What does a quantum computer actually do? - EMF 2026

Thank you. [applause] Thanks so much. Uh thanks for inviting me to talk. It's an honor honor to join. Um I must start by uh noting to maybe to the organizers or whoever did this. Um, my talk is actually talk number 42. Uh, and it's about a futuristic computer. So, I hope that that's a deliberate reference. I thought that was excellent. Um, so what does a quantum meter actually do? A very short disclaimer at the start. The views and opinions expressed in this talk are my own and do not necessarily reflect those views or positions of my employer. So, why am I doing this talk? So, I spent the best part of 15 years working in electronic engineering. uh mostly programming chips and writing uh signal processing stuff and I decided 18 months ago to make a change and I joined a quantum computing startup. Uh and I joined to help build a control system um which is the bit I kind of vaguely understand but the physics side absolute blackbox to me had no idea whatsoever. Uh so I thought what better way to learn than to force myself to commit to doing a talk at EMF about the subject and see what happens. Um, so you can be the judge of how successful that's been. Any physicists in the audience, please forgive me for what I'm about to do. Um, fingers crossed. So, what is a trapped ion quantum computer? So, I'm highlighting the trapped ion bit because there are a few different types. Uh, the most famous types that IBM and Google are building are actually superconducting uh cubit quantum computers which are a little bit different. um they've got a chip and they're the ones with those big spiraly coils um that you've seen in all the photos. Um but there are a few competing technologies. Trapion is another type that I'm going to talk about and um some of the one of the main advantages of that is there's more ideas on how they'll scale up. So when we have our massive computer that's actually going to do a useful job for us um there's good potential in trap iron ones. So I'll start by showing you what what they look like. Uh so these are pictures from a few different companies. Uh so on the outside we have the the vacuum chambers. They almost all of them have well in fact they all have a big vacuum chamber uh that uh is you know a vacuum and um it gets down to very cool temperatures. Um the other common theme is the uh the pictures along the middle which are the chips. So they're they're microchips ultimately that they're made with standard processes generally speaking. Um, and that's where you trap your ions. So, that's the the key bit. And all that paraphernalia around it, all of that stuff is just to move these ions around and get them in the right place. Um, you'll also see in the bottom right, uh, whole bunch of lasers or like lenses and things. Lasers are a common theme as well. They're used all over the place. There's lots of different frequencies used. Uh, and so that is what they look like. But what is going on? What are we doing with all of this stuff? This is what I'm trying to cover today. So, I'll step back just for a little second. A quick reminder, what a computer is supposed to do some maths. Um, on a standard computer, you can store information with just a not or a one. So, you only have two options. Um, on and then on the right hand side, there's a bit of a difference. In a quantum computer, you have a few more degrees of freedom in your bit. you're still storing a n and a one, but you can also hold your your cubit in a whole bunch of states in between. Uh, and it can be visualized by drawing a point on a sphere. Uh, and the the state can exist anywhere on the surface of that sphere. It's just a picture. It's not it's not actually doing that. There's no actual rotating physically going on on the thing itself, but that's how you visualize the information that's stored. and and you can you can say that there's a superp position which is not not that easy to understand as a word but you that just means you're between somewhere between your zero and your one state. Um so that's the theory side. So for a trapped ion quantum computer what actually is the physical medium where are we storing our information and as the name suggests in ions. Um so that's an atom which has been charged. Uh and this picture is um actually a superp position of lots of um captured uh locations of electrons. So this is an electron field. Um so it's quite hard to show a picture of an atom really. They um you can't you can't shine lots of light on it to see what it looks like. It will do something different each time you shine a light. So this is like a probability field. This is if you took thousands of pictures of where an electron was in the field and put them all on top of each other and that's what the atom would look like. Um so what properties of the atom are we using to store our information? Uh atoms have quite a lot of different properties. They have momentum, they have spin, they have uh all sorts of things. Um for trap iron quantum computers generally the thing that we are using to store information is the relative spin of the electron and the nucleus of the atom. What is spin? I will ask let you ask a physicist that it is an innate property of uh um protons and electrons for example. Nothing's physically spinning. Uh it is just a property that they have. Uh and spin is a useful reference because it kind of describes how they behave. Uh but they are absolutely not spinning. Uh I I tried to get this