The 2025 Nobel Prize in Physics (quantum tunnelling) - Sixty Symbols

0:00 The 2025 Nobel Prize in Physics has been awarded.

0:03 So it's been awarded to three guys, John Clark, John Martinez,

0:07 and Michelle Devet for work done long time ago, 40 years ago, 1985.

0:13 And I think it's been awarded to them partly for the work they did then,

0:16 also for the enormous legacy that it's had,

0:18 what it's turned into, and the role they've played in developing that legacy.

0:23 It's got all the words I like in here.

0:25 So scientific background to the Nobel Prize in physics

0:28 2025 for the discovery of well we'll get

0:30 back to macroscopic that's an interesting word quantum

0:34 mechanical tunneling and energy quantization in an electrical circuit.

0:40 This is a field dream.

0:41 Yeah absolutely bang in the the area

0:43 we we love which is quantum mechanical tunneling.

0:46 So if I can Oh I can get back to the old

0:49 There we go.

0:49 You got your vows

0:50 from I think the last time we used one of these was a decade ago.

0:53 Brady.

0:53 So, classically, not enough energy for it to go through the wall.

0:58 Break the ball.

0:59 Wow.

1:00 I've been using these in your videos for so long.

1:01 So, what we have here is a ball.

1:03 And you imagine if I take this ball and bounce it off the table,

1:06 what will happen is that it'll hit the the same point on the wall.

1:08 So, it won't go through the wall.

1:09 That's that's not a very um interesting experiment to do.

1:13 If we shrink this all the way down to the quantum level

1:15 and we've got a barrier to the the particle's motion at the quantum level,

1:20 this particle can under certain circumstances actually pass through

1:24 that wall or that barrier as if it wasn't there.

1:28 We know that large scale objects, footballs,

1:31 things like that obey the everyday rules

1:35 that we call classical mechanics in physics.

1:37 very very small objects behave completely differently have very counterintuitive

1:42 properties obey a set of rules that we call quantum mechanics.

1:45 The point is that the football um basically you can always describe it

1:49 by saying it's at this position at this time and it's going this fast.

1:53 Much smaller objects they have this property

1:56 that they can be in superp position.

1:57 So they can be in more than one state at the same time.

2:01 The other thing they have is that they have only specific energies.

2:04 So they have what are called discrete energy levels.

2:06 This is a real property of atoms.

2:08 And this is what these guys were doing in this experiment.

2:11 They were taking an electrical circuit with billions of electrons

2:15 and seeing whether collectively those electrons behave basically like an atom.

2:21 That collective word is exactly it.

2:22 It's about how electrons behave together collectively.

2:26 In this particular case, they use something which is called

2:30 a Josephson junction which is a superconductor,

2:33 an insulator and then a superconductor and made

2:36 rings of these things and use that to explore

2:39 how deoized or how big these quantum states

2:42 can be and still show quantum mechanical effects.

2:45 So one very nice way of seeing

2:47 these type of macroscopic quantum effects is superconductors.

2:51 The reason we have superc conductivity is that the electrons pair up together.

2:56 Now normally they are charged well the electrons

3:00 are charged and you have an interaction

3:02 and you think the last thing those electrons want to do is pair up.

3:04 In superconductors they do and they pair up

3:07 into what are called cooper pairs and then

3:09 those cooper pairs can flow effectively without

3:12 resistance and that's why we have the superconductivity.

3:15 Those cooper pairs again by themselves are not independent.

3:18 They all come together to form one massive quantum state.

3:23 [Music]

3:28 I always hear about quantum computing and it's going to be the next big thing.

3:31 How does this work bring quantum computing closer to us?

3:34 If you think about normal computing there

3:36 there are different levels at which it works.

3:39 So there's actual programming.

3:41 So there's writing code uh that you know lots of people do.

3:46 Now then there's the idea of logic and boolean

3:50 logic and the idea of gates ands uh knots

3:54 zeros and ones and then underneath that there's a physical

3:58 electrical circuit which is realizing the the logical uh

4:04 operations that you want basically the contribution of uh

4:09 the Nobel prize winners is at this bottom level so

4:12 it's basically it's the hardware underlying quant Quantum computing

4:16 what you need is something analogist to a classical bit.

4:21 Exactly.

4:21 In a quantum computer it's a quantum bit.

4:24 It's a cubit and it's not just on or off.

4:27 It can be in a super position of those states at the same time.

4:30 It does different things.

4:32 It operates in a very different way.

4:34 And what that means is that for certain

4:36 problems it can be dramatically faster basically.

4:41 But I would say we probably don't know the limits of what it can do.

