Okay, if anyone can, uh, decode what's going on here. Um, Andrea will offer you a job or a position in touch. We'll We'll talk about job opportunities. If you can actually decipher what's on there, let's do it.

Hi! I'm here with Andrea Melo who you've no doubt seen a previous video of about Quantum computers. We've done a one and a half hour talking Compu It was very comfortable back then on. the Red Velvet Lounge in my house time is going to be we have to move a little bit more, but hopefully it's going to be even more exciting because you can see it's actually very lucky. You can see both a Uh Quantum computer experiment completely open and accessible and a quantum computer experiment all closed up cold and vacuum and so on but functioning so with functioning Quantum bits right there ready to look at.

Awesome! And where are we? today? we are in the fundamental Quantum Technologies Laboratory at the University of New South Wales in Sydney Australia Australia for the win. All right, and you're the head Professor Here you're in charge of the group. Yes, yes, I'm the Professor of Quantum Engineering in the School of Electrical Engineering and Telecommunications at Unsw and I'm also one of the key people who have set up the world's first undergraduate degree in Quantum Engineering. Fantastic actually should.

teaching this stuff as far as I know. Well, there's another one in for Germany yes, Y and things are popping up in the US But you know there's there's a growing consensus that this thing needs to be addressed because there's a lot of jobs in Quantum technology that need to be filled up by qualified people. And such. Qualified people do not grow on trees.

Got it? So someone like us need to do something about it. So that's what. Usually it's part of another course. It's usually it's not a dedicated course, it's part of an E degree, some branch of Yeah.

So in the old days, people were able to take Quantum engineering courses as elective within an E degree or they would come through a Physics degree and they would do quantum physics through the Physics degree. And then lots of people did physics and E as a dual degree. so those people were quite well sorted. And um, another really killer degree is for example: Quantum Engineering and Computer Science.

That, as you'll see in a moment where the money's at, that's where Mone where think the money's going to be at? Yeah, yeah, yeah, no that that's really killer. Killer app. Yeah, got it. So you're going to show us a real quantum computer or is this mainly a research rig? What do you? Well, this is definitely a research rig.

Uh, look, how can I say this politely? Okay, let let me let me just not go out. of. There is no truly useful quantum computer in the world. The most advanced, the most useful Quantum Comp computers are also still used for basic research for Hardware development and also though to help people learn how to program quantum computers.

Probably if you ask me today, what is the most useful application of a quantum computer is for people to learn how to use them and program them. So that's kind of where we're at in anticipation that the hardware will eventually exact become practical. Exactly what we do here is we develop from the ground up Cradle to grave I Like to say from the design, the theory, the nanofabrication, the measurement, the analysis of the data, Everything from beginning to end. Uh silicon based quantum computer chips.

So what we have in these Uh devices, this one is actually about to be removed. This is a, let me call it a two Cubit Quantum computer. It's a single Phosphoros atom and the two cubits are one atom of, well, one nucleus of the phosphorus atom and the electron that is bound to that Phosphoros at that's a two Cubit system. In this other setup, we have an atom of Antimony.

Antimony is a heavier, bigger atom. It's two rows down from phosphorus in the periodic table. so it has a bigger nucleus and the nucleus has eight. Quantum Mechanical States eight is 2 to the^ three.

so that nucleus alone is the equivalent of three cubits. Nice. And then there's the electron. Makes it four.

So technically, that's a four Cubit System Y We've done that. We've covered all this in the previous video. So y here to look at the hardware. All right.

let's look at the hardware. So if you type quantum computer in a web search, what you will find is something like this. This is not, of course a quantum computer itself. This is the refrigerator that cools down the quantum computer chip.

The quantum computer chip is actually down there. So what you see here is a is a copper box which is there to protect Uh to Shield electromagnetic radiation. both for actual, you know, electromagnetic noise, uh, reduction, but also to prevent any high frequency photons which are carriers of energy from hitting the chip and heating it up right? So the rig itself is called a dilution refrigerator. It's a refrigerator that uses, in fact, it uses Quantum quantum physics to reach 0 0 1 above absolute zero.

Wow, that's right. So maybe we want to take a step back and see how the whole thing sticks together. In the end, this will be a a vacuum enclosure. You can see the the remaining vacuum cans there at the back right have been just removed and you can also see those uh silverplated Shields Right there are the white painted.

those are the external vacuum cans and then the silverplated shields. Those are radiation. Shields Fields So those are to prevent the warm surfaces on the outside of the refrigerator from radiating hot black body radiation into the inside, which would provide a heat load and make it difficult to cool down the inside to the temperature we want it to go down to. So the system works in let's say two stages.

First there is what's called a pulse tube cooler which you can probably hear. this machine is off, but the other one is on. You hear this. that's basically a cycle of compression and expansion of helium gas which gives a cooling power that allows the plate that you can see if you look from here.

So this plate here can go to about 4 Kelvin So minus 269 De Celsius or 4 above absolute zero. So that is cooled down by. let me call it crudely speaking the you know know 10 kilow version of the refrigerator you have at home, right? Your fridge at home to keep the food cold. also has a little compressor and a gas and you hear it going.

It turns on periodically. That's right, and this is like the giant version of that using helium gas. The reason it uses helium gas is because helium is the only substance that never solidifies. Even at Absolute Zero temperature, it remains.

so it goes liquid at 4.2 Dees Kelvin. But it then, uh, it never solidifies unless you put 30 atmospheres of pressure on that. There's a holster I could keep you for hours on that. but let's not.

let's not get distracted. How do you physically verify it's at that temperature? Or is it based on the fundamental physics? You know it's at that temperature. We have thermometers all the way through and funnily enough, those thermometers are just chip resistors. So a lot of the little surface amount resistors that you use in normal Electronics Um, they are actually optimized to have a very flat temperature coefficient around room temperature, right? You build an electronic circuit and you want it to be you know, temperature stable in case your circuit changes temperature a little bit.

And so these, um, these resistors are made out of some special oxides and they have a temperature coefficient that changes signs. so the the resistance goes more or less flat around room temperature, then increases again if you heat them up again and then increases again if you cool them down. so it's like a it's like a parabola. So if you cool them down to you know very low temperatures you can take you know I Just know from memory a 1 and a half kilm resistor will go about 20 kilm when you are at 0.01 degrees above absolute Z.

