Hi last week I sat down with Professor Andrea Morello from the University of New South Wales is one of the world's leading quantum computing researchers and a fellow electrical engineer. So I thought we'd discuss quantum computing. all aspects of quantum computing. The rabbit hole goes super deep on this one, trust me.

and but do it from an electrical engineering perspective, so that's where we're gonna come from. So I've got a full one hour and 45 minute talk with Andre Oh, it's absolutely fantastic. You've got to go watch it. It's over on my Eevee Discover channel, but because I've only got limited subscribers over there.

I Thought: I'd put around the first star 25 minutes or so plus some bonus teasers at the end as well for what the full interview is like. So if you love this content, please go over to my Eevee Discover channel to watch the full thing because it's absolutely fantastic. So anyway, I'm with the show. if you liked it.

give it a big thumbs up. You know all that sort of stuff. Catch you next time. Come in today.

it's okay. Computing department? Well, Electrical Engineering department? Really? Yes, Yes, Yes, that's the department amount and we're creating. You know, the quantum engineering of the future. so it's all blended together.

Would you explain quantum computing to electrical engineers? All right. So electrical engineers will know that a classical computer that we use every day and that maybe some of your audience has helped developing. The micro electronics engineers in particular are built with the transistors and when they are used for logic, they act essentially as switches that have two states. You know a low voltage state and high voltage States.

So that's your zeros and ones in digital logic. And and you build a processor where you have you know a large interconnected array of nowadays billions of those transistors. And those are the chips that you use today to do classical computations. So information is encoded in the electrical state of a nano scale transistor in silicon.

it's encoded in a in a binary mode, zeros and ones corresponds to lower high voltages, and then you do logic operations by having essentially the state of a transistor, switching or not, depending on the state of another transistor. Okay, a quantum computer is something that retains the binary logic, so it's still based upon zeros and ones. But those zeros and ones are not the high or low voltage state of a transistor, but they are one of the two quantum states of a suitable quantum mechanical object. Yeah, so the simplest example one can give is that of an electron that can jump between two atoms.

So in my particular research, I work with dopant atoms in silicon Again, hopefully an electrical engineer will have done in their second year Electronics some introduction to what a semiconductor devices and how it works. You take a crystal of silicon, introduce dopants which can be phosphorus or arsenic or am I am, but also antimony for other reasons that I can go into if you're curious. but so they're n-type dopants. So normally that dopant will donate, it's a donor.

It will donate an electron to the conduction band of of silicon. And now imagine you set up your electronic device in such a way that you have two dopants close to each other and just one electron. right? And you could say okay, I'm going to encode a bit of information here: I call a zero, the electron on the left and the one the elect on the right. Okay, it's an A system that can have two options.

Another possibility, which is the one that I actually work on, is to use the spin of the electron electron not only as a charge, but also as a spin. The spin is the fundamental microscopic magnetic dipole of elementary particles like electrons, protons, and neutrons. And so if I place this electron in a magnetic field, the spin will have two bases: quantum mechanical States pointing up or pointing down. So I can call spin down zero and spin up the one for example.

So I could make digital logic that way. But an electron is not just like a transistor, it is a genuine quantum object. So again, think of the two atoms and one electron shared between them. That electron doesn't need to be choosing one autumn or the other.

it can be in a quantum superposition of being on both. Which again, when you say that way, people go all crazy. Oh, this counterintuitive will work one two. This is actually completely logical, right? If you have two identical atoms and one electron and the system is completely symmetric, which atom will the electron choose, it's going to choose either both.

The logical natural answer is that it spreads out across both. So I Never let anyone get away with saying the quantum mechanics is counter intuitive. You know it's actually completely logical. You choose both when you have equal equal opportunities and equal choices.

So that means that you can make a quantum bit that is in the zero in one state at the same time. Yeah, now this is a moment. No, no, it's nothing that superposition, sorry, superposition entanglement is the next step. And that's what it gets.

Really interesting Again, for the benefit of our electrical engineering friends. I quite often get the question from electrical engineer and say, okay, so you have this want to bid that can be in an arbitrary superposition of being between zero and one. Isn't that the same as an analog circuit, right? So if I take an Alan analog amplifier that can have a output voltage between zero and five volts, I can have any range of voltages between zero and five volts. So does that mean I've made a quantum computer? No.

And to see why that is, you need to take it to the next step which is the entanglement. So the entanglement is a little bit more complicated. but again it's You have to think of the the naturality of it. Okay, so now let's say that Mmm, let me do the example.

What do you think is best the spin or the charge spin Spin. Okay, let's do the spin more familiar. Good. Fantastic.

