Transcript: The Mind-Bending Reality Of Quantum Mechanics – Jim Al Khalili

Niels Bohr at this point is only in the secondary. He’s not even one of the grandees yet. But then you get the young guns: Heisenberg and Pauli and Schrödinger. Anyone who was anyone working in this new theory of the atomic world was at that conference.

And often the way history is taught, it would be suggested that quantum mechanics was complete. It was finished. They’d sorted it all out. Now let’s go and use it to understand physics and chemistry and build the modern world by this time.

Einstein vs. Bohr: The Famous Debate About Reality

Famously, the 1927 Solvay Conference was also a time when the two giants of the field, namely Einstein and Niels Bohr, had their famous debate about the nature of reality. And simplified versions of history would say, well, look, Bohr won the argument. Bohr had his famous institute in Copenhagen, and his view, together with his group of young geniuses—Heisenberg and Pauli—created what we today call the Copenhagen picture, which is the view of reality they argued was forced upon us by the mathematics, by the theory and by experimental evidence.

This suggested that the atomic world wasn’t something that we could picture. We couldn’t talk about an atom as a miniature solar system with electrons buzzing around nucleus. You couldn’t even imagine what an atom was like. It was maths. It was abstract. What quantum mechanics could give you, they argued, was predictions about the results of measurements if you were to look. If you look to see what an atom is doing or where an electron is, then this is what quantum mechanics tells you you will find. And sure enough, that’s what they found.

And quantum mechanics explains so many experimental results which until then people hadn’t been able to figure out. It turned out quantum mechanics got it right. But at its heart, there was this mystery about really, can we really not understand what’s really going on? Can we not have a picture?

Einstein famously said, “If everything ultimately is made of quantum particles, then is the moon not there when I’m not looking? Do I only bring it into existence when I observe it, as you’re telling me happens when I observe atoms?”

Well, people working in quantum mechanics said, “Yeah, well, that’s philosophy, right? Let’s get on with things.” And they did.

The Progression of Quantum Mechanics Through the 20th Century

If you think about the progression of quantum mechanics throughout the 20th century, we start with the old quantum theory of 1900. By the 1920s, we have fully-fledged quantum mechanics. By the late ’20s, people like Paul Dirac were already developing advances on quantum mechanics, saying, “Well, the other big theory in physics is relativity theory—special relativity, the idea that nothing can go faster than light, E equals mc squared, and all that business, time is the fourth dimension.”

People like Paul Dirac started to build on the foundations of quantum mechanics. And we get the first ideas in quantum field theory. It develops after the 1940s. People like the great American physicist Richard Feynman, along with Schwinger and Tomonaga, they develop quantum electrodynamics. Quantum electrodynamics applies to all matter and all electromagnetic radiation—light and radiation—how they interact with each other. Quantum electrodynamics is by far the most accurate theory in the whole of science. It’s incredibly powerful. Most of the phenomena around us that we see today, we can account for using quantum electrodynamics.

Further, physicists have gone further. There are other forces in nature. There are forces that hold the atomic nucleus together. In the 1960s, they were able to combine one of those forces, the weak nuclear force, with quantum electrodynamics to get what they called, together with electromagnetism, what we call the electroweak theory. They didn’t stop there. There was the other force in the nucleus, the strong nuclear force. What if we apply quantum theory to that? And that developed what is called quantum chromodynamics.

The Standard Model of Particle Physics

In the 1970s, all those forces were brought together in what we call today the Standard Model of particle physics—a terrible name, suggests it’s boring physics. But actually, from what we know, it accounts for all forces, all phenomena that we see, apart from gravity. And even now, certainly over the past few decades, the biggest challenge in quantum physics is how do you bring in gravity? That’s the Holy Grail of theoretical physics, something that hasn’t yet been achieved in a quantum mechanical way.

Well, physicists were using quantum mechanics. It was working, it was powerful, they were developing more powerful theories, but not everyone was happy.