summarized even better than that. Honestly, I I couldn't I think there's I'm reading textbooks, watching YouTube, I'm struggling with spin. But all we need to know because I'm an engineer is what can you do with it and how do you measure it? So that's what I'll be focusing on. Um, so we have these uh spins uh relative spins. When they're opposite each other, we're in state zero. And when they're aligned with each other, we're in state one. But we can be anywhere in between those two states on the surface of this sphere. How do you actually change the state? You shine lasers or microwaves at your atom. Uh, and everything that we're doing manipulating it will be pretty much those two things. Uh so we've decided we want an atom but which which element should we use? Um there are three main elements that used durbium, calcium and barerium. Turbium's bottom right down there. Um they they are metals. Um they're chosen because they happen to have useful properties for trapping and um changing state. And we can use lasers that we can actually build to change the state because the wavelength of the laser matches nicely to the properties of the atom. Uh so I'm going to be focusing on itium and that's a picture of what the metal looks like in the bottom right there. Um so we've got this idea of storing an atom uh and we want to do things with it. The first thing we have to do is get rid of all the other atoms. Uh because they'll just plow into it and knock it out of the way, change its state. We want to have as little other stuff going on. So, you need a vacuum. You need a really good vacuum. You need about the same pressure that you have uh on the surface of the moon. Um and actually to get there is an is a real pain. So, you build a chamber. They've got to have really thick walls. Um you've got to bake it in an oven for like several days while you're pulling the air out. Uh because the metals in this chamber have absorbed loads of air and hydrogen and they'll just gas that off during your experiment and completely ruin it if you don't bake it for a long time. So, a right pain. So whenever we make a mistake uh we have to like degass it, do everything again and then got a few more days before the next time we can get inside the chamber. So very irritating. The other property we need is for it to be really cold uh because then the atoms are vibrating less and that means you've got better um precision, less error in your measurement. So we've got this idea, we've got a chamber um we want to get atoms into it and we want to do something with them. Um, so there's a a component which is there's a picture of in the top there called an oven. Uh, it it literally has a a small bit of the metal in there. Um, and then a coil of wire around it to heat up the metal. Uh, and it shoots uh individual atoms um, a few at a time out the front of the uh, oven. Uh, and so they're uncharged at this point. Uh, so we need to knock an electron off to make it positively charged. Um, so we do that with a specific laser on the top there. Uh, and we also need to call it down. It's going way too fast at this point. Uh, so we have a different laser, different frequency that we're far at it to slow it down to a point where we can catch it. So hopefully this animates all right. So I talked about the fact that we have this chip which lives inside the the vacuum chamber. Uh so on the surface of this chip there's a whole bunch of electrodes that we can pass currents through to to manipulate the atom. Um the key uh key concept is these these rails that run along the middle. So the the gold color and the yellow colored rails um have high frequency high power current running through and it's like a electric field. So it runs along like a cylindrical way and it expands and contracts like this so fast that the atom doesn't have time to escape or go back down. So it kind of gets held in a well of moving electric field. So it will only move backwards and forwards in a linear fashion. So with this radiating field, it will move back and forward like this. The colored electrodes in your picture are to hold it in place in that straight line. Um, so where you can see a blue color, we've got a low voltage, negative voltage that holds it in. And where you can see a red color, we've got a positive voltage. And on the bottom there, that's a picture of all the different voltages we need to send in. So you can imagine how many cables we need to get into this vacuum chamber to uh manipulate a large number of ions. This is one of the big problems that you got to deal with when we scale up quantum computers. How do we get thousands and thousands of individual electrodes uh moving? At the moment, we've only built small ones. So, we're we're in the process of tackling that. Uh so, that's one ion, but we can't do much with just one. So, you can chain them together. And quite a useful property is we've charged it now, so they repel each other. So, you can actually queue up a long chain of them or move them around. And it's quite easy to separate them by manipulating these electric fields. Um, and we'll be using this property later. That's actually the key of how we entangle keep these um, bits talking to each other is this shared motion that they have when they're trapped together. So, we talked about the fact that we have a zero state and a one state uh, and many states in between. When we start our experiment, we want to know what the state is of our our ion. Um, so we want to get it down to