The quantum revolution: ‘Spooky action’

This is an audio transcript of the Tech Tonic podcast episode: ‘The quantum revolution: ‘Spooky action’’

Madhumita Murgia
Hi, my name is Madhumita Murgia, and I’m one of the presenters of Tech Tonic. We’re looking for some feedback from our listeners about the show. So if you have a second, please fill out our brief listener survey, which you can find at ft.com/techtonicsurvey.

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In this season of Tech Tonic, we’ve been talking about quantum computers and why some people think they’re so revolutionary. But so far we’ve mainly talked about the things quantum computers can do, or at least what they might be able to do in the future that makes them so groundbreaking: performing calculations that should take centuries in minutes, cracking the unbreakable codes of the internet, dramatically speeding up the development of new drugs and materials. But what we haven’t done yet is look at why they’re able to do these things. What’s going on inside a quantum computer that makes them so extraordinary, so completely different to any computer that’s come before.

So in this episode, we’re stepping inside the machine. (Futuristic sound) And this is where things get a little spooky, where our sense of reality gets kind of warped because stepping inside a quantum computer involves stepping into the strange world of quantum physics. Quantum physics is what gives a quantum computer its power and its magic. And quantum physics is really weird. (Futuristic sound)

To explain that weirdness. I want to start with a letter written from one famous physicist to another about a cat. (Cat meowing) It’s 1935, and the renowned physicist Erwin Schrödinger has left Germany to escape the rise of the Nazis, and he’s moved to Oxford in the UK. And he sits down at his desk. He’s writing a letter to a friend, the physicist Albert Einstein (sound of someone writing). Imagine you had a cat, Schrödinger writes to Einstein. You put the cat in a metal box and close the lid. (Sound of box being closed, followed by a meow) And in the box was a contraption that might or might not release some poisonous gas and kill the cat (sound of hissing gas). Without opening the box to look: Is that cat alive or dead? (Cat meowing)

In the world of quantum physics, Schrödinger says, the answer seems to be both. The cat is both dead and alive at the same time. By this point in the mid-’30s, Schrödinger and Einstein had both spent more than a decade working on a revolutionary new theory called quantum mechanics. The theory described how subatomic particles, things like electrons, behave. And it worked. Their equations describe the subatomic world in a way that the existing laws of physics just couldn’t. And in 1933, Schrödinger won the Nobel Prize. (Sound of applause) But now both Schrödinger and Einstein were starting to worry. They thought that something might be deeply wrong with their theory. The equations seem to suggest that something really weird was going on in the subatomic world. When you looked for them, subatomic particles could be found in a particular place. But quantum mechanics said that when you weren’t looking at them, they seemed to be in lots of different places all at the same time.

Sean Carroll
There’s something called classical mechanics, which is what Isaac Newton bequeathed to us, according to which the world is sort of a definite clockwork mechanism. There’s particles. They have positions, they have velocities. We can predict everything. Quantum mechanics says that that’s what you see when you look at the world. But that’s not what really exists when you’re not looking at the world.

Madhumita Murgia
Sean Carroll is a theoretical physicist.

Sean Carroll
If you have an electron, a tiny little particle, right, and you look at it, you measure where it is, you will see it to be somewhere. That is a fact about the world. And quantum mechanics says when you’re not measuring it, it’s spread out. It’s in a superposition of being in all these different possible locations.

Madhumita Murgia
This idea of a superposition, that a particle could be in lots of different places at the same time, was completely new. And so was the idea that particles seem to decide on a specific location only when someone looked at them. It was such a weird idea that even some of the greatest minds of the 20th century thought this was too strange to be really true. So in his letter to Einstein, Schrodinger comes up with this thought experiment.

Sean Carroll
So he sort of came up with a way to amplify a microscopic superposition, like we might have for an electron or an atom, and amplify it up to a macroscopic thing like a cat.