Einstein’s Objections and the EPR Paradox

Einstein, for example, never gave up the fight. Despite being the founding father himself, having come up with his two theories of relativity, he spent the rest of his life trying to attack and undermine the probabilistic nature of quantum mechanics. In the 1930s, he published a very famous paper with two colleagues, Boris Podolsky and Nathan Rosen. Their names probably don’t mean much to you—they’re not as famous as Einstein—but this paper is famous because it’s known as the EPR paper, after the initials of the three colleagues: Einstein, Podolsky, and Rosen. And in that paper, they came up with their problem with quantum mechanics, which today is referred to as the EPR paradox. I want to describe this because it’s going to come back again when I talk about our quantum technologies going forward.

So what is the EPR paradox? Well, I’m going to describe it. It’s probably not the way they describe it in their paper, but you can look online—there are many ways of describing it. But imagine you have some device that produces two light particles, two photons, and it spits them out, back to back, traveling in opposite directions with opposite momenta, opposite energies. I’ve labeled them here as photon one and photon two. Now, if a particle is detected in space, then it reveals its nature as a particle rather than its nature as a wave. We know that light also behaves as a wave.

In fact, here on the desk is the first edition of the Lectures by Thomas Young, who’s often claimed to be the last great polymath. He did everything. But over 200 years ago, here at the Royal Institution, he showed that light behaves as a wave. He shone light from two slits with the light showing an interference pattern when the light went through two slits. There are beautiful diagrams in this book. During the 19th century, it was established that light behaves as a wave. Well, Einstein had shown that light behaves as a particle. So I’m trying to reconcile wave-particle duality, Young and Einstein. It’s a particle because it’s detected, it doesn’t smear out, but it also behaves as a wave.

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We say they’re entangled with a photon. They’re excited by the same photon source, a single photon source that splits that photon in two. A single photon can behave like a wave or a photon, and depending on your ability, it’s not one. It’s two. It’s entangled. The same thing with photon 2. Photon 1 needs to make up its mind effectively about what photon 2 is doing—quantum entangled.

Einstein’s Problem with Quantum Mechanics

Well, the EPR paper wasn’t concerned with the fact that you can’t know the position and the momentum of a particle at the same time. This is what’s held in Heisenberg’s uncertainty principle. Einstein didn’t like that at first, but he’d come to terms with the idea by 1935. That wasn’t his problem. His problem was the way that Photon 2 would have gotten this property from the beginning, and the fact that different observers wouldn’t know it. Then there was something happening between different observers, right? That was complete. In fact, in the last sentence in their paper, we have shown that the quantum mechanical description does not provide a complete description of physical reality, and we’ve chosen—the question is whether that’s complete. We believe, however, that such a choice is possible.

The followers of Niels Bohr said, “No, we’re going on with that. It’s not possible. You can describe these two photons in every way they relate to a single particle observer.” Well, the paper caused quite a stir. In fact, it made the front page of the New York Times. In 1935, Einstein was concerned with the photons—not concerned with the two other authors of the paper, Einstein and two collaborators. Einstein, at this time, is the most famous physicist in the world, so he’s going to get credit for it. But, in fact, Einstein didn’t write that paper. Boris Podolsky wrote that paper, and Einstein wasn’t happy to say that Boris Podolsky had spoken to the New York Times about it either. So he didn’t like the idea of being published in that way. He didn’t disagree with the ideas of the paper, but he didn’t write it.

Schrödinger’s Cat and Quantum Entanglement

Corresponding with Einstein was the other physicist, Erwin Schrödinger, the Austrian physicist. In the same year, not long after the EPR paper came out, he was writing a set of papers actually. This is where he first described the cat in the box—the cat that’s alive and dead at the same time until you open the box to look. Now, the Schrödinger project wasn’t concerned with the weirdness of quantum mechanics, it wasn’t concerned with the idea of uncertainty. It was concerned with what happens when you make a measurement. How does this thing that’s able to be in multiple states and do many things at once give you a definite result when you look? What is the environment? And so he uses the cat as an example. How can the cat be alive and dead at the same time?

What’s important in the context of what I want to say is that it’s in that paper where he first introduces quantum entanglement. “Verschränkung” is what I’m saying in German. Actually, a few years later, Einstein writes a letter to another physicist, Max Born, and he describes what happens in quantum entanglement in his EPR paper. This is where he first uses the term “Spooky Action at a Distance”—Spukhafte Fernwirkung. It’s perfect German in every way I’m concerned with this.