Let's do spin which is my baby. Okay, so now let's say you have two of these electrons close to each other, right? So they have a spin that you know in its simple state can be up or down, but it can also be in a superposition. Okay, let's say that this pin is pointing up right. And really, you can take the classical image that you've seen in all your little geography books when you were a kid of the magnetic field produced by the earth that makes these lines of magnetic field like these that come out of the North Pole and wind around and get into the South Pole.

Okay, so if you ever spin pointing up this way, it makes a magnetic field that goes up and then whines back down on the side right? What scale are we talking about? Not only this, it's not ambrosial. Well, I mean the field spreads off to infinity, but it becomes infinitely small as you go away so you know to have a significant effect you need to be nanometers close. Okay, so let's pin point in this way up and then I have another spin here, right? So this pin will be subjected to the magnetic field produced by the first spin. So on the side, the magnetic field is pointing down.

So this pin will prefer to point this way because that's the lower energy, the lower energy state of the two magnetically coupled spin. Alright, so this is the preferred orientation for these two, but what if I turn them this way then it's equally enjoyable as before, right? Because this is now making a film that both happy. So now what is the natural quantum state of these two spins that coupled through this magnetic interaction? Is it this one or is it that one? They don't care. Okay, it's both.

But these are not a little bit more cheeky because now if I ask you in that state where they are at the same time like this and like that, which direction is this pin pointing, it's gonna be always opposite to the other one. Correct. So if you know one, you know the other Yes. Hence why entanglement works.

Yes, Is that correct? Yes, it's correct. But the point is this pin doesn't have a direction of its own anymore. So if you ask me which way is this pin pointing, the correct answer is nowhere nowhere. And if you actually do the calculation, it's really a simple calculation that I teach in third-year to electrical engineers.

You can calculate very simply what is the expected value of the spin orientation and it's zero in every direction. The spin has essentially evaporated, so the number pops er. zero for erection for all directions. right? right? That's why you can't Not.

That's why you have something that a classical system cannot reproduce. Correct, right? So if you now take two analog circuits and you couple them together, you will always have some voltage you can measure at the output of that circuit, whereas here, you can't Right? So once you get to entanglement, that's where you really see the difference between classical. You know continuous variables and quantum quantum systems. Now, this quantum state here, where they are in the up, down, and down up state at the same time constitutes a completely legitimate digital code for a quantum computer.

Okay, so in a quantum computer with two quantum bits, I can encode for different combinations that are completely legitimate. So I can have the down, down, the up, up, the combination of down-up and up-down where they are opposite to each other, and then there's another one where they are parallel to each other, but they point nowhere in the equatorial plane and that's the extra yeah, basic point. Like digital point, Exactly. So these entangled codes and now and now.

If you want to tell me which of the four combinations is that set of two quantum bits taking, you need to give me the coefficient of each one of those four combinations. So to completely describe those two quantum bits, you need to give me four numbers. You need to give me the coefficient of the down, down, the up, up, the this one. and this one.

and that number is a piece of information. Yes, so you need four pieces of information. If I had three, you need eight if you have for you at sixteen. And so you see that the density of information contained in a set of N quantum bits is 2 to the power n versus n in a classical computing.

So this is why you only need say 300 odd bits or something as an exam. Three hundred Qubits We're talking about These is is a qubit 1 cubed is 1 bit. Yeah, right. Is there other words other terms for like 2 bits and 3? No.

But people use the word you did. You did it for a d-dimensional system. so you were asking me before. You normally use phosphorus as the dopant.

We encode the information actually A Yours Also, use Antimony because antimony, from an electrical point of view is equivalent to phosphor. It's on the same column of the periodic table, right? But the nuclear spin of Antimony has a spin 7 1/2 which means it has eight possible orientation of the spin of the nucleus. So that becomes an eighth dimensional quantum system. So you have eight possibilities instead of two.

So that's a qubit with D equal eight dimensions. Equatorial. Is there any other advantage to that apart from information density? Well, for quantum computing, you would I don't know if I if I can call it an advantage, it's a difference. One important aspect of quantum computing is how resilient they are to noise.

So quantum states are very fragile. So if you imagine having meant, let's say we then eight dimensional spin U is the equivalent of having three qubits, right? Because 2 to the 3 is 8. So one atom of one nucleus of Antimony is equivalent to three nuclei of Phosphorus equivalent, but is different because the way they will be subjected to noise will be different. Imagine you have magnetic field noise.