Madhumita Murgia
So imagine you have a cat in a metal box that may or may not have been gassed to death. When you open the box, you will discover that the cat is either alive or dead. But according to the theory of quantum mechanics, Schrödinger said, until the moment you open the box (sound of box being opened) and observe either the living cat or the dead cat, the cat is in a superposition of both alive and dead. It’s not that you just don’t know yet if the cat is alive or dead. Quantum mechanics says it’s literally both at the same time.

Sean Carroll
Apparently there’s a quote from his daughter who said that her father was just not a cat person. So . . . (Madhumita Murgia laughs) That’s why he chose to put the cat in this torturous device. But the idea was not to say, like, isn’t quantum mechanics cool? The idea was to take this idea of a superposition as being real and say, surely you don’t really believe this. You telling me that it’s in a superposition until I open the box and look at it? That’s just crazy. (Laughter) And everyone else said, yep, that’s what we’re telling you. (Laughter)

Madhumita Murgia
The world, according to quantum mechanics, is a strange and counterintuitive place where particles behave like waves, and waves behave like particles, where things appear to exist in different places and states at the same time, where the laws of physics that we experience in our everyday lives do not apply. But it’s these features of quantum mechanics, things like superpositions, that are being used to build quantum computers today. It’s what makes quantum computers so powerful and so extraordinary and so deeply strange all at the same time.

This is Tech Tonic from the Financial Times. I’m Madhumita Murgia. This season is about quantum computers and how they might revolutionise computing and change the world. This episode is about the science behind quantum computers, quantum physics and how you can use it to build a computer.

First off, let’s acknowledge here that quantum mechanics is notoriously difficult to get your head around. We’re usually talking about things happening on a subatomic scale, things we never really see happening and that our brains aren’t used to dealing with. So to get a grip on what quantum mechanics tells us about the world and how that relates to computers, I reached out to an expert, Sean Carroll, a theoretical physicist and philosopher from Johns Hopkins University in the US. So how did you get interested in quantum physics?

Sean Carroll
It sort of was a roundabout way. You know, I was always interested, since I was a kid, in physics, in theoretical physics, but my favourite thing was cosmology and gravity, black holes, the Big Bang. But, you know, as a professional physicist, you at some point step back and say, OK, like, how can I really do the most important work there is to be done? Like, what is the biggest question that I could help understand?

Madhumita Murgia
Sean says that to get a sense of how features of quantum mechanics like superpositions found their way into computers, you have to go back to the start of the 20th century. Back then, scientists were puzzled by what atoms, the basic building blocks of the universe, really looked like.

Sean Carroll
Think about the very famous, traditional, iconic picture of an atom, right? You have an atom with a nucleus at the middle, and there are electrons orbiting the atom — almost like a little solar system, right? There’s electrons, with that are dots that are moving in circles around the nucleus. And that’s just wrong. That picture, even though it’s, you know, the iconic image of physics in many people’s minds, it can’t be right.

Madhumita Murgia
Even back then, scientists realised that if electrons really did orbit the nucleus like planets orbiting a sun, they would immediately spiral into the nucleus and the atom would collapse. But atoms don’t collapse. So they had to come up with a different picture.

Sean Carroll
What actually is going on is the electron is a cloud. It’s a, what we call a wave function. It’s a terrible name for such an important thing. But rather than being a dot that is in an orbit, it’s sort of spread out in a very definite shape around that atom.

Madhumita Murgia
But the trouble was that when you took a photograph of an electron, it didn’t look like a wave or a cloud. It looked like a particle. So they came up with the theory that the electron is in a cloud made up of all its possible locations when you’re not observing it. And it becomes a particle in a specific place when you measure it. Just like Schrödinger’s cat. (Meow) All possible versions of dead and alive until you look at it and it becomes either dead or alive. Schrödinger and even Albert Einstein might have found this idea disturbing. But the idea of subatomic particles in superpositions is central to understanding how quantum computers are different to classical computers. Again, let’s start with the basics.