Because how can Photon 2 and Photon 1 communicate with each other instantaneously, and Photon 1 says, “They’ve measured my position, you’d better be in a wave. They’ve measured my momentum, you’d better be in a particle.” Einstein says, “How can that be? Surely there’s nothing that can communicate between the two faster than the speed of light.” But in the quantum world, that seems to happen.

This is quantum entanglement, and we’ll see later how quantum entanglement is helping us in our revolution, the technological revolution today.

The Einstein-Bohr Debate and Its Aftermath

The establishment of quantum mechanics—most physicists seemed to accept the fact that Bohr had won the argument. Indeed, quantum entanglement seemed to be, Einstein was wrong, and the establishment of quantum mechanics works. Einstein, of course, went to his grave, I guess he was right in taking his view, didn’t want to engage with quantum entanglement. He wasn’t going to accept quantum entanglement.

Most physicists, along with men like Eugene Wigner and John Wheeler in the 1950s said, “Is the entanglement explanation the correct explanation? Are we happy that we understand what’s going on in quantum entanglement?” They seemed to be more open to young physicists exploring, and I use that term, indeed, they were exploring alternatives to the Copenhagen school—people like David Bohm and Hugh Everett. They were coming up with alternative explanations which said, “Well, the EPR paper, which said that the cat in the box, thought experiments, which challenged the Copenhagen interpretation which says the only thing that matters is the results of experiments. Quantum mechanics predicts what you will get when you do an experiment, and you can’t say anything about what’s happening when you’re not looking,” said the interpretations.

So that was the majority view, and one of the minority voices in the 1960s was the Irish physicist John Bell. Now, John Bell certainly didn’t like the Copenhagen interpretation. He talks about a time when he was being taught by the great physicists, and he says he didn’t get the courage to speak up about the Copenhagen interpretation and say he wasn’t entirely happy with that.

A Personal Encounter with John Bell

Well, many years later, in the 80s, I got access to John Bell. I met him at an American Physical Society meeting in Baltimore in 1989. I was doing my PhD and looking at interpretations in America. I came out of a talk on different interpretations by someone who seemed to be presenting some new idea I thought was different, and I came out of the talk with John Bell, and I liked John Bell. He didn’t get the courage to challenge the American Physical Society. I got the courage because I was presenting with John Bell, so he was happy.

So I said, “Oh, that last talk, I knew it had been at the talk as well,” and I said, “Yeah,” and I said, “Yeah, I agree with you.” He hadn’t heard of that interpretation, I know, obviously he didn’t understand social conventions. So I said, “I said, I said, I said, I said, I said…”

Bell’s Theorem and Locality

Now, locality means you can only interact with things close to you, and if it’s far away, then it takes time before you can reach it. At most, the fastest you can get information is through sending light signals. And you can’t interact with something immediately if it’s far away. And I said, “So, if, if, if I’m right and Nature is local, then there’s a formula which says that there’s a limit to how much, for example, in the EPR photons, how much correlation the two photons could have with each other because they’re only interacting, they start together. Connected through interaction at the beginning, for example. They can’t change what they’re doing when they’re far apart afterwards if they don’t have time to tell each other what’s happening.”

He said, “That formula, it was put into, as an inequality, by two other physicists, Clauser, Horne, Shimony, and Holt, who gave the inequality CHSH its name,” put into a nice mathematical formulation that you can test experimentally, that experimenters can go out and test. So can we finally prove Einstein wrong?

You measure these things in an experiment and you get a number more than two, they must be communicating after they’ve left. There must be some, what’s called non-local connection between them, instantaneous quantum entanglement. Well, that experiment was first done by John Clauser at Berkeley in the early 1970s, then repeated in the early 80s by Alan Aspect and his collaborators in Orsay in France, and then many other experiments, particularly by an Austrian physicist, Anton Zeilinger, and in fact, all three of those men had to wait a long time. 2022, they won the Nobel Prize for Tests of Bell’s Inequality. What did they find? Quantum mechanics is right. You get a number more than two. Einstein couldn’t be right. Quantum entanglement, however weird you might think it is, is real.