4:44 So it's almost like at the very very bottom

4:47 level of computing like it's a new rocket fuel.

4:49 It's like a supercharged rocket fuel that and everything

4:52 above it we do that you and I consider

4:53 normal like sending our emails or coding and stuff

4:56 like that suddenly has something much more capable underneath.

5:00 Yeah.

5:00 But I don't think anyone's going to use a quantum computer for email.

5:04 No.

5:04 Because it because it there are certain tasks that it's very good at.

5:09 Yeah.

5:07 Um and it can do uh much faster.

5:10 One of the things physicists like is that basically you

5:13 can probably use these things to simulate complicated physical systems.

5:19 Um, which means that we can then use

5:22 it as a tool to hopefully understand physics better.

5:24 Predict the weather.

5:26 Well, I don't know about that.

5:29 We've talked about particles tunneling.

5:31 Is it just a chance thing that happens?

5:33 Do you throw them against the wall and see

5:35 which ones go through and which ones don't?

5:37 or can you kind of bias the system and make it more likely

5:41 that the particles will tunnel and do this magical thing we like to see them do?

5:45 Great question.

5:46 Um, I've got some simulations we could have a look at just on that point,

5:49 but just to answer it briefly, um,

5:52 it depends on the height of the barrier and it depends

5:53 on the energy of the particle and you can control what

5:56 we call the transmission coefficient

5:58 and the reflection coefficient and those have

5:59 got to add to one because we're not creating new particles.

6:03 So this is almost perfect reflection.

6:05 In this case, the barrier is set up so as compared

6:07 to the energy of the particle is very very high.

6:09 So the vast majority of the particle gets knocked back and you can

6:12 even see the ripples in the in the waves as it as it approaches.

6:15 So that's one extreme.

6:16 And then in this case in terms

6:18 of the energy of the particle compared to the barrier,

6:20 the barrier is much lower and you can see

6:23 the vast majority of the particle just tunnels through.

6:26 In fact in this case it's pretty much 100% transmission.

6:29 But now and this is where it get this is where the the quantumness

6:32 really starts to bite is let's choose somewhere between those two extremes.

6:36 So we set a barrier which allows us transmission coefficient

6:40 of 50 a 50% a reflection coefficient of about 50%.

6:43 So some of it's going to go through.

6:45 You see this pile up as the waves get scattered at the edge.

6:48 The really critical thing here is that's not two particles that's one particle.

6:52 We haven't created another particle.

6:54 We've just taken to use the quantum mechanical language,

6:56 we've taken the wave function, the original particle,

6:58 and we've got a different spatial distribution of that wave function,

7:01 but it's still one particle.

7:03 There's still only one electron.

7:04 So only one of those two situations is So at the moment, we're not right.

7:08 So you got to mention this is unfolding without

7:10 us observing it because as soon as we observe it,

7:12 we're going to get either the left hand side or the right hand side.

7:16 Yeah.

7:16 So I don't I I think you guys get a bit carried away with this whole,

7:19 oh, isn't it amazing two things happen at once?

7:21 It is much more than just probabilities in that sense.

7:24 There is we can make the physical measurements

7:26 and we will see that we get this result

7:28 or that result which if it were two

7:30 separate particles the statistics would be completely different.

7:34 You can actually now you can log in to a quantum computer

7:40 a a sort of a toy quantum computer and run by different companies in different

7:44 places over the internet and you can basically do a whole series of experiments

7:48 that at the time were absolutely worldleading

7:51 just from the comfort of your office.

7:53 I I first did this in the pandemic

7:55 with students and I nearly died of excitement because I

7:58 was thinking about how hard this was and the impact

8:01 that some of these things had at the time.

8:03 Um, and you know, you the first time you do it,

8:06 the students were really, really excited,

8:07 but after a while it becomes a little bit

8:09 difficult to get across just how profound what's actually going

8:14 on when it's just a web interface because we're all

8:17 used to web interfaces and it all just looks so easy.

8:20 So, part of my job is to try and unpackage all of that.

8:28 This is your lab.

8:29 Yeah, we love this system.

8:30 This is from a a company called Unisoko.

8:32 So in terms of connections to the Nobel Prize,

8:35 there are tons of connections here.

8:36 So this is a scanning tunneling i.e.

8:39 quantum mechanical tunneling microscope which is down

8:42 right at the bottom of this magnet under that magnet or in that magnet

8:46 in the magnet right in the in the middle.

8:48 You can't see it.

8:49 So that big green thing that's the big magnet.

8:51 That's the big magnet.

8:51 So at the moment this is filled with liquid helium

8:54 and the microscope is all the way down here and we

8:57 cool that microscope down to at the moment it's 4K

9:00 but the lowest temperature that we've got to is uh 333 mill.