So there are some absolute calibration that are done with various melting points. Yeah, yeah, there traceability. There's ways to verify that you calibrate this simple chip thermometers against these other reference thermometers and then you place them at strategic points along the fridge. and then you have just a resistance bridge that measures the resistance.

Can we physically see those in here? Yes, they are. Um, here. so they're actually held. These are the heaters actually the thermometers.

See, They're basically they're just attached to some little copper P little chip resistors cased into a copper thing and then attach. And then there are these little looms. You see those ribbons. They're basically Twisted pairs.

Put it in a loom and then they go all the way up and you connect to this instrument. Let me I'll show you the one on this rack which is the same. So this thing here is a resistance Bridge is an AC resistance. Bridge So with this thing, you can measure the resistance of those little chip thermometers with a power dissipation of order.

PCO Wats right, That's important because the cooling power of this refrigerator at the lowest temperature is about 10 microwatts. So you need to make sure that anything else you attach there doesn't start pumping power. Otherwise, it hits up course. Yes! And so the resistance measurement itself has to be very, very low power in order to in order to be uh, set up.

What is that Wrap around that post there? It's like that's a that's thermalization. a thermal Equalization braid. Or exactly So the the braid is is the loom. so those are the signal wires for the thermometers.

Oh okay, but the thing is because they come from room temperature and you know they are conductive wires. So by the Vidman France law, if you have an electrical conductivity, you also have a thermal conductivity right? so you want to make sure. And these things are not copper wires. These things are some, uh, they're usually um, um, Manganin or Constantan.

There are some lousily lousily conductive thing. low thermal cond lmal that's right. And so at every stage, you want to hook them up to a copper post and wrap them around and soak them in. Uh, something called G varnish, which is a special kind of it's insulating varnish, but it has some decent thermal conductivity and it's like a goo that used like like a glue.

and it thermally anchors the wires at the various temperature stages to make sure there isn't too much heat coming down. and the same is done. as you can kind of see uh with the coal cables. So these ones are semi- rigid coaxial cables that are used for the high frequency lines.

Yes, and you see, they all get broken down at these posts here. and so there. The body of the cable is attached to a to a bulk bulkhead and that bulkhead is, you know, goldplated and thermally anchored to the plate. so the cable gets thermalized there.

The problem with COA cable is that the inner conductor is actually not very well thermally anchored, right? So that's why you always give them Loops Because by giving Loops um, the Teflon dialectric that's in that's inside will pull when you cool it down. You know, if you have ever tried putting some Teflon Cable in a nitrogen vessel will shrink a lot. so this thing actually pulls and that that pressure helps a little bit. Um, with thermalizing the cable and then uh, let me see if I have it here somewhere.

where are there should be an attenuator? Uh, yes, probably some right Here Let me see where I can see attenuator isor an RF attenuator. That's right, right. The role of the RF attenuator is to actually thermalize the inner conductor so it cuts off some of the high frequency radiation and it provides a galvanically conductive path between the inner conductor and the ground. which Again by the same principle is also a thermal link so that the interc conductor gets cooled down a little bit and then the they see.

The thing you see here is something we built ourselves, designed and built. It's a filter box. So this is a box that contains RC filters and LC filters and again serves the job of thermalizing and and filtering away all the high frequency radiation by the plank Einstein relation that the energy is the plan constants times the frequency of a photon. A high frequency signal carries photons with a lot of energy, so you want those photons to not be there unless you really need them.

So everything about doing low temperature experiments is a game of. We do need high frequency signals for certain purposes, but where we don't need them, we have to filter them out completely. Got it. Are there any vibrational problems like Tribo electric effects from your Te from your Teflon Co Oh yes.

I'm glad you asked. I'm glad you asked. We Are The World experts on this? Oh, really? yes we are. So Um, you see these black cables here? Yes, so those are um, graphite coated.

They're black because they're graphite coated these ones. y They're basically flexible coaxial cables. They have um, copper nickel inner conductor, a Teflon dialectric, a coating of graphite, and then a braided Cooper Niel outer conductor, and then very importantly, another Teflon gasket on the outside. So what happens is this if you once you cool down your coaxial cable, the Teflon shrinks right and so take a semi- rigid cable like this.

Once the Teflon shrink, the part that's inside the rigid outer conductor can just move around as a result of the vibration caused by that cryocooler. That right? Yeah, Yeah! And so we actually analyzed this and we found out that this is like a noise source with an internal impedance of 100 kilm. So on the high frequency lines which are 50 ohm in p as matched on all sides, that doesn't matter, right? The voltage resist. The voltage.

division between 50 ohm and 100 Kil means that the signal you get here is basically nothing but on the lines that we use to measure the transistor, which is what reads out the quantum bit. The transistor itself is switching between 100 Kil and open circuit and on the other side there is a trans impedance amplifier which some people incorrectly call a current amplifier. but you know what I mean Yes, we don't. Yeah, we know that has a very small input impedance so that voltage source with 100 kiloohm input res sorry internal resistance will put will create a current that goes straight into the input of the transimpedance amplifier.

so we'll see all of that. So these cables have been designed to kill that problem and they do so by having one, the graphite which is a solid lubricant and two the outer Teflon gasket is like a corset. It also shrinks and it keeps it all in. So we actually we have a whole very detailed paper from a few years ago where we when we first started using these systems.

So these kind of cryogen free dilution refrigerators. They're relatively new in the old days. people used actual liquid helium like a bath of liquid helium to arrive at full Kelvin and that means there's no vibrations there on once it's started using these things, that's when a whole hell broke loose right? how we deal with this. And so that's why.

actually we also bought this specific Um vibration isolation setup. This is quite extreme. so you see there it is. Yeah, this is a very solid frame on which rests a very thick aluminum plate on air suspensions.

This is this is what holds the actual refrigerator, then all the components of the pulse tube cooler, the ones that make the noise and vibration are on a separate frame, right? The two things are decoupled, mechanically decoupled, and then all the pumping lines. There is a pump that circulates helium gas. They are held on a separate frame that has a 250 kg concrete block and the lines are cast into the concrete block. No, that's the effort you had to go.