Quantum ideas and some of these weirdnesses of quantum mechanics continue to this day, and in fact, only this year, three other quantum physicists won the Nobel Prize of Physics for another weird aspect of the quantum world, namely quantum tunneling.

Superposition and Interference

Okay, well, I want to move on to what is weird about quantum mechanics. I want to go through this quickly. Superposition and interference. Now, superposition and interference are properties of waves. Thomas Young would have been very clear about light waves, but we can think of water waves or sound waves behaving in this way. When we say waves superimpose on each other, we mean this sort of thing. Drop two pebbles in water and the waves, the concentric circles that move outwards, will overlap and you start to get some sort of weird crests and troughs canceling out and superimposing and so on. That’s what we call interference.

Well, in the quantum world, particles also can have this interference effect. But it’s a superposition, not of waves. You can have superpositions of particles, for example, having two energies at the same time or being in two locations in space at the same time. If we think about a light switch that’s on, so for US audiences, that would probably be off. I don’t know why they have it the wrong way around. They drive on the wrong side of the road and they don’t know how to fix their light switches. But, so that’ll be on and that’ll be off. In the quantum world, it would be like having something that is both on and off at the same time. The cat that’s dead and alive at the same time. But we don’t worry about cats. That was a thought experiment that we’re talking here about is objects down in the quantum realm. Particles, electrons, photons, atoms, and so on. So, and there can be, what we say, there can be in different quantum states and they can have different values of energy and so on. And not just two, not just on and off. They can have multiple states at the same time.

And you can have lots of particles all together all being in a superposition at the same time. These sort of weirdnesses were known about and understood right from the early days of quantum mechanics. And this is what we mean when we say quantum mechanics is perplexing and counterintuitive.

The Quantum Skier Analogy

My famous example is the quantum skier. The tree looks fine. There’s no reason not to think he could father children. Hasn’t done him any damage. But you see something like this, no, that’s impossible, right? In the quantum world, that’s what happens.

The Two-Slit Experiment

Famously, it’s what happens in the two-slit experiment. Now, Thomas Young was the first to carry out the two-slit experiment for light. But we now know we can do the quantum version of it using not light. You can use light, but you can also do it using atoms. I’m not going to go into this in detail. And I’ll explain why.

My last RI discourse was in 2013. And there I am talking about the two-slit experiment. Now, I explained how it went and I went through all the stages of how just how weird and wacky and exploding brain it is. And then I made a mistake. I said, anyone in the audience who thinks they have a common sense explanation of how the atom gets, so this is with light, but I then go on to show with atoms. Firing one atom at a time, you still get the interference pattern. How does the atom go through both slits at the same time? It does, but then if you look to see where it’s going, it seems to only go through one or the other. It seems to know if you’re spying on it.

I said, if you have a common sense explanation, get in touch with me because the king of Sweden might want to give you a phone call. To this day, I don’t know why I thought the king of Sweden would listen to my recommendations from some random people. Give your majesty, give that chap a Nobel Prize, he sorted out.

But my real mistake was I gave a lecture in this Saturday lecture theater, talking to a few hundred people, forgetting that the lecture was being streamed online and then goes onto the Royal Institution YouTube channel. That clip has been viewed several million times. I, to this day, 12 years later, I get one or two emails a week from people saying, “I have solved the two-slit experiment, give me my Nobel Prize.” And I had to think, no, no, you haven’t, you haven’t solved it. It’s sad, but you know, it’s great that people are thinking about science in this way, but not so great when they think that they’ve solved a problem that many people have thought very hard about and can’t really figure out as common sense view.

Quantum Tunneling

So quantum tunneling is another aspect of the quantum world that’s strange. The example that I like to give is, that many people like to give is, if you have a ball and a hill, you have to kick the ball up the hill hard enough if you want to get it to the top and over to the other side. If you don’t kick it hard enough, it’s going to go halfway up the hill and then come back down again.

