How Physicists Proved Everything is Quantum - Nobel Physics Prize 2025 Explained

0:00 The 2025 Nobel Prize in Physics was just awarded to John Clark, Michael H.

0:03 Devay, and John M.

0:05 Martinez for their experimental work,

0:06 revealing that the weird world of quantum mechanics can be

0:09 seen not just at the level of atoms and particles,

0:12 but even at sizes that human beings are more familiar with.

0:15 The heart of their discovery is one of the most

0:17 fundamental and frankly weirdest features of quantum mechanics,

0:20 something called quantum tunneling.

0:22 If I throw this ball or anything against a wall, it bounces back.

0:26 It's a lemon.

0:26 It doesn't bounce very well every single time.

0:29 But if we take this into the quantum

0:31 realm by shrinking this example down to the subatomic

0:33 level and instead of a ball imagine an electron

0:36 and make our wall very very thin with each

0:39 bounce against the wall there is some small

0:41 chance that the electron doesn't bounce back but instead

0:44 passes straight through the wall and appears on the other

0:47 side without ever being present inside the wall.

0:50 That other than my favorite explanation as to how Santa gets down small

0:53 chimneys is the idea that shows us how weird the quantum world can be.

0:57 And thinking about quantum objects as tiny billyard balls isn't accurate.

1:00 Instead, we need to think about them

1:02 as objects without any definite edges, as waves.

1:06 In quantum mechanics, a particle doesn't have one exact position.

1:10 Instead, it can be described by a wave function,

1:12 a mathematical shape that tells you where it's most likely to be found.

1:16 The taller the wave, the higher the chance that you'll detect it there.

1:19 The wave fades at its edges, but it never quite reaches zero.

1:22 Meaning that there is always a small

1:24 chance that that particle could be somewhere unexpected.

1:27 So when that probability wave meets a barrier,

1:29 part of it is reflected, but part of it can seep through,

1:32 carrying the tiny possibility that that particle can

1:35 appear on the other side as if by magic.

1:37 That means that for an electron traveling down a wire,

1:39 if there is a break in the wire,

1:41 there is a small chance that that electron could hop over the gap

1:44 and continue on its journey as if the wire wasn't broken at all.

1:48 In fact, we are engineering computer chips down

1:50 to the sizes where that is becoming a real problem.

1:52 Components are so close together,

1:54 electrons can jump from one circuit to another,

1:56 even when we don't really want them to.

1:58 Weird, but that is the quantum worldview.

2:00 And for most of the 20th century, quantum effects were believed to be limited

2:03 to this tiny regime exclusively to electrons,

2:06 photons, atoms, and things small since

2:08 larger objects always appear to behave classically.

2:10 But in the 1980s at the University of California,

2:13 Berkeley, John Clog, Michael Devay, and John Martinez,

2:16 Martineis set out to find if these sorts

2:18 of strange behaviors could also happen at the macro scale.

2:21 They turned to this idea of the electron jumping across a gap,

2:24 but in a specific form called a Josephson junction,

2:27 where two superconducting wires are sandwiched

2:29 either side of an insulating barrier, which acts as the gap,

2:33 creating an impenetrable wall that classical

2:35 electrons shouldn't be able to cross.

2:37 In a normal wire, as electrons are negatively charged,

2:39 they repel each other as well as scatter off of other atoms,

2:42 constantly losing momentum.

2:44 That scattering is what causes electrical resistance

2:46 and produces heat and light in our electronics.

2:48 But in a superconductor, when cooled down to some of the lowest

2:51 temperatures in the universe, everything changes.

2:54 As an electron moves, it slightly distorts the latis of metal ions around it,

2:59 leaving behind a small region of positive charge.

3:02 Another electron can feel that positive region

3:05 and is attracted to it following behind

3:07 the first electrons path and linking these two

3:09 electrons together into something we call Kooper pairs.

3:12 Inside the superconductor,

3:13 billions of these pairs move together without any resistance

3:16 at all described by a single collective wave function.

3:19 When that wave function reached the thin

3:21 insulating barrier of the Josephson junction,

3:23 it can extend into the barrier overlapping

3:25 with the wave function on the other side.

3:28 That overlap allows Cooper pairs to tunnel through,

3:30 creating a steady supercurren with zero voltage,

3:34 the hallmark of superc conductivity.