That's the whole thing thing. Wow. Funnny enough. You can't buy this thing anymore.

The company doesn't want to sell it because it's an absolute nightmare to put it together. You have these, you know, half a ton frames. You have to line them up to a millimeter right? So the installation Engineers were going absolutely crazy So we are very lucky we managed to get some of those uh models while they were still willing to make them. so you not moving Labs anytime soon.

Not this ones. At least don't move. not this ones. Yeah.

So we're running a experiment in there at the moment and you said that. You've got a like I'm here on the lucky day. Yes, where what experiment are you running you doing groundbreaking research at the moment? Yes, we are. It's It's going to be a little bit complicated to show you the actual ground break it side of things because it's really quite.

Um, it's quite Advanced quantum mechanics. But my student Steve whom I would like to invite on camera here. he has set up a couple of little scripts to show you how we do the basic um operations on these Quantum bits. So first of all, um I'd like to dispel this myth right? People have this idea sometimes of the scientist as you know, the guy with a white lap coat and the thick glasses there and some dark bunker you know handling.

you know little knobs and stuff because there nothing like that right Nowadays doing an experiment means writing. you know scripts Scripts that control a bunch of instruments Kitty you're just a scrip Kitty Well look, you know there is a there is a there is a part of the job that consists of, you know, setting up that whole thing, connectorized, making the device, testing it, connecting all the instrument you know, making sure everything is calibrated. But once it's all up and running, it's basically run from behind the python script. How often do you tweak the set like the physical setup for? like, not often.

So when we find the perfect chip, it may stay in a fridge for a year, right? There is so much research. you can do research and new science to be done with one prototype chip that it often stays called for a year. Some have been called for four years. Four years.

Yeah, wow yeah, wow. whereas sometimes that fridge, for example, is the one we use for more of a fast turnaround. So we'll try something for a couple of weeks and then we'll swap chip. but it just depends.

Can you cycle them? What happens if you thermally cycle them? Does that cause issues? Can you turn them off? or do you have to leave them? They tend to come back. So if you do a thermal cycle, Um, what happens is this: These devices are single charged devices. Okay, we are H We are addressing single electrons and their spins. So the presence or absence of a single electron charge within let's say 50 nanometers of the charge that we want to look at will slightly change.

its Behavior So it just takes one electron to be stuck at a dialectric surface in the vicinity to slightly shift how this device behaves. But in the end, when we go and look at the spin Cubit the actual object that carries the quantum information, that one is pretty solid. So the thermal cycle and other electrical you know effect X will shift a little bit where we have to go to observe that single electron. but once we have it, it tends to behave the same way because an electron is an electron within.

You know there a subtleties. but yeah so maybe I Just talk you through this one. This is a map that was taken just this morning. So what you see here is in color scale it says DC voltage.

That's the DC voltage that comes are of the amplifier chain. but you can think of this as being the current through the transistor. right? So bright yellow or green means High current by High I mean half a nanoamp. Yep, blue means zero de because your sensor is basically a one mosfet which you guys have designed modified manufactured.

That's right, it's manufactured designed by us. a high electron Mobility transistor Is that actually? No, the electrom mobility is not that great. Okay, yeah, it's is not is not a particular High Electrom Mobility What makes it special is that instead of being a continuous mosfet Channel there are two barriers across the channel. That Isol isolate a puddle of about 100 electrons and then the conduction instead of being a continuous conduction path between source and drain of the mosfet has to go by.

Quantum Tunneling through those barrier. So any electron that wants to flow through the transistor needs to arrive at the barrier Quantum mechanically tunnel through the barrier into the Little P of 100 electron and then tunnel out. But that's based on probability. Yes, but that probability can be controlled Ah That's a secret Source Yes, that's a secret source so that probability can be killed off completely such that there is no current.

That's the dark blue you see here for most situations. And then as you scan the gate voltage on the transistor, you will find a situation where basically for an electron to be on that island or not be on that island, there is no energetic difference. So the electron can be there or not be there. It makes no difference.

And so that is the only place where you get current because you can get an electron that goes onto the island and then once it's there, it can escape again one at a time. and then a L electron goes onto the island, stays for a bit and escapes again. And by tuning the transparency of those tunnel barriers, we can tune the current we get through so we can tune the probability to give us the current that we want. So this is one of the simple constants of nature.

You know that the electron charge is 1.6 10us 19 Kum What that means is that you have 6.2 24 * 108 electrons per second in one amp. Yes, right now no one remembers that. But if you're people like us, you do remember that because it means you get 6.24 million electrons per second in a Picco amp, right? So a pamp of current mean there is 6.24 million electrons per second going through. So if you want to get a Pico amp of car, we actually want more than that.

But just to keep the number simple, if you want a P amp of car and you need to tune the Pro: the quantum mechanical tunneling probability for an electron to come on and off that Island such that there are 6.24 million electron on average per second that make it through. Got it right? So what you're seeing here If you just scan this gate along this axis, you see these peaks of current, right? So every time you move from here to there, so let's say you go up here, you're going more positive, so more positive voltage makes it more desirable for an electron which is a negative charge to be on that paddle of electrons. So every between every Peak you've added an extra electron to the island. One by one, you can actually count them.

Why are they not Uh SP Consistent, Equally spaced. Yeah, because that island is not. uh, very large. So if you made a larger Island they will be perfectly Equis Space: This island is is actually small enough that you get a combination of a classical coolum charging energy.

So to to put an electron a single electron on a capacitor of capacitance C you need to pay an energy e squ over 2C Right now, this thing is A it is a capacitor, right? It's a little capacitor plate that everything has capacitance, but in addition, it's also a Quantum confinement potential. So you're getting a bit of a mix of classical Kum charging energy with quantum mechanical level spacing, so that's why they're not quite completely uniform. There's a lot to unpack in there. but anyway, the key point I Want to draw your attention to is this: look for example.

at this point here, this line you see here, we've got all these: Peaks They all follow each other not quite regularly for the reason we Now understand. But then if you move left here, these are two different Gates Like there's more than one gate around the transistor, there's a few of them. It breaks. The whole pattern breaks and shifts right.