9:05 Where's the tunneling happening then?

9:07 The tunneling h is happening actually down here.

9:09 You can't see it.

9:10 This flange here the STM is right at the bottom of that.

9:14 There's what we call an insert.

9:16 So all these connections lead down to the STM

9:18 and this is where we measure tunneling.

9:19 So, Phil, as I understand it,

9:21 the tunneling you're doing is placing a tip and a sample near each other

9:25 and the one thing shouldn't be able to jump from the other, but they do.

9:30 Yeah.

9:30 Electrons tunnel.

9:31 Yeah.

9:31 When they tunnel.

9:32 So there sort of the barrier or the wall

9:34 that you're running through is actually just a gap.

9:38 Exactly.

9:38 That's exact.

9:38 It's just a vacuum gap.

9:40 And it literally is a cuz it's in a vacuum chamber.

9:42 So how does that connect to the Nobel Prize?

9:44 So the Nobel Prize a key experimental circuit

9:47 in there was something called a Josephson's injunction.

9:50 So generally we have a metal tip which is tungsten or some other metal and then

9:55 like a needle like a really

9:56 it's it's a really sharp needle atomically sharp needle.

9:59 Therefore you have normal metal gap or insulator and superconductor.

10:03 What you can do is go drag it around and cover this with the superconductor.

10:09 So then you have superconductor gap superconductor.

10:12 That's a Josephson junction.

10:14 So if we just have a metal tip and we have our superconducting sample,

10:19 um if the electrons are below the energy what we call the superconducting gap,

10:23 they can't tunnel in because it's almost like it's

10:26 a little bit difficult to come up with a metaphor,

10:27 but it's almost like somebody get crashing a party where

10:31 everybody's coupled up and paired in and they just get rejected.

10:34 They can't go because everything else is paired in.

10:36 However, if you then coat the tip with lead superconductor,

10:40 then what will tunnel is not the electrons, but the the bzons, the cooper pairs,

10:45 and then you see what's called a Joseph current.

10:49 Where is it?

10:50 There we go.

10:51 Now, that's the gap, right?

10:52 So, this is what's called the superconducting gap.

10:54 We have a superconducting gap on the tip

10:56 and we have a superconducting gap on the sample,

10:58 which is why we've got the string structure.

10:59 But ultimately, that's the gap.

11:01 And that means electrons cannot tunnel into the superconductor

11:04 within if they've got energies within its range.

11:06 This is the the bias voltage here.

11:09 However, if you get in close enough and because you've

11:11 got a superconductor on the tip as well as the sample,

11:13 then what you start to see is this current

11:15 at zero bias and that's that's our Josephson peak.

11:20 How do you feel about the Nobel Prize as a scientist?

11:22 Is it something that excites you, interests you?

11:24 Are you cynical about it?

11:26 What what do you what's your personal feelings about the prize as a whole?

11:30 Well, I think it serves the purpose of focusing attention on physics.

11:35 So, there's a lot of publicity around it.

11:37 There's a lot of interest.

11:39 Everybody knows that what they did was very impressive.

11:42 I think you know it's a bit like say the Nobel

11:45 Prize in literature though there are lots of great books.

11:48 There are lots of great novelists who whoever

11:51 wins you know that they've written great books.

11:53 It doesn't mean that there aren't other people.

11:55 For me, one reason why it's really great is I

11:58 basically teach a module on this material and I know I

12:01 can go into the students and I can kick it off

12:04 and I can get them excited from day one by saying,

12:06 "Look, that Nobel Prize announced three weeks ago

12:09 because it's going to start in a few weeks.

12:11 It's all about this module basically."

12:14 As always, a lot of these Nobel prizes have been won by men.

12:16 This is three men as well, is it?

12:18 Yeah.

12:19 I think you have to see this in the context

12:20 of the fact that it's being awarded for something done in 1985.

12:24 It's just a historical fact that there

12:27 were far fewer women active in physics because

12:31 of the way society worked back in 1985

12:34 and the prizes being awarded for work done then.

12:38 So it's like a trip back in time.

12:40 future Nobel prizes will uh be awarded for work done

12:45 more recently and as that happens I think we're going

12:47 to see uh a much more diverse range of winners reflecting

12:51 the fact that the physics community has become much more diverse

12:56 in the orange makes it smell of oranges

12:58 in nature inside my body inside your body we have

13:04 enzymes which are really quite complicated molecules that can

13:09 make molecules of one hand rather than the other.

13:13 They act as catalysts.

13:15 They bring together the reactants so that they

13:18 react to make the handed molecule you