Sean Carroll
So in classical computers, you start with a set of bits. A bit is just some register in your computer that is either 0 or 1, and you can use a string of bits to make a binary number and represent anything that you’d like.

Madhumita Murgia
All the computers in the world, from the biggest supercomputers to the phone in your pocket, encode all their information in bits. No matter how complex the information is, you can boil it down to 0s and 1s. But what if your bits didn’t have to be just 0 or 1? What if, as quantum mechanics says, things can be in more than one state at the same time?

Sean Carroll
Quantum mechanics comes along and says, actually, instead of a bit you have what is called a quantum bit or a qubit.

Madhumita Murgia
Remember qubits? We heard a lot about them in episodes one and two.

Sean Carroll
And a quantum bit can be in any superposition of 0 and 1. So actually that’s a lot more information, right? Rather than just 0 or 1 as digits, it’s a combination of 0 and 1. So that’s already more information than in the classical bit. (Sound of coin dropped)

Madhumita Murgia
One way to imagine this is that the bits in a classical computer are coins that are either heads or tails. (Sound of a spinning coin) But a qubit in a quantum computer is a spinning coin. It’s kind of heads and tails at the same time. That means the computer isn’t limited to representing just one string of 1s and 0s. It can represent lots of combinations of 1s and 0s simultaneously. That gives it the potential for much more computing power. But what physically is a qubit? In classical computers, bits are generated using transistors. Electrical switches that are either on or off to represent 1 or 0.

Sean Carroll
Well, just like a classical bit, a qubit is basically an idea. And then you have to figure out how are you going to represent that idea. So a classical bit maybe represented by literally a switch like in an abacus. Something is up or down, or in a computer, maybe it’s a voltage, it’s high or low. There’s an infinite number of ways to physically embody the abstract idea of a bit of a 0 or 1. Likewise, there’s many different ways to physically embody a qubit.

Madhumita Murgia
One way you could make a qubit is to take advantage of the fact that one of the features of subatomic particles that you can measure is their spin.

Sean Carroll
Conceptually, the simplest thing is just the spin of an electron, right? Electron spin is a superposition of spin up or spin down so we can label spin up, 0, and spin down, 1. There you have the qubit.

Madhumita Murgia
The people building quantum computers today use a variety of methods using the spin of electrons, ions trapped in a vacuum, or photons, particles of light. But the idea is always to trap a particle and use it to run calculations while it’s in the superposition of 1 and 0.

OK, so let’s say you’ve got some qubits, but that’s only the first step in building a quantum computer. For the computer to work, you need to get the qubits to work together. And this involves another really bizarre feature of the quantum world, a feature called entanglement. Quantum entanglement is actually quite a simple concept, but it gets weird quite quickly. So stay with me here. We’re stepping deep into the machine.

So let’s say you have two qubits in a quantum computer. Instead of them both being in separate superpositions of 1 and 0, you can entangle them so they are together in a superposition of all the possible combinations of 1 and 0. That’s four states: 11, 00, 10 and 01. This is the real secret of quantum computing’s potentially massive power. Every extra qubit that you entangle exponentially increases the number of states that your quantum computer can represent. But here’s the bizarre bit. The idea that different particles can be entangled in this way has some really odd consequences.

Sean Carroll
Yeah, again, it’s a fascinating story that was only brought into focus because of the sceptics about quantum mechanics. In this case it was Einstein. Einstein came up with this idea which he called spooky action at a distance, the idea that if I have two particles that are entangled, I can move them very far away from each other. Measure one and I instantly know what the result will be at the other one. Somehow the other particle knows what answer I got. Einstein hated this. He thought that was clearly wrong.