What's happening there? What's happening There is that. as you go left again, the voltage goes positive this way. So as you go more negative on this voltage, you are making it less desirable for electrons to be there there. And at some point at that point being exactly here, there is one electron somewhere that just pops out of whever it is and doesn't come back.

So the whole pattern shifts. it goes into the drain, goes into the drain of the transistor. It just okay, vanishes up. The point is that electron belongs to a single antimony atom that we have implanted into the Silicon chip.

So we've put in there an antimon atom. That antimon atom is a group five donor. It's it's an Nend type donor. Right when you go to low enough temperature, instead of donating an electron to the conduction band, it holds the electron there.

It behaves like a hydrogen atom, so the nucleus is like a positive charge and the electron is the negative charge that's bound to it. But here, it's like having an hydrogen atom in in the middle of some electrodes that allow you to rip the electron off from that at. So here on the right hand side of this brake, there is an electron bound to the atom. As you pass to the left hand side of the brake, the electron gets popped out and just vanishes out into the drain.

So you dope it with one atom. Yes, one one there say is only one atom in the whole well. In this particular device, we have about 20 atoms and you can see there's more than one of these braks, right? So there's one here. There's one here, then one here.

there's there's a few of them. We actually do have the technology to put one and only one atom. We are right now in the process of making cubic devices with deterministic single ion implantation, so that is desirable. To have a single, it is desirable to scale up.

So for the purpose of early experiments, it's actually quite nice to have more than one because you can kind of find the one that's in the right spot. you know, like for early experiments, it's actually convenient long term. If you want to actually build a deterministic large scale quantum computer, you want to have one in every spot and nothing else WR nothing else. y because it gives you more freed Cubit block one in every cell of the array.

So think of the quantum computer as an array of physical cubits Like you know, classical computer chip is an array of transistors that act as you know, 01 switches So there you will want to have one at every side. But for this early sort of pro prototype devices, it's actually quite convenient to have more than one. but there's not a lot of them. There's maybe 20 of them in total.

This is where the magic happens Exactly at this corner here, right? Okay, so what happens there is that if you were in zero magnetic field, if you sat right there at that corner, that electron would have the same preference to be on the atom and be off the atom. So you will get this situation that it's actually a random Telegraph signal which many electrical engineers are familiar with. You would have an electron that jumps on and off. Electon actually comes on and off that island of electrons and then from there it may go away into the drain, but it first go onto the other so it just goes back and forth.

It physically goes back and forth physically goes back and for so not the probability we're not. Well, when you're talking probability, you can actually see it on the screen. We'll see it in a moment. Um, you can see when it does So all right by the Swit switching of the current in the transistor, right? So when the electron switches on and off, you're basically switching from being in the dark blue.

Let's go up here. It's a little brighter from the dark blue to the green. Right? right? We mean zero current to finite current. Now, when does that happen? That's the randomness and that's why it's called random Telegraph signal.

It's not a square wave, right? It's got some Randomness in the interval between the switches, so it's almost randomly pulse withth Yeah, Yeah, yeah, but there is. There is a time scale, right? So you have slow random Telegraph signal. Fast random Telegraph signal. So you can actually show Steve you want to show Yeah, Uh, I And this is true random.

That's true Random random. No, No, that's true random. That's real random. Yeah, so what we can do is okay.

we just go to this point and zoom in this point and show a live scan. Okay, yeah, yeah, we can show a live scan right there. Yeah, so zoom into that area. Okay So we're just running a Python script at the moment.

Yeah, yeah. so as you what you can see here, Okay, it's this line. Okay and then okay, you can see the blips. Okay, you have the spikes and this is what Andrea just mentioned.

You have the T event of the electron onto the donor of the door, right? and this is pretty random where they happen right? Also, the duration. some are a bit longer, some are really sharp. Yeah, notice that. Yeah, so that's a 2d Live Scan This is done reasonably fast.

just two fast sort of so tooth waves. Yeah, but you're doing a research experiment. Yes, so this is. Let me put it this way, the way we Um address and encode Quantum Information in these spins is by magnetic resonance.

Okay, now doing magnetic resonance on one spin is actually not harder than doing it on a quadrillion spins. like what happens to you when you go and take an MR scan. Yeah, right? right? So that technique I'm not saying it's easy, especially when you do it in a refrigerator like this is. You know, there's all sorts of complications, but but the real hard part is to read out a single spin right once you have read out.

Once you have a physical access to the to the physical property of that Cubit the rest. I'm not going to say it's easy, but it it's kind of. You know, the flood gates open right? So this was this is what we got done in 2010. It was the first spin read out in Silicon done right here.

Um, that really opened the flood gates for everything we were able to do after that, right? Got and you're working on Silicon Because you think that's going to be the most practical in the future? That's one of the reasons the other reason is that Silicon is a Um semiconductor that is made out of Um Three Natural Isotopes Silicon 28 Silicon 29 silicon 30 of which the most abundant is Silicon 28 that has zero nuclear spin. So if where a Silicon 29 has a nuclear spin of 1/2 so it's actually a slightly magnetic nucleus because the quantum information is encoded in the magnetic state of the electron and the nucleus, Any other nuclei that carry a spin act as sources of noise. Noise? Yes, so we have access to specially isotopically purified silicon material where those Silicon 29 spins been almost completely eliminated. So that gives us a some people call it a semiconductor vacuum, right? These atoms implanted in Silicon are really almost the same as being atoms in vacuum despite being in a semiconductor with you know, Gates and Nano electronics and stuff we can connect to do these: Silicon 29 Isotopes Do they cause a problem on regular silicon chips? No, it's only for this Quantum world.

Well, well, little curiosity. So Silicon 29 for us is a problem because of its nuclear spin. There has been research I think probably about 30 years ago in the classical semiconductor electronic industry to see whether a purified Silicon 28 material without the Silicon 29 and the Silicon 30 would have better thermal Management Properties So Silicon 29 and Silicon 30 S 29 not only has a spin, but it also has a different Mass. So if you look at how does heat propagate through a crystal right Um, it's I'm oversimplifying here, but it's almost as if as the difference between electricity propagating through a pure metal versus an alloy, right? right? So silicon having three different Mass Isotopes The crystal vibrations which carry the heat are modified and made more complex by the presence of three different Mass Isotopes So it is true that if you have a single isotope silicon crystal, it has a better thermal conductivity.