3:36 This effect won Brian Josephson the junction's

3:38 name the Nobel Prize back in 1973.

3:41 The Berkeley team though wanted to take this one step further.

3:44 They wanted to see if the whole wave function representing billions of Koopa

3:47 pairs within the wire could tunnel through

3:50 the barrier as a single quantum object.

3:52 That would be macroscopic quantum tunneling.

3:54 tunneling on a range that is relevant to human beings.

3:57 To run the experiment, they needed to measure the tiniest possible

4:00 changes in current and voltage across the junction.

4:03 They used a dilution refrigerator to cool

4:05 the junction to just a few tens of millichelvin,

4:07 temperatures colder than interstellar space,

4:10 and surrounded the setup with layers

4:11 of magnetic shielding and microwave filtering.

4:13 Then, as they passed a precisely controlled current through the junction,

4:17 at low currents, everything behaved as superconductivity predicts.

4:20 a steadystate supercurren flowed producing no voltage

4:24 at all as Koopa pairs tunnneled across the barrier.

4:27 But as the current continued to increase at a certain critical value,

4:31 counterintuitively to superconducting systems,

4:34 suddenly a voltage started to appear.

4:36 That spike meant that the collective quantum state had

4:39 escaped its confinement and was tunneling across the junction.

4:43 Now, it turns out the microscopic quantum tunneling of a wave

4:45 function representing billions of electrons is kind of hard to draw.

4:48 So, if that doesn't mean much to you, I'm kind of right there with you.

4:51 Without relying on just talking through the math,

4:53 let's talk about why that voltage is arising and why

4:55 that tells us this is a macroscopic quantum tunneling event.

4:58 As the wave function escapes, its relative overlap compared to the wave

5:02 function on the right hand side changes.

5:04 That continual variant is what's driving a voltage.

5:06 And that's what tells us it's

5:08 something about this macroscopic wave function that's

5:10 actually moving and is different from individual

5:12 cooper pairs just tunneling over the junction.

5:13 Maybe the more important question to ask here is how do

5:16 we actually know this is a quantum tunneling process that is happening?

5:19 Because there are situations where classical physics can explain this switch.

5:23 At higher temperatures,

5:24 random thermal energy can jolt the wave function over the barrier,

5:27 a process called thermal activation.

5:29 Actually, at higher temperatures,

5:30 we see a strong correlation between temperature

5:32 and the current required to see this phenomenon.

5:35 What tells us though that this is a quantum

5:36 process is that as these temperatures are decreased,

5:39 the escape rate stops showing any dependence on temperature at all,

5:44 moving it out of how classical systems behave at these temperatures.

5:47 This process can only be the quantum system

5:49 representing billions of Koopa pairs tunneling through the barrier,

5:53 proving to us for the very first time that quantum properties

5:56 of the universe happen even at scales above the individual particle.

6:01 Back in 1935, Shreddinger was talking about a cat in a box to make a point

6:04 about how absurd quantum theory sounds when

6:06 you take it out of the microscopic world.

6:09 But here we are.

6:10 Thanks to Clark Devet and Martinez, we've seen actually it is entirely possible.

6:15 It's not the zombie cap that we were promised,

6:16 but it laid the foundations for superconducting cubits that we use today where

6:20 the same principles of these early Joseph

6:22 and junctions are harnessed to control quantum states.

6:25 This discovery often gets so much visibility

6:27 and celebration because it is the underpinning part of so much of what quantum

6:31 mechanics and quantum computation now relies upon.

6:34 But keep in mind that this is all the output of that discovery.

6:38 This took decades to finally prove and was driven purely by curiosity

6:42 and a need to know without necessarily

6:45 understanding any of what might be potentially unlocked.

6:48 Just testing a theory out against the behavior of the universe

6:50 until we proved it one way or the other.

6:52 and it's why this work feels so worthy of a Nobel Prize to me.

6:55 If you made it this far without your wave function collapsing

6:57 and would like to support the channel and the content that we make,

7:00 hit the like button and join our Patreon if you'd like.

7:02 You'll see some updates about a quantum computing company that I just visited.

7:05 Congratulations to the rest of this year's Nobel laureates.

7:07 Thank you for watching.

7:08 I'll see you in the next one.