In the quantum world, replace ball with atom or electron or whatever, and replace hill with some energy barrier. You can give that atom, say, enough energy to get halfway up the hill and it won’t roll back down again. There’s a certain probability that it gets halfway up, disappears, reappears on the other and rolls down. Like a ghost phantom walking through a solid wall. If we saw it in our everyday world, we’d say that’s ridiculous. You’ve clearly lost the plot, you’re not doing science now, you’re just doing silly stuff. Quantum tunneling is real, not just because the guys won a Nobel Prize for it this year.

The reason we are here today is because we get life, life gets energy, light and warmth from the sun and the sun shines because of quantum tunneling. Hydrogen nuclei, protons, can quantum tunnel together in the first step towards nuclear fusion to make helium and that’s what creates a thermonuclear fusion, the energy of the sun. So the sun shines because of quantum tunneling.

How do we talk about it in quantum language? Well, it’s basically particles are waves and waves can leak through the barrier. So there’s a certain probability, so part of the wave bounces off and the other part travels through. What is it that this tells us? Well, it gives us the chances, the probability that the particle is going to get through and that probability depends on just how big the wave is that passes through. The math all works, you see, and the experiments that we do to check the math all agree with what the math predicts. It’s the logic, the common sense picture that we struggle with.

Quantum Entanglement: The Source Code of Reality

Okay, and then there’s quantum entanglement. Remember the EPR, spooky action as distance? Quantum entanglement was seen to be, for many years, this really extremely outlandish counterintuitive aspect of quantum mechanics. It’s far weirder than quantum tunneling or quantum superposition interference. Today we know that entanglement is so prevalent that it’s probably the most important feature of physical reality.

We still don’t teach quantum entanglement seriously to undergraduates in physics at university and I think we should because it is so important. And current theories, the fundamentals of physics are even suggesting that quantum entanglement may be the source code base of reality. Even time and space themselves may emerge from quantum entanglement is one suggestion.

But more importantly, many of the technologies I’m going to mention in a moment rely on quantum entanglement and use quantum entanglement. It’s not just weird. However weird you might think it is, it’s being used. We’re developing devices that are going to benefit humanity because quantum mechanics is real.

Decoherence: Entanglement’s Annoying Little Brother

Okay, an entanglement comes along with something else called decoherence. That’s its annoying little brother. Decoherence is when our everyday world messes with, disturbs quantum entanglement. Quantum entanglement, the two photons back to back, that’s a very delicate quantum state. If you measure it, you open Schrödinger’s box, for example, or you detect whether photon one is a particle or a wave, you destroy the entanglement. You force the other particle to do what it’s told, what it should do, to complement the first one.

So decoherence destroys entanglement. Decoherence is something that was developed mainly by D’Souza and Wojciech Zurek up to half a century ago now. And it’s a very well-developed idea in physics. It solves things like the Schrödinger’s cat paradox. Now we understand a lot more about decoherence and entanglement.

What Have We Done With Quantum Mechanics?

Okay, part three. What have we done with quantum mechanics then? Well in physics, of course, we’ve used quantum mechanics to understand the structure of matter. We understand, so the cartoon of the atom with electrons orbiting around it is wrong, but we still use it to somehow depict the atom in a simplified way. We understand the nucleus of the atom is made of protons and neutrons, and the protons and neutrons are made of quarks and gluons.

We’ve built larger and larger particle accelerators to smash matter together at ever higher energies to break it up into tinier and tinier constituents, and we arrive at the standard model, which I mentioned earlier, which tells us the building blocks of matter as far as we know so far. There are still a few things we’ve got to iron out, like what is dark matter and what is dark energy? But anyway. So that’s physics.

Chemistry. This is a modern periodic table of the elements, as you would see on any school wall in the world. A modern one because it’s, well, certainly it’s far more modern than Mendeleev’s original periodic table from the 19th century. He only knew about half the elements that we know of today. But this one contains lots of new elements you may not even have heard.