So in the context of you know, super high density chips, you would imagine, oh, that'd be great. It turns out that Improvement in thermal conductivity is most significant at low temperatures. I'm going by memory, probably minus 100 Cel or something like that at room temperature. It's it's not much.

so you're talking talking some percent. Yeah, so it. There's no. there's no commercial reason to go through the effort of of doing that.

But people looked it up. People actually tried and did experiments because in principle it does change thermal conductivity. Interesting, That's because of the mass, not because of the spin. We care about the spin.

Yeah. and so when we reading this information, we put up that live view again. Yeah, Okay, so so so it's all happening. regardless of whether or not you start this script.

It's all. you're just actually measuring it. Uh, no, no, no. So what's We're always measuring the current.

But what's happening? The script makes two voltage sweeps on the gates. That's right. Yeah, got it. For example, if I just stop this one and you can actually see from the picoscope or the soloscope, this is what happening.

Okay this point. Yeah. So in this case, you don't need to Swip to voltages? Yeah, but just sit at the radar point and then you have the P there. The thing I was telling you before, if you just sit there, you will see electrons just randomly hopping on and off.

Now, this is tuned in a place where it's much more often on the atom than it is off, so it very rarely pops out. But for the most part, it stays on the Atle, so when you read it out, you've destroyed that information. It's not. Yes.

Okay, we haven't actually done readout yet. So to do the readout now, let's see you intrusively: no, no, no, I'm just I'm just looking at the charge. Well, in a sense I'm I'm reading out the charge state I'm reading out where the charge is, but there's no useful information. That's not my Quantum bit got it.

My Quantum bit is the spin. So the spin um is a Quantum to level system. There is a spin down and a spin up energy level and we place this thing in the refrigerator at 0.01 Kelvin and in a magnetic field of about one Tesla which is a big magnetic field. the Earth magnetic field is 50 micro Tesla to give you a sense.

um so that the energy difference between the spin down and spin up state is more than the thermal energy of anything in its surrounding, right? Okay, and I don't know if you want to have a look, it's actually over there. From the other setup that's been dismantled, you can see the superconducting magnet that is used to create the magnetic field. Okay, so that's so. this is a it's a solenoid of super conducting wire.

It goes Super conducting at about 10 Kelvin What's the material makeup of that Wi? this is a Niobium Niobium Titanium, right? It's it's an alloy Niobium titanium, so it's a big solenoid. It has an inductance of about 10 Henry M For the electrical engineers: Among Us Who know how big that is? a very big inductor, and um, it's run by those power supplies there, so they it takes about. With about 100 amps of current, you get about six Tesla magnetic field from this magnet. Now the interesting thing is that because it's a superconductor, you don't need to keep it powered up all the time so you can charge up the inductor to the current that you want.

and then there is a super conducting switch that you can use to Short Circuit the coil right and then you can turn off the current and that super current will flow on forever. Yes, that's actually what happens at the hospital. When you go to see an MRI machine, you will not see a power supply there, right? someone? The installation engineer came one day with a rack of power supplies charged up that magnet. Thison connected and walked away and went to charge up the next magnet at the next Hospital got it.

And but if they remove the power, they screwed. No. If you remove the cooling, you remove the cooling your stuff. and so for those of you are interested, you can go and look for Magnet Quench on the Internet.

Magnet Quench Quench. Yes! So Magnet Quench is what happens when there is a hot spot along the superconducting wires such that the wire goes from being superconductor to being a normal resistor. and once you get that, it starts to dissipate and so it run. It's a Runway process because it Heats it Heats out the parts of the wire that are next to the hotspot and that hotspot propagates until the whole wire goes normal.

So you have a 10 inductor charged with you know, 50 or 100 amp that is suddenly becoming a normal resistor. In fact, a fairly like this is not a high conductance wire. Now Obum Tianium is is an alloy. It's not like copper or gold, you know, and so there's a huge amount of power dissipation and it boils off the whole helium bath in which the magnet is immersed right.

So go on the Internet, look for Magnet NMR Magnet Quench and you'll see this thing where the this big puff of cold helium gas blows out because the whole thing has gone resistive and blows it out. Got it? That relay that shorts out the superc conducting relay that shorts out the coil. What happens to that if you short it out? Well, no, it's actually not a relay. That's the interesting part.

All that is right: I'm I'm I'm Physically think know it's a very good you're very good question Dave It's actually a piece of the exact same super conducting wire put across the inductor. Yeah, but in normal operation, when you want the switch to be open, you wrap a little resistive wire around that piece of super conducting short to make it go normal. So that thing will be a few Ohm resistor which is nothing compared, which is in parallel to zero, so nothing goes through it, right? right? Whereas if you stop heating it up, then it goes Super conducting. So now you have a fully closed zero resistance circuit.

Got it? Yeah, Okay, that makes sense. Yeah, and that's built somewhere in there. There's a little yeah, so you will see. Yeah, So you see this little thing here.

Couple little cables. that's the cable that feed. Needs that little heater to turn the switch on and off. Got it? Yeah, you got some serious ground bond in here.

Yes, that's a building ground. That's a thing. Yeah, No, no, no, that's a clean ground. So we want to have all our instruments uh, connected to a ground that is separate from all the noisy ground that all the rest of the building runs off.

So these are Big copper strips that go to a Stak in the ground that's been cast with a special electrically conducting jet and so on. and so all our instruments are an isolation. Transformer It's like a star connection to the ground to that clean building. R Got it.

Just little power. Hygiene: Um, Emi is not an issue, it is. It is. I Mean of course this thing is itself a faraday cage right? So and then the other thing.

youve already seen this. you know we try to Shield as well as we can. Everything. That's not to say that Emi No, no, you still pick up stuff.

These things things are so sensitive. you know you, you see things. I'm not I'm not saying that we have complete UT you know Emi protection. But the thing is, we also we need to put signals in right? So there are certain experiments where you only have very low frequency signals and you can people build shielded rooms.

Like fully shielded rooms. Like the whole system is inside a shielded room. all the cables go through a copper fit through and you can do all that. but we have to run 40 G microwave signals into the system.

