Name: 量子チェシャ猫 - 60のシンボル (Quantum Cheshire Cat - Sixty Symbols)
Uploaded: 2021-01-14T10:33:08.000Z
Duration: 11 min 51 s
Description: VoiceTubeの動画で発音を聞きながら英語表現を覚えよう！学べる英語：

様態の副詞

I'm reliably informed that go down very well on YouTube.

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

程度の副詞

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.

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.

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.

They have physically separated the particle from its magnetic moment.

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.

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.

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?

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.

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.

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.

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.

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.

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

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

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

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.

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.

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.

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

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.

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.

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.

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

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

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.

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量子チェシャ猫 - 60のシンボル (Quantum Cheshire Cat - Sixty Symbols)

weird

obvious

subtle

basically