Okay, so there is a fluctuating magnetic field in the environment. If you have three Phosphorus atoms side by side, the magnetic field might be slightly different on each one of them. So you have noise, which may be uncorrelated, whereas in that single nucleus of Antimony, the noise is by definition correlated. You know all the levels see because it's one atom, they all see the same noise.

So this can be a bad things in certain encodings. It can be a good thing in Kony's depending on how you run it. So this is in the subtleties. I Probably don't want to go into, but it's I Wouldn't call it better.

Worse, it's different. Okay, but for this discussion, we'll stick with the let's stick with a few bits, which is a simple thing, right? So can we think of a cubit as a storage register? Would that be an accurate? Is it? Is it a storage element? It is with one cave yet that you cannot clone the information. You cannot make a copy. This is a fundamental theorem of quantum mechanics called the no-cloning theorem.

you can transfer. So for example, I can encode a bit of quantum information on one phosphorus atom here. and if I have another phosphorus atom next to it, I can transfer the information from here to there. But once I've done that, this one is erased.

There is nothing left on this. Got it? So it's non-volatile as long as you don't touch it. Yeah, as long as you don't measure it. Yeah, but you can't duplicate it.

you can't duplicate it, right? You can transfer it, but you lose the original. You only ever have one copy, you can copy, but you lose the original. Yeah. So most people think, ok, a qubit is where we store the information that we're going to process.

Yeah, now Quantum I'll be your Soviet processes on the qubit. Yes, the processing. This is what we need to get into. Ok, before we get into how the processing works, the actual computation.

How does the quantum measurement works? Because you affect the state of it by measuring it? Yes. So this is a hopefully a nice example for our electrical engineering audience. So here is where we use actual transistors for the measurements. So the technology that I use is based upon using the dopant atoms as the qubits, the spin of the atom.

but the readout device is actually a essentially modified MOSFET so small transistor that we fabricate in our cleanroom. It's about 50 by 100 nanometers in size. So it's it's small, is not even as small as the ones you have in the chip in your camera? Probably. But you know that's what we can do now.

That transistor is designed in a way that we can make it very nonlinear in its response. So it's not is not acting like a linear amplifier. it's it's a switch that switches from the change in position of even a single electron in its vicinity. This is actually not as hard as it sounds.

Okay moving one electron in the vicinity of, you know, 50 nanometer sized transistor actually has a significant effect on the bias point of that transistor. It's it's equivalent to moving anything. It's equivalent to applying, you know, some bottom milli volt transistor because you're looking at nanometer distance. It's just an electron charge, but an electron charge At that distance it has, You know matters.

And then this whole system is cooled down to near absolute zero temperatures. So it's you know the system is extremely sensitive. and so this transition transistor can switch from off to on by simply displacing one electron in its vicinity. Okay, and then what we do is something that's called spin to charge conversions.

Essentially, we make the displacement of the electron dependent on the orientation of the spin. So the idea is this and and that probably already answers the question that you may have had for later on. Which is why do you need to go to near absolute zero temperature and why do you need to do the things you do? So if you came to my lab, you will see some giant refrigerator that cools to 0.01 degrees above absolute zero. and you will see a rack of electronics that is full of you know, high frequency, you know, microwave generators and you know very sensitive amplifiers and super quiet voltage sources.

And one of the things are like the most when I explain it to the students is that you can look at this whole rack of instruments and refrigerators and everything is there. In there is the result of ratios of constants of nature, right? It's the Magneton Planck constant and Boltzmann constant. Given those numbers, you can, you can understand why you need that rack of instruments. Why you need that refrigerator.

So it is this: if you take the spin of an electron and you place it in a magnetic field of 1 Tesla 1 Tesla is a fairly strong magnetic field. Ok, so it's so the the Earth's magnetic field in Sidney is about 60 micro Tesla something like that. So so you put a 1 Tesla magnetic field which we do either with a superconducting magnet or with we're using nowadays some small arrays of permanent magnets. If you take a strong neodymium magnet, it's actually 1.3 Tesla So it's such it about right.

And so we make some little arrays and we bold them to that to them call this point in this refrigerator. So an electron spin in a 1 Tesla magnetic field has an energy difference between the spin down and the spin up state that is equivalent to 1.3 Kelvin. That is why you can't have anything about 1.3 Kelvin because you could see the different see the different, won't see the difference. Now Is this a fundamental will? This always be a fundamental limitation of quantum computers? Or is there some? Do you guys have some grand vision to overcome this at like will room temperature So computers The question is a little more subtle than this.

So okay, maybe let me just finish explaining how I use the transistor and then I'll tell you how you can do some other things. Okay, so you have this pin when it's down. it's in the lowest energy state. When it's up, it's 1.3 kelvin above the lowest energy state in energy and in frequency units.