There's all sorts of fast pulses, so in a sense, the Emi from the environment is not even the biggest problem given the kind of signals that we pump into the system. So we try to do the best we can. But you know, and you have to use the 40 gig odd frequencies due to uh, the value of the Pl constant constant, right? So very precise. Yes, absolutely yes.

So this is one my favorite things to. Uh, when I teach my Quantum engineering course, you know I just tell St there are some numbers you need to remember like your date of birth. One is that one pamp is 6.24 electrons per second. that's the electron charge.

The other one is that one Kelvin is 20.84% Photon of frequency 2084 GHz and that one Tesla is 1.34 kelv on a free electron. So if I put a magnetic field of one Tesla on just a free electron, that energy splitting is 1 34 Kelvin in energy and it's 28 GHz in frequency. So now you know why I need this. So we normally operate at 1 to 1 1.4 Tesla but the higher the better the more separation you have between the energy levels.

Buying Vector microwave sources above 40 gz becomes a very expensive exercise and also the microwave engineering of that is really starting to become. make those to order. You probably couldn't get off the shelf at that point. Yeah, so it starts to get really challenging.

40 GHz is enough. So 40 Gz is what 2 Kelvin in energy we are at. Realistic Ally 100 Melvin 0.1 Kelvin So you know 40 GHz and it corresponds to 1.4 Tesla magnetic field. So it's funny how just the ratio of constants of nature gives you the shopping list for what you need to do to run an experiment like this.

I don't have that you know I don't I don't get to choose those constants of nature of what they are. That's why I need this this Beast right? And I Think it's important to note that this is not the operating frequency of the Quantum processor which is only down in the MHz way. Way slower. Yeah, Actually, we might see how fast it goes in a moment.

Yes, Okay, let's go back to the we got a little diversion here. but let's get back to Okay so what we're doing here is this: We are applying microwave pulses at different frequency. You see the frequency AIS Here it goes from 38.6 to 39.2 GHz So what we is this: We start with the electron electrochemical potential at a position in that corner where I showed you before in a position where only an electron spin down can go onto the atom. whereas an electron spin up doesn't have, there's not enough energy to populate the spin up level.

That's why we go so cold. If we went warmer then you would populate both. 50/50 probability. That's what the refrigerator is for.

At this temperature, we can with almost certainty populate the spin down level. Okay, then what we do is magnetic resonance. We apply electromagnetic radiation at the exact frequency corresponding to the energy difference between the spin up and spin down States And when we hit the right frequency, that electron spin down, get excited to the spin up State. Once it does, so, it has enough energy to escape the atom again and give us the blip of current right and make the transistor switch.

So what you're seeing here. Now this is the scan. Again, you know bright means High Current Blue means low current as a function of the frequency, And here we're kind of repeating as we go. So you see there are some specific frequencies at which you get a bright line.

What's happening there is that there is more than one frequency at which that electron responds. Why is that? Because there's a nucleus attached to it? Oh, so that nuclear spin. This is in particular and you'll see it in a moment. We populate them all.

This is an eight level nuclear spin. It's Antimony 123 right? It has a spin 7even half. so it has eight Quantum level. So it has eight possible orientations, and every orientation of the nuclear spin corresponds to a different effective internal magnetic field that is applied to the electron that shifts the frequency at which it responds.

So depending on the nuclear spin orientation, we will get a different frequency at which the electron responds. So you will. You won't get that for the phosphorus. One Phosphor will get only two two.

Sorry, Yes, here we get eight, so that would offer greater information. Dens, that's right. So an antimony atom has eight level instead of two8 is 2 to the^ three. So it's the equivalent of three cubits in one atom.

So what Steve is doing here is basically scanning the electron resonance frequency and as he flips the nucleus as he goes so he can see all the various resonances, they are perfectly even spaced almost per almost perfectly. Again, for reasons that have to do with with Um, they're almost almost perfectly evenly spaced. They would be evenly exactly evenly spaced if you went in the limit of infinite external infinite external magnetic field. Got okay that so if you if you turn, turn the field lower and lower, you will lose the ability to measure it.

But imagine you could do it. You will see that these things become more unevenly spaced for reasons. Got it? And then they start overlapping, overlapping and you can't tell the difference. Well, what happens is that you start to entangle the electron with the nucleus.

Oh oh, quantum entanglement. Quantum entanglement. So no, no, Well, we do sometimes. But in this specific case, we want to work in a regime where if left in peace, the electron and the nucleus are disentangled so that the electron acts as a we call it an anilla is an anilla device to read out the nuclear State You see what you're doing here.

I Mean when you think about it, you're actually watching the quantum state of a single nucleus by seeing at what frequency the electron responds. You know which way this single nucleus of Antimony is oriented among a different possibilities? And just think about. you know we we take it for granted. But you know, think about it for you're watching.

You know you're watching a current through a mosfet, right? And by doing so, you are watching the magnetic orientation of one nucleus. Yes, and you're getting all this information from the IV curve of a single, not even the IV curve just for the current. So we we we. We set it up at a specific voltage.

Just the instant instaneous current current because one electron is is enough to switch the transition on and off and you can easily measure that. CU It's in the order of yeah, it's Nan Yeah, it's n and that's easy to. It's fine. You can use off the shelf stuff off the shelf.

Yeah. Wow. Okay, so the information is destroyed when you read it out like that. Correct? All right.

So let's let's say you can't copy it because you can't copy Quantum Um, information. You can't let me try and put it this way. So let's say I had prepar the quantum superposition of the nucleus being in this state and in that state you, you've set it up that way. I've set it up that way and then I went to measure at what frequency the electron responds.

I will find only one of them. Which means that nucleus, the quantum state of their nucleus has been projected to the state that corresponds to the frequency at which the electron responded right. So given the initial Quantum superposition state of the nucleus, there could be two frequencies at which the electron responds. I go and interrogate the electron.

If it gives me a blip at 39.2 GHz then I know that the nucleus is in the highest spin Direction And so even though before I did the measurement, it was in a super position of 72 and 5 half spin projection after after I see this blep of current in the transistor. then nucleus is collapsed into that specific orientation. Got it? Are you running applications on this quantum computer? No, it's too small. There is no application of any kind at this scale.