Madhumita Murgia
The problem Einstein had with entanglement was that you could have two particles when knowing the state of one would actually determine the state of the other. So here’s a way of thinking about that. Imagine you had a pair of gloves in separate boxes. If you opened one box and found a left hand glove, you would know that the other was a right hand glove. But imagine, for the sake of argument, that these two gloves were entangled quantum particles, one spinning left and the other spinning right. Quantum mechanics says that until you look at them, they haven’t decided yet which one is left and which one is right. So that means when you open one box and see a left spinning particle, it actually makes the other particle spin right. And this effect would work over great distances, so you could move the entangled particles to opposite sides of the world. And observing a left spinning particle on one side of the world would make the other particle on the other side spin right. Einstein thought this was bizarre and spooky because it basically breaks the laws of physics that are supposed to govern the universe. For one thing, it suggests information can move between two particles faster than the speed of light, which is supposed to be impossible.

So let’s just recap here. We have particles existing in two places or states at the same time, which seems completely counter-intuitive. And now we also have particles that are entangled with other particles in a way that basically enables a form of information teleportation, which sounds impossible. But it turns out that this is how the world at its fundamental level really does work. Superpositions are real, and you can entangle particles together so that they influence each other. And the idea of entangling qubits in superpositions together is the key to the quantum computers being built today.

Quantum mechanics all feels very abstract. But what’s amazing about quantum computing is that you can take those abstract concepts and make them physical. I was reminded of this when a few years ago my husband, who was doing a PhD in quantum computing, came home excitedly holding a photograph in his hand. The picture showed him next to a huge jumble of wires and machines and lasers. Right in the middle of it was a tiny, bright dot — that he explained to me was an ion, a subatomic particle that could form the basis of a quantum computer. Taking particles like that one and turning them into computers is the day job of people like Michelle Simmons.

Michelle Simmons
The key essence of quantum computing is you have essentially two different things that you require. The first one is the superposition, the ability to be somewhere in between the 1s and 0s. But the second one is literally putting atoms in place and bringing them close enough together that we can create that entanglement.

Madhumita Murgia
Michelle is professor of quantum physics at the University of New South Wales in Australia. She’s a pioneer in the field of building electronic devices at a really small scale, and she’s using that expertise to build a quantum computer.

Michelle Simmons
In the quantum world, what you’re trying to do is get down to the smallest. You’re essentially getting rid of everything and just using a single quantum particle. And so really it’s that ability to control nature at that tiny, minuscule, microscopic scale that allows you to access those quantum states.

Madhumita Murgia
To make qubits, Michelle uses the spin of electrons. She takes the electrons in a phosphorus atom encased in a silicon crystal, and she’s able to manipulate the way these electrons spin, putting them into superpositions and entangling them together.

Michelle Simmons
In our case, we literally physically bring the electron wave functions into close proximity so that when you bring them close enough together, the electron wave functions of the atoms overlap. That gives us the entanglement so that essentially you create a new state where if you try and manipulate one of the spins in this entangled state, it automatically affects the other spin.

Madhumita Murgia
For Michelle, there’s nothing particularly mysterious or weird about the idea of an electron being in a superposition state, or of two electrons being entangled in the spooky way that worried people like Einstein. Using a special microscope, she can even see it happen.

Michelle Simmons
We’re using a scanning tunnelling microscope, so it is actually a microscope to image the actual position of the atom, but also the wave function. And the wave function essentially represents the quantum state that we’re trying to capture and control so we can get really absolute precision, accuracy and knowledge of exactly where the atom is in the crystal, what the wave function looks like. And so we’ve got a very deep understanding of our qubit states directly.

Madhumita Murgia
Michelle and her team are at the cutting edge of quantum computing technology today, but manipulating quantum states at the subatomic level is hard. Quantum states are really, really delicate and easily disturbed. Part of the problem is the fact that particles only appear to be in quantum states when you’re not measuring them. As soon as you measure them, the quantum state seems to collapse. This is just like Schrödinger’s cat, which collapses into the state of being dead or the state of being alive only when you open the box. (Sound of box being opened) This may sound outlandish, but it has some real implications. In a quantum computer, that means that your qubits lose their quantum magic as soon as you observe them. And when we say observe, we don’t really mean looking at them in the way that Schrödinger imagines us looking at his cat. Literally any other particle that isn’t part of the computer’s quantum state can measure a qubit simply by bumping into it. Here’s Sean Carroll again.