So now you divide the energy by the Planck constant. that corresponds to 28 gigahertz. So if you come to my lab, you will see a 40 gig Earth Microwave generator because that's what we need that's given by the Planck constant. So you have to set the frequency right by the Planck constant.

Given the magnetic field, how tight does that have to be Very tight? Because these bins are extremely coherent. Meaning the resolution we have on what is the frequency at which they respond is about 1 kilo. Hertz This is very, very sharp and that's exactly what we want because the uncertainty on that frequency corresponds to an uncertainty on the quantum state as it evolves in time. It's like a clock, right? So you want to keep track of all the clocks you have in your system and if the clocks start to go slow or fast, then you lost.

You lost the relation between the phase of the clocks. Yeah, so then we have these. We have the spin that can be you know, down or up. If it's up, its 1.3 Kelvin which is 120 microvolts for electrical engineers.

120 microvolts above the lowest energy state and these electron is in the proximity of the transistor. And when the electron is in the high energy state, it has just enough energy to escape the atom and be sunk into the drain of the transistor. Oh no no no, it goes into the drain. The gate is isolated.

The gate is isolated. So think of, you know, second-year electronics transistor. You got a source and a drain. You get a silicon oxide insulator and the gate is on the top.

The gate controls the potential but is electrical insulator and then in the body of the silicon you get the source and the drain. This particular transistor, a little difference called a single electron transistors got a little island of electrons between the source and the drain. That's what makes it so nonlinear. but you know, for the purpose of this discussion, we can kind of forget about it.

Just imagine the electron bound to the atom if it's in the high energy state, can escape into the drain of the transistor and just fly away. So now you have a positive charge in the vicinity of the transistor. That positive charge will shift the bias point of the transistor and make it conduct. And when it conducts, we will give us about a nano amp of current that we can measure with a sensing and Cremation on-ramp We can measure it in real time so you can watch in real time with your eyes the quantum state of a single spin by watching a step in the in fact, a blip in the current through a transistor so you can watch it on your digital.

Yes, essentially it's just a blip on your Scylla scope that digitizes the output of a current amplifier. You flipping one bit conditional on the state of the other, but because the other can be made in a superposition, then the flipping is also in a superposition of happening and not happening. And that's how you create entanglement. Same stuff.

Yeah, they don't need to. That's a good, they don't need to. This is for our engineer friends. You know what there is.

It is a genuine engineering problem. There are tolerances. Like in any engineering design, the tolerance is not zero. It's finite.

It's a quantized nonlinear oscillator. So imagine you make an LC oscillator just an LC circuit, right? It will oscillate at a certain frequency. So if you just take a capacitor and an inductor and you put it here in a breadboard, it's a tank resonant circuit. and at room temperature we will have you know billions of for microwave photons in it.

Now imagine this circuit resonates at ten gigahertz. Ten gigahertz is the equivalent of half a Kelvin in temperature. So if you now cool down this tank circuit to 20 millikelvin how many photons are in there Zero. It's essentially zero.

So it's an L In it's an electrical circuit. It's an LC oscillator in its quantum mechanical ground state. Okay, is that like a wave? No, no. the wave does it in a different So this is the example I gave you is what Google and IBM and some others are using.

New Wave does something different. The wave uses what's called a flux qubit. So it's also superconducting circuit. But you have to imagine it's a loop.

the loop of superconductor. It is important and so if it were sorted then of course you find it. But if it's unsorted and and the reason this algorithm is intellectually important is because it's one of the very few where we know mathematically that there is a quantum advantage. It's amazing If you look at the form of that equation is like the Schrodinger equation of quantum mechanics.

Oh, we're the Planck constant is the degree of arbitrage. Yeah, it pops up. So the quantum uncertainty we have in the Schrodinger equation in the economics model is the arbitrage is the uncertainty in the in the exchange. right? Mind-blowing right? You see.

My my wild dream is to you know have developed the technology to make you know quantum bits in silicon. have it all under control. Have it, you know. Have some deal with some foundry and we get it all manufactured with the latest equipment, the fanciest technology and we make one hundred qubits circuit and you run it and it works.

You make a thousand cubed circuit and you run it and it works. You make a ten thousand cubed circuit scales and then eventually it just stops. That would be amazing. That would be amazing.

They would be like to actually watch in an alley in an engineered electronic device. A new law of physics? You all could. could electronics engineering listen to this my friends. Electronics Engineering enabling the discovery of the new law of physics.

That's what I was. That's what I work for Wow There's a Nobel Prize in that one. I'm still alive you.