At at what scale does it start to become practical? Well, a trillion dollar question. So um, it starts to become practical. Depends what application you're looking at. Let's say that the earliest applications are probably going to be the ones where you use a quantum computer to simulate other Quantum systems such as molecules for example.

Pharmaceuticals right? right? Yes! So why can't you design a cancer cure on a computer? Just say oh, you know This is what the oncologist said I have I've got this kind of cancer? Write a Python script that gives me the that Kills the cancer. You can't do that. Why? Because whatever. Even that.

imagine that drug exists. Maybe it doesn't. but let's say if it existed, the classical computational complexity of calculating the behavior of all the nuclei and electron in a reasonably complex molecule is far beyond the capability of even the bigger superc computer. An interesting Factor One of the biggest molecules that you can you know manageably simulate on a classical computer is Cap Caffeine.

Oh okay, caffeine is not very big. look it up going and type caffeine molecule structure. It's a fairly simple molecule and that's because it's a quantum mechanical problem. All those atoms and orbitals and the chemical bonds and how they interact with other things such as the cancer cells in the body is a Quantum problem.

So why don't we use a Quantum system to understand a Quantum problem? That's that's that's the logic of it. Sim So for that you'll probably need I should know this better. but I I'm guessing. Okay, don't call me up on this.

Please don't cancel me on the line. Um, over some 100 to a thousand very good cubits Very good. Cubit What's the difference between a very good and a crappy Cubit A very good Cubit is one that you can run you know thousands of operations on without errors along the way. So what I call a very good cub? Cubit is a cubit that can be operated on for thousands and thousands or possibly more operations without errors.

Now, this is very difficult to achieve in the physical world in the practice. So what people are trying to set up is some redundancy where one bit of quantum information is encoded not in one, for example atom, but in many of them. And there are some operations and measurement sequences that allow you to both operate and change the quantum information on that encoded Cubit, but also detect and correct errors as they occur. Now, there's an interesting thing about this: the absolute minimum amount of redundancy you need to have to make any kind of quantum error detection and correction is three right? Now, how many equivalent cubes do we have in this atom? Oh, you've got the three, right, right? So that's why this is really exciting.

Okay, there are some very clever colleagues of mine who came up with ideas to encode a what we call a logical cubid. So cubid that has enough redundancy to detect and correct errors in a single nucleus instead of having to piece together different physical cubes that are spread out across the chip. And you couldn't do that in the phosphorous one. No, there's not enough.

There's only one cubit there. There's not enough. So you'd have to multiple cubits to do the same operation as a Cub. Here we with Antimony, we can do it in a single nucleus, right? Got.

But is that going to be more complex to manufacture? No, No, no, You just change the mass selector in the ion implanter. Oh, this is exactly the same as a phosphorus device. If you look at them from the outside, they're indistinguishable. So why is anyone working on phosphorus anymore? Well, um, look, it's it's easier at the beginning to do the early experiments right.

Uh, I mean I Have to say we are basically the only group that's doing this right. So this these experiments are very hard. especially the fabrication is very very very hard. What we have here at Unsw is really an amazing nanofabrication facility which is actually part of the national Uh collaborative research infrastructure set up.

you know, by the Australia government. very very cleverly. it's called the Australian National fabrication facility. so it's actually it's it's a it's a accessible, user accessible fabrication facility.

But here at Unsw there has been more than 20 years of really focused, um, investment in getting the specific tools you need to get silicon nanoscale Quantum devices done properly. And this stuff is not easy. It's not cheap, there has to be really critical mass of people who want to do it, but we have it here. So that's why we are able to make these devices.

You can't just wake up one morning and say, oh, let me do this. That's all right, Okay, but with the Phosphorous one, it's easier. You can smaller groups can Muck around the device is actually the same, right? It's actually the same. so even making the phosphorus one is hard.

right? From a nanofabrication point of view, it's exactly the same. Moving to Antimony, the microwave engineering of it the the. In fact, the engineering of it becomes more complicated. but even that has been made easier in more recent time by the progress of Fpga W from generators we know I Don't think we're going to be able to show you this.

It gets really complicated. but basically to control this eight level nuclear spin, you need to have seven radio frequency uh, signals, right? There are seven differences in energy between the eight states. So you need seven. You need A.

Basically you need an RF generator that makes seven signals at different frequency and each one of them completely face coherent. So I need to be able to make a pulse at frequency one, then a pulse at frequency two, then a pulse at frequency three and then come back to frequency one and be in a perfect phase relation with the first pulse. I did a millisecond ago. That is actually not that simple, but it is becoming possible now with the latest generation Fpga away from generators.

Yeah, very so. 10 years ago it would have been a real pain right now. You can buy a commercial machine is Not is not off the shelf. It's A It's A.

It's a startup that makes machines just for Quantum Control. You can buy it and it's already the Fpga program. There are some you know, uh, coding instructions you can use and you can make all these multifrequency signals all face coherent with each other and you can operate this whole large multi-dimensional Quantum system like that. Okay, so what Steve is going to show us now is the rotation of a single electron spin.

So we start it in the spin down state and then we apply a burst of microwaves at what is 39 GHz and the electron, the probability of finding the electron in the UP State will oscillate. Remember, it's always a probability. Yes, Okay, so we start down that we can do deterministically. But then we start to make a superposition of down and up that has a heavier and heavier weight of up.

Oh, until it goes all the way up and then it comes back down. It's weighted. Yeah, it's A There are. It's a weighted probability.

Yeah. So this is done by applying a burst of microwaves that approximately 39 GHz and changing the duration of that microwave burst. The longer you leave it on, the more the electron rotates. so you can see it going all the way from down to up and back.

What sort of period are we talking about? So we are talking 8 micros. Eight micros? Okay, rotation. This is for a full rotation. Yeah, and so what Steve is doing here is just repeating the experiment multiple times.

You can see all the traces and remember so this is on the vertical scale I Don't know if you can see it on your camera. it's it says up proportion. Which means how many times you get a spin up. Our measurement is binary classical.

