is that those same particles magnetic moments of following the lower e think that stuff something thing smacks of a mistake.

You think they just how they had the experiment the wrong way out or something?

Maybe the top magnet wasn't working.

It's the same magnet.

So you just move it from the top to the bottom.

So it's weird, right?

That when you put attenuate is in that tells you that the product was that you're detecting over here, we're following the upper root when you put magnets in that shows you that actually their magnetic moments from traveling along the route.

And so that's this whole thing of the Cheshire cat, right?

You separated the cat from its mild.

In this case, you've separated the neutron from its magnetic moment.

It turns out that might actually be a use for it.

Do you want to use for it?

Little cryptography?

No, not quick.

Target people change.

The use for it is you might have.

You might want to study some very fundamental property of a particle.

But the fact that it has a magnetic moment masks the very subtle effects you're looking for.

Because you know what the experiment you're trying to do end up interacting with the magnetic moment of the particle and rather than the very subtle property of the particle you actually after potentially.

If you can use this kind of apparatus to separate physically, separate the particle from its magnetic moment, you can now do your experiments on the particle, the naked particle without the magnetic moment and from that, actually learn very fundamental properties of nature that you're not really well defined when the two are set on top of each other.