the zero state. Um, we could do this by, again, it's a laser, fire a laser at it. Um, but in this point, we're trying to get it actually into state one as aggressively as possible. Um, and then it will when you stop shining the light on it, it will drop down to state zero. If you shine, if you point to one long enough and uh then it will eventually drop down to one, down to zero, and then all of them will end up in this state because state zero can't see this laser. So at this point, we've got all of our atoms in a good state. Going to talk about gates now. So if you remember that picture, that block sphere of all of the different states that we can have our ion in, uh we can manipulate uh the position of the vector on that ion in different dimensions. So it's kind it's a 3D sphere. So we can move in the x direction um which is actually up and down to between the one and the zero state. We can move in the y direction which is 90° from that. And then uh there's I've got that wrong. The z axis is between the zero and one. Sorry. Um so those are our three individual uh ion state changes. And that's all done with laser pulses. So we got a whole bunch of uh lenses and lasers that big racks of lasers that are all fiberally fed into chains of mirrors. um and physically getting it in there is through a bit of glass through a window. Um [snorts] so that's how we manipulate the individual ion states. The final gate is called an entanglement gate. Um it's a mulmer sorenson gate. And this is the interesting one because you can't do much if you only have um you can individually manipulate the bits. Then you basically have a normal computer that's not particularly advantageous. In fact, we've made a very expensive, extremely difficult to look after computer that's much worse than than your phone. Um, so this entanglement is the key part. Um, again, it is done with lasers. We get our ions in the same trap and we shine uh laser frequencies either side of the normal frequency that we do for a gate change. Um, this if you do it for the right amount of time, this entangles our bits together and then we get this sense of this quantum scaling. So when you put two cubits together, you go from having two possible states to four possible states. Then with three cubits, that's squared doubled again and that keeps doubling. So when people tell you you can start stringing together all of these ions and do one big calculation that this is what you need to do. You need to be entangling your gates and then any operation you do will apply to all of your cubits. Um you can't measure what's going on though. Once you've set it in your zero state, you need to do all of your operations and never look at the state of them again. Soon as you do, they'll collapse to a one or a zero again. So, you need to do a whole chain of operations without inspecting them, which does present some challenges. Uh how do we do the actual measurement at the end that I just talked about? Um you it's a laser, surprise surprise. um you shine it on the uh the ion is a specific frequency to illuminate it if f say it's one or you won't see any reflections back if or you won't see any emissions if f is zero. Uh so this is called a photo multiplier tube but there's a few other approaches. Um you need uh it's quite dim. It's only an atom in size. So you do need a good ability to capture uh those photons that come out. So, we have all the ingredients. That's actually all you need. Those gates I talked about let you do any quantum computing operation, which is pretty cool. Um, so I'll go through them again quickly. We have a a vacuum, keep our atoms out. Uh, it's very cold, so our atoms don't vibrate too much. We've got our aturbium atoms from our oven. They repel each other. We can trap them and we can move them around with electric fields. Uh, we've set all our ions to the zero state, so we're ready to go. We've got our ability to change the state, and we've got our ability to antangle our ions. So, let's do something. Uh, as I said, you you can do any operation with a combination of these gates. So, we need to pick something to calculate now. Uh so the really interesting exciting applications for quantum computing that are coming they're not quite there yet are things like drug discovery and material science basically simulating systems that are already quantum in nature. um I think are the what will be the most interesting applications. At the moment the computers we have are not quite powerful enough to do that. Um and actually one of the most famous algorithms is more about RSA code breaking. Um and there's an algorithm called Shaw's algorithm. We can't do really big RSA encryption keys yet. We can only do very small ones. Um but we can look at the maths and see what happens. I'm not going to go too much into shores because I know there's a talk tomorrow on the subject. Uh so please do go and see that if you have time in your diaries. So the two-minute version of what we're doing with this RSA key decryption. So RSA creates a public and a private key. The public shared, the private isn't. Uh the public key is made of a very large prime number that's very hard to find factors for. So that non-reversibility is what makes it um useful. until quantum computers exist or good enough ones. Um so the private key is made from these prime factors. Uh we have the public key and we want to find the prime factors that form our private key. And the public key we're going to look at today is 15. So it's