We only get a zero or a one. We get zero current or high current. There's nothing in between. So how do you know if a spin is pointing halfway between up and down? You repeat the experiment 30 times.

and if you get 15 time times down and 15 times up, that tells you it was in a super position of 50/50 super position of being up and down. That's effectly what you have to do. Are you controlling the period of that? How are you controlling the with the power? So if you crank up the amplitude of the microwave drive, this thing will oscillate faster. Got it? Yeah right.

And that relates to the kind of the effective Quantum process in time of a quantum computer. It kind of does. That's right. So for example if you want to do to do a not gate yes on the bit that will take four microc.

Correct in this. So the clock this will be you can call it 100 KZ clock speed but the advantage eventually will be the parallel of it is the is the complex. So you will use a different algorithm to arrive at the result and that algorithm will have a much smaller number of steps. So even though every step individually may be slower because we're running 100 Kilz clock instead of a 2 gz clock.

if there is an exponentially smaller number of steps to arrive the results, we'll still get there sooner. Uh, then we can do the check Nucleus Spin: Yeah, so Steve has written a couple of scripts that are already demonstration demonstration ready. ASR is that equivalent series resistance no Electron SP Resonance Electron Okay, so you see this was blue for several shots and then it switched to Orange right? So that's Sr1 Sr2. What it means is that he was checking all the eight possible frequency at which the electron responds and for sometimes it was always this one.

Then it switched to this one and then it switched black back to the blue one. What it means is that the nuclear spin has switched between two states. I'm amazed that you can do this with just like a current and the basic signal is what you see here on oscope screen. It's just the spikes of current.

That's nuts because you know what frequency and timing you're exciting it at and it doesn't have to be that quick. The fact it needs to be at the 40 GHz or the 28 GHz due to the fundamental uh constants, but you're controlling that. Yes, let me go back to something we discussed before, which is why silicon, right? I Told you we can get silicon 28 and Rich material that removes the Silicon 29 spins that give you noise. Now if if I was doing the the same experiment on a natural silicon chip like the ones you have in every phone, this wouldn't work because the spin would lose its Quantum State quicker than I can address it.

Got it? So the reason I can take my time and have the clock speed of 100 Kilz to do that rotation in a couple of microsc is because that spin knows its Quantum State and remains it's its precession is coherent. So the phase of the procession, the frequency at which it process is constant over time scales of hundreds of micros seconds. If I did that in natural silicon, it would be a Nanc range. Oh okay, so it's T in theory possible, but but very hard.

very hard. We made here the very first Quantum beating silicon in 2012 and that was in natural silicon and that was a hero experiment. We had to put a lot of power down to do the things really fast and you can barely see the oscillation once we move to the isotopically purified silicon. It's Paradise magic.

And how does that relate to the magnetic field strength? Uh, it doesn't. It doesn't. The magnetic field strength only tells you at what frequency needs to be the signal that flips the spin, but you were asking me before about is these a high electrom Mobility transistor. Many of them are made in 35 semiconductor like Gallum Arsenide, right? People have made spin cubits in Gallum Aride, but again, they suffer the same problem.

In fact, even worse. Gallum Aride has only isotopes with a spin, both Gallum and Arsenic have a nucleus spin. So in that material, even though from a charge point of view, from an electrical conduction point of view, it's super clean. You can get this super high Mobility Transistor from a electrical point of view.

From a spin point of view, they're very noisy and very hard to operate, so we don't. We much prefer having a silicon mosfet where the mobility is not that great, but the spin properties are fantastic rather than the other way around. and so again, just just I Want to, You know, especially your viewers to think in what's going on here? Just watching. And this thing is literally just counting how many blips we have here, right? This is a counter renormalized between zero and one.

When you see the thing switching, you are seeing a single antimony nucleus pin switching. Direction Wow, that's happening. Live before your eyes. And that's just by counting how many blips you're going to do and the other seven or whatever you've got in there.

they're all low. We actually check all of them, You see? Oh okay, we're checking all all eight of them. and that's important. There's only ever one of them that's high, right? right? You can only ever see it in one Quantum state.

All the other ones are low and one of them is high. Got it right? Interes so you'll never see two. What you want to show the Nuclear Rabi: Okay, so now we're doing the same thing we did with the electron before. So to make the electron rotate now, we do it on the nucleus.

Now here, this is done at a much lower frequency. The nuclear spin has a much smaller gyromagnetic ratio. The gyromagnetic ratio is the frequency at which the spin processes in a certain magnetic field. So for an electron spin, that's 28 gahz per Tesla one.

Tesla Magnetic field gives 28 GHz for a nuclear spin. Well, depends on the spin. In fact, every nucleus has a different Garam magnetic ratio and that's what people use in chemistry to do chemical analysis with nuclear magnetic resonance. Oh right.

they can tell what's in a molecule by looking at what frequency resp. I Never looked into it That make sense. Every spin has a different gyro. Magnetic Antimony has a 5.55 mahz per Tesla jar Magnetic Ra.

So here it will be running at what? 7 and a half? Mehz. More or less. 5 by five? Yeah five. Sorry, there's a triple Five timer joke running.

Yeah, that's right. So let's see. this will take a little bit longer to run. So the nuclear spin has an even slower clock speed than the electron for the same reason.

Now, what sets that clock speed is the So when we apply that radio frequency or microwave signal to the spin, we are creating an oscillating magnetic field. The amplitude of that oscillating magnetic field sets the time scale over which the spin gets rotated from up to down. So if I apply a one microtesla oscillating magnetic field at the 39 GHz frequency to the electron. Yeah, Yeah, right.

Uh, it's an AM Um. So if I apply a micro Tesla it will give me a 28 Kilz oh rate at which the electron goes up and down. So by seeing that that was at Uh was about yeah, 200 Kilz it means we were running something less than 10 Micro Tesla oscillating magnetic field at 39 G Wow. Okay, got it.

now with the nuclear spin Because the gyromagnetic ratio is more than a thousand times smaller, it actually goes much slower. Now, it doesn't go a thousand times slower because being at Megz instead of 40 Gigz, we have much less losses along the coaction line. So we actually able to put a lot more power down. so it goes about maybe 100 times lower.

So that's right. Yeah, so this excitation signal to flip the nucleus is slow enough. You can see it on on a simple oscilloscope. Why is it multi level there? It