So this OG is the heaviest element synthesized, Oganesson, element 118. The reason why the periodic table is classified in this way according to the physical and chemical properties of the elements is because of quantum mechanics. Quantum mechanics explains the rules for how electrons arrange themselves around atomic nuclei and give us the properties of the elements. Without quantum mechanics, you wouldn’t understand why and how the periodic table looks the way it does.

The First Quantum Revolution

In terms of technology, of course, what we now call the first quantum revolution involved developing many devices that relied on these ideas in quantum mechanics. The laser is a famous one, but of course we have the diode, LEDs, integrated circuits, the computer, GPS, internet, smartphones. Everything that we rely on and take for granted in our everyday world relies on our understanding of the quantum world. Without that understanding of the quantum world, we wouldn’t have any of these things, let alone things like the electron microscope, the photomultiplier, which is a device that many of you will not have even heard of but is used in so many other instruments, MRI machines. They’re quantum. It’s not just sticking a patient in a strong magnetic field. It works because of quantum mechanics.

All of these, in fact, the whole of modern electronics is really thanks to our understanding of the quantum world because it gave us an understanding of semiconductors and semiconductors, the understanding of chips and so on, and we have computers. All of that relies on the quantum mechanics that was developed in the early 20th century, quantum tunneling, quantum interference, and so on, but we now have a second quantum revolution. Sometimes it’s called quantum 2.0, but that suggests there’s a 3.0, and it’s like Windows. It’s not. It’s the second quantum revolution, and I want to go through a few of the technologies that are emerging today because of this revolution.

Quantum Timing and Atomic Clocks

Quantum timing is the first one, by which I mean quantum clocks. The atomic clock. Now, the original atomic clocks, they rely on an early feature of the quantum world, namely this idea that electrons and atoms can’t have any old energy. They can’t orbit around any distance from the nucleus. They have to have discrete energies, and that is used in order to generate, in the early atomic clocks, light, well, microwave radiation coming from a certain type of atom that you can measure the wavelength of very, very accurately. If you can measure the wavelength, it’s how many waves pass through a certain point. That gives you a counter. It gives you a way of measuring time, not a pendulum swinging or quartz crystal vibrating, but microwave radiation emitted by atoms that pass through this, what’s called an optical frequency cone, and that gives us very accurate timing to tiny, tiny millions and millions of a second.

These sorts of atomic clocks are the basis of global navigation systems, GPS systems. The reason your smartphone, the reason you can use Google Earth and find out, or Google Maps to find out where you are, because those satellites that are sending radio signals to your phone have atomic clocks on board. Those atomic clocks are needed to measure time very, very accurately in order for us to measure the distance from those satellites very accurately, because that’s the only way you can triangulate where you are.

Optical Lattice Clocks

Atomic clocks over the last decade or two have been miniaturized, as do many technologies. Here’s one alongside a single coffee bean. But also, they’re becoming more accurate. So there are now what are called optical lattice clocks. These are atomic clocks that use lasers to pump energy into atoms, and then those atoms give off that same energy very, very precisely, but it’s not in the microwave region of the electromagnetic spectrum. It’s much higher energy in the optical region. So higher energy means higher frequency. Higher frequency means more waves passing through this counter, the checkpoint, and that means much more accurate timekeeping.

The talk now is that these atomic clocks are so accurate, they lose something like less than a second over the age of the entire universe. I mean, just ridiculous, ridiculous accuracy, and yet it’s needed for many technologies, that level of accuracy. So even now advances in rather than atomic clocks, but nuclear clocks, looking at pumping the nucleus of the atom with energy and seeing how that gives off its energy at much higher frequencies to give even more accuracy.

Quantum Imaging and Quantum Sensing

These are fun advances in technology. Quantum imaging, well, the example I wanted to say, I want to go through this relatively quickly. So ghost imaging is this wonderful idea where you can look at something while you’re actually not looking at it. You send light to probe it, and then light in another direction goes to a camera, and then the camera is the image of the thing that you weren’t looking at. I know. I mean, it’s crackers, right?

But it relies on this idea of quantum entanglement, the fact that the two photons that go back to back, they’re still entangled, they’re still talking to each other. The basic idea is this. If you have some sort of device that fires photons of light, high energy, they pass through a crystal, a material that sometimes, not all those photons, but sometimes can absorb those photons, and then that energy that is absorbed, it gives out again out the other end as two photons, just like the two back-to-back photons in the EPR.