quite a small one admittedly. That is four bits in size. Uh RSA that was used on the internet now is more like 48 bits. So we're 444 to go. Um but if I tried to show you operation by operation that process I don't think uh we would be here we would be here quite a long time. Um so lucky [clears throat] for me there are already um quantum compilers that have been written. This one is called kisskit. Uh, and it breaks down um a mass problem into a sequence of uh laser pulses and made of the four gates that we talked about the RX, R Y, RZ and the entangling gate. Uh so we get to enjoy watching this calculation go on. Uh so there are eight ions uh in this uh calculation. So that's literally the number of atoms that you'd need in reality. the error level on existing computers might be too high. So you might have to have some extra ones as error checking. And that error correction is another big topic of study at the moment. Um it's coming along. But um we're nearly there, I think. [panting] [gasps] Um so this is just factorizing 15. I mean this is something that you could all do in your heads, I think. And we finally got to our measurement case. So we're measuring 1 2 3 four bits at the end. And all we will get from that is a zero or one. Um so this is actually at the end of a FIA transform. So that each bit um each bit represents one two four and eight. Um so you can get your number out of that. Uh so yes uh and I actually have a different visualization which we'll see if I can bring up. Hopefully that works. Yes. Um so this is a bit closer to what you'd actually see on inside the trap. So, we've got our eight ions there, and you'd be firing your different lasers at them. Uh, so hopefully it gives you a different different idea of what it might look like. Um, I think we'll be here a while, so let's just speed that up a little bit. [panting] Uh, you can see that one of the things is in there's the entanglement gate, and you can see all of the ions vibrating together. What's cool about it is you can you only need to fire lasers on the ones that you want to entangle, but all of them talk to each other. and they're like sharing information through a through their motion. Um, which is quite a cool property. We'll move on from there. I can get my mouse back to the right place. Brilliant. Um, so I talked about the fact that you only get a one and a zero back. So if you have a really complex problem, uh, where the answer is more than just a zero or one, uh, you need to do it several times. And also it it's probabilistic. This is a quantum computer. You could just get a random zero. That would be absolutely within the realms of possibility. Um because all you're storing on your block sphere is probability. So ultimately we're going to have to run each each test thousands of times. In this case for Shaw's algorithm, you'll have to trust me that the right answer is R is four. Again, hopefully tomorrow that will become clear. Um but you would do it 2,000 times and you might get 500 of each of these results. 0 1 0 0 1 1 0 0. uh we can pick out the right answer from that. [snorts] So we've got our very small calculation here. Even though it looked like a lot of operations, just compare it to your your your computer to run to draw a picture of a cat. I can guarantee it would be thousands thousands of times more operations you'd actually have to do. So computers are fast. Trap time ones are slower, but they're still reasonably fast. We can fire these irons around. Um so you you've still got a reasonable chance of doing a complex thing. Don't be scared off by how long it took to do. Um, but how do we get to the useful ones? Um, there exist today um a few trap iron quantum computer companies that have gone up to about 100 cubits. That's the the most the most that people have managed. We need something like 100,000 to do reasonably useful stuff and more like 10 million for really really useful. Uh and so scaling this up is what all of these these companies are focusing on at the moment. Um uh but they're really interesting problems. Um that that chain of um ions that I talked about, you can't keep adding to that chain. The errors become too big. So you need to do different things. You need to start moving your ions around. Um and there's a few few ideas for that. The other challenge is the number of lasers that you'd need if you if you want to have thousands of cubits on the surface of your chip. Are you going to be able to pipe all these lasers around efficiently and reliably? Quite a challenge. So other ideas are to use wave guides inside the chip to get them exactly where you want. [snorts] Uh other scaling thoughts are there's only so big that you can make a microchip. Um so you'd be limited how many ions you can physically fit on that surface. So you can start tiling them up. Uh and then the hard bit is the transition between the tiles. Um all of these things are being worked on. So, I hope that was helpful. That's just quick recap. Quantum is pretty weird. Uh, I did not tackle some of the harder stuff like explaining what spin is, but we can use their properties without understanding them. Luckily for me, um, you can break it down into reasonable set of steps and that's what we've been trying to do the last few months and I think it's been a really helpful exercise for me, hopefully for you as well. And I'm looking forward to all the engineering challenges I will have at work in the future. Thank you so much for listening. [applause] [applause]