Sean Carroll
If even a single photon comes in and interacts with our qubits, you’ve lost it. You’ve broken the coherence of the entangled qubits that you started with. That’s the problem. And there’s a lot of photons out there in the universe. Everything around us is glowing because it has a temperature. And that’s why you need to build a quantum computer at a really, really, really low temperature to prevent it from bumping into all the photons that are flying around trying to ”decohere”.

Madhumita Murgia
These are the huge technical challenges that scientists have to contend with. You need to capture and entangle millions of qubits in order to make a quantum computer that can actually do something useful. And you need to stop the environment from accidentally measuring your qubits all the time and collapsing those delicate quantum states. But what really strikes me about quantum computer builders like Michelle is that they don’t spend a lot of time worrying about the weirdness of the quantum world. If you’re building a quantum computer, you don’t need to worry about any of this because quantum mechanics just works.

Michelle Simmons
The mysteries of quantum mechanics are not something that I consider every day. I see it as really just a hardcore engineering challenge. We’re really, as I say, very kind of practical at the level of doing controlled experiments, and most of the time it works out that it makes sense. And we understand it fairly easily. And it’s, you know, it’s more of an engineering challenge rather than, you know, understanding the universe at this stage.

Madhumita Murgia
If you find the features of the quantum world baffling, don’t worry, you’re not alone. Back in the 1960s, the renowned physicist Richard Feynman was one of the first people to come up with the concept of a quantum computer. And he said that no one really understands quantum mechanics. Sean Carroll says that this is still true even among physicists today.

Sean Carroll
The weird thing, the thing that is actually sort of most bizarre from the professional physicist’s point of view is that you don’t need to understand quantum mechanics at a super deep level to use it and to make predictions.

Madhumita Murgia
For decades, Sean says, scientists and engineers have used the equations of quantum mechanics to develop new physics theories and to build quantum computers without worrying too much about what it all tells us about the nature of reality. But the fact that the universe does operate according to the rules of quantum physics is part of what makes quantum computers so exciting. If the world around us is really a quantum world, then quantum computers are the first machines we’ve had that actually work in the same way. And if you want to model the universe starting from the smallest particles, a quantum computer should be able to do it better than anything else. Sean is open-minded about whether quantum computers will bring about a technology revolution, but he thinks this ability to represent the quantum nature of the universe holds the most promise.

Sean Carroll
I think that there’s absolutely a chance that quantum computing is going to be transformative, be like a really amazing kind of technology that will be something very different than what we’ve had before. But we’re not sure. I think that what will, is most likely, sort of the most robust and believable prediction is that quantum computing helps us understand quantum mechanical systems. So chemistry and physics and things like that I think will be enormously helped by quantum computing. Whether or not things like traffic flow or encryption will be helped is a harder problem that I don’t know about yet.

Madhumita Murgia
In the next episode of Tech Tonic: Out of the quantum world and into the real world in all its messy unpredictability.

Matt Schrap
So for a driver, nobody wants to sit around waiting. If you’re a driver, you want to drive. That windshield time behind the wheel is something that you cherish.

Madhumita Murgia
We take a trip to the Port of Los Angeles to see quantum computers at work on the ground.

Alan Baratz
We are not talking about something that will become available in the future. We are talking about technology systems and services that are available for our customers’ use today.

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Madhumita Murgia
I’m Madhumita Murgia. Tech Tonic is presented by me and John Thornhill. Our senior producer is Edwin Lane, and our producer is Josh Gabert-Doyon. Our executive producer is Manuela Saragosa. Sound design by Samantha Giovinco and Breen Turner. Original music by Metaphor Music. The FT’s global head of audio is Cheryl Brumley.