But what’s different here is that these two photons don’t have to have the same energy, right, or the same color. So for example, one can be an infrared photon, and one could be a visible light photon, higher energy. As long as the energy, their combined energy, is the same as the original photon that was sent in, you’re conserving energy, and everything’s fine. So one could be visible light, and the other can be infrared. They are still quantum entangled. So what happens to one influences what happens to the other.

And so if the infrared photon interacts with and passes through or measures, looks at an object, then we see the image with the visible light photon there, sort of time-correlated, which means that when you know which photons arrive and which one it’s paired with. This is what’s called ghost imaging.

The Entanglement Camera

And it gets crazier. I’ll tell you something about the entanglement camera. So this idea uses entanglement and interference experiment and interference patterns. Well, let’s see if I can talk you through quickly. So you’ve got the photon comes in, so now imagine it’s coming vertically rather than horizontally. It goes through a first crystal. It is what’s called down conversion. It creates these two photons of different energies. You’ve got the visible light and the infrared light. They’re bounced off mirrors, bounce back again. Meanwhile, another photon has passed through and down converted in a second crystal. And that creates another infrared photon and a visible light photon in the opposite direction.

And you get an interference pattern. You see these interference fringes. This is the two slit experiment fringes or Young’s fringes. You’ve got the infrared one and you’ve got the visible light one. Now if you stick an object in the path of the first infrared photon, that changes the interference pattern here. Because they’re entangled, you get the picture on the other side. It’s quite incredible that you’re looking at something, seeing something without looking at it.

It’s not wacky science fiction. This is now being used to pick up tiny details in biopsies of tumours in breast cancer. They can pick up far more detail than you could just with infrared cameras, for example, because visible light cameras are much more precise, much sharper images. It is quite incredible what it can do. Already these cameras are being developed.

Now, a camera that relies on quantum entanglement, what would you call it? Yeah, a film cam. Perfect. And indeed, so here’s the demonstration device.

Here the researchers at the Imperial College and the University of Birmingham who are working on these ideas are showing it off at this year’s, this summer’s, summer exhibition at the Royal Society just up the road from here.

Quantum Sensing Applications

Okay. Quantum sensing is another technology that is already with us today. It has many applications. For example, you can use it to measure Earth’s gravity very, very accurately. You can even measure it out at sea. These devices are very versatile or down underground. There are lots and lots of different applications that we can think of that require measuring Earth’s gravity very accurately. How do they work? Well, rather than sending photons through the two slits, they work on what’s called atom interferometry. So rather than one photon becoming two and bouncing off mirrors, now you have atoms that are quantum entangled. That’s my bum and the microphone touching the table. I’d better not do that again.

On the right is a brain scanner, so like a cycling helmet using lasers and atoms and quantum entanglement that is so sensitive, they can pick up the weakest of magnetic fields, even the magnetic fields created by the firing of a single neuron in the brain. So rather than sticking someone in an MRI scanner, which is a very scary process laying inside there without moving for an hour, particularly for young children, but studying lots of neurological disorders by just wearing this cycling helmet and you being able to pick up brain waves is absolutely remarkable. These devices, these instruments, these technologies are already with us today.

The Promise and Reality of Quantum Computing

Quantum computing, of course, is the famous one, and it relies on this idea of superposition that rather than having a logic gate that’s zero or one binary digits, now we have zero and one at the same time. Have we built quantum computers yet? Well, we’re getting there. Not quite as advanced as the imaging and the sensing technologies. What will they do for us? Quantum computers, I don’t want to say too much about them because we’ve heard a lot about quantum computers, but certainly if and when they arrive, they’re going to help us with drug discovery in medicine, maybe designing better batteries or solar panels to help combat climate change. They’re going to be working in finance and logistics. And for researchers, for physicists and chemists and material scientists, they might be able to help us simulate the subatomic world. It makes sense if you’re trying to describe a quantum system that you can use a quantum computer running a quantum algorithm to do the job.

Just last year, I published a paper with my PhD student, Lance, and my colleague, Paul Stevenson, where we’re trying to study the atomic nucleus. So although there’s no quantum computer yet, we can use a classical computer pretending to be a quantum computer to see if that gives it an advantage in solving some of these important problems in theoretical physics.

Challenges and Timeline for Quantum Computers

How far away then is quantum computing? Unlike sensors and images that we have already, well, there are problems, there are challenges. First of all, you need lots of qubits, quantum bits of information to work. You have to make sure that decoherence doesn’t kill your quantum entanglement very quickly. This is something called error correction. We don’t even know what to build a quantum computer with. What we make it out of, I’ll show you a few examples next. And then the software. We need to write the algorithms that would run on a quantum computer. We have a few, but not many just yet.

You’ll hear the big corporations working at Google and IBM and others saying, you know, “We’ve done it, we’ve cracked it, it’s here, it’ll be here next year or the year after.” Realistically, we’re talking about one or two decades before quantum computers actually arrive.

So how do you build one? Well, there are lots of candidates. One of them is using what are called superconducting qubits. So this is a famous chandelier cryostat developed by Google.

Essentially, what you’re looking at is basically the wiring that cools down the real thing that’s doing the quantum computation down to near absolute zero. So this is an example provided by a researcher at the Quantum Centre for Quantum Computing, Nye Baker. This is a four qubit quantum processor. These blue squares are the qubits. They’re quantum entangled, they’re linked together, and you have to feed in information. They do their job, and then they send the information out.

A four qubit quantum computer is a start. But of course, a lot of these companies are going further than that. They need more qubits in order to do something useful. A lot of the technology developed here with these superconducting quantum computers relies on several aspects of the quantum world. For example, superconductivity is something that I haven’t had a chance to say anything about. Materials cooled down to near absolute zero lose their electrical resistance in their wiring, and so there’s no reason so the current can flow through without any hindrance, and that makes it a very powerful device.

They’re developing their processors with more and more qubits, and this is, I did have a, I’ve forgotten the name for this one. Probably people in the audience will know what it is. Anyway, that’s the latest processor.

Alternative Quantum Computing Technologies

There are other candidate quantum computers using light, using photons called photonic qubits. There are neutral atom qubits. So this is another 3D printed model. Another researcher working quantum computing, Luke Fernley, has learned this to me. So this is, the idea here is that rather than using superconducting wires or photons, it uses atoms, clouds of atoms, that are cooled down by lasers.

So you get the cloud of atoms, usually something like cesium atoms at the bottom. We’re using lasers, which are from the first quantum revolution, to cool the atoms down. Then they’re sent up, they’re held in place by a magnetic field, and then other lasers cool them down further. Then you’ve got other lasers that move and manipulate them around. Then you’ve got other lasers that pump them with energy and put them into a quantum superposition, and then they do their quantum magic. So all of it is to do with lasers, in this sense.

And then there are others like trapped ion quantum computers. So they’re like neutral atoms, but an ion, remember, is an atom that’s gained or lost electrons. So it’s charged. The advantage there is that you can use electromagnetic fields to hold them in place.

We don’t know which of these quantum computers is going to be the one that ends up being the most efficient, the most powerful, the one that’s going to give us quantum computing in the years to come. It’s not easy. I mean, look at the mess. I can’t even connect my Skybox to my TV, wiring at the back. I don’t know how. It’s lasers and mirrors and wires. This is why I became a theoretical physicist, because that would give me absolute nightmares.

Looking to the Future

I want to very quickly then end with talking about quantum communication, quantum encryption. I don’t want to say too much about these, these are things in the future. The quantum internet would link computers together. See, I was so determined to finish on time. But the future is very close, it’s coming to us very clearly, very fast. Some of these devices are with us today.

I want to end by thanking… I did give a talk on quantum biology at a previous discourse, so I won’t have to say anything about that now. But particularly I would like to thank the IOP for giving me the honour of giving this lecture, culmination of the UK contribution to International Year of Quantum Science and Technology. And it’s been a pleasure, so thank you very much indeed. Thank you very much.

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