Quantum Computer Breakthrough: The Crosstalk Problem in Silicon Qubits

<v Speaker 1>Welcome to the quarre Side Quantum Physics Podcast, an exploration
<v Speaker 1>of the fundamental structure of reality. We're quantum laws govern matter, energy,
<v Speaker 1>and information. Here, uncertainty is a feature, not a flaw,
<v Speaker 1>and understanding begins at the smallest scales.
<v Speaker 2>Welcome back today. We are going to try to wrap
<v Speaker 2>our heads around something that is simultaneously incredibly small and
<v Speaker 2>absolutely massive in terms of its implications.
<v Speaker 3>That's a good way to put it.
<v Speaker 2>We're talking about the future of computing, but you know,
<v Speaker 2>not the shiny marketing brochure version of it.
<v Speaker 3>No, we are getting into the messy reality, the plumbing
<v Speaker 3>behind the palace, so to speak.
<v Speaker 2>We always hear the grand promise, right We hear that
<v Speaker 2>quantum computers are going to solve these impossible problems revolutionized medicine,
<v Speaker 2>crack encryption codes that would take a normal computer a
<v Speaker 2>billion years to solve. It's painted is this sci fi
<v Speaker 2>utopia where the machine just knows the answer.
<v Speaker 3>It is the ultimate promise of computational power. It's elaked
<v Speaker 3>not just a step. But the gap between that promise
<v Speaker 3>and where we are today is well, it's filled with
<v Speaker 3>some of the most difficult engineering problems in human history.
<v Speaker 2>And today, we are going to zoom in on one
<v Speaker 2>of those problems.
<v Speaker 3>Way way in, right down to the component.
<v Speaker 2>Level, past the headlines, past the hype, and right down
<v Speaker 2>to the microscopic reality of what it actually takes to
<v Speaker 2>build one of these machines. Because it turns out building
<v Speaker 2>the future isn't just about big ideas. Wow, It's about
<v Speaker 2>controlling individual electrons in spaces that are only tens of
<v Speaker 2>nanometers wide.
<v Speaker 3>It is an engineering challenge as much as it is
<v Speaker 3>a physics challenge. You are you're literally wrestling with nature
<v Speaker 3>at its most fundamental level. Howso well, you're trying to
<v Speaker 3>force sub atomic particles to behave against their natural chaotic instincts,
<v Speaker 3>to hold them still and get them to do useful
<v Speaker 3>work for you.
<v Speaker 2>And we have a specific tension to look at today.
<v Speaker 2>We want to pack these things called quantum dots closer
<v Speaker 2>and closer together to build powerful computers, because a computer
<v Speaker 2>with two bits isn't very useful, not at all. But
<v Speaker 2>new research shows that these tiny neighbors are starting to
<v Speaker 2>well shout over each other.
<v Speaker 3>Shouting is a very vivid way to put it. In
<v Speaker 3>the quantum world, shouting is more about energy exchange and
<v Speaker 3>frequency shows but the effect is the same distraction, interference noise.
<v Speaker 2>And we're basing today's discussion on a brand news study
<v Speaker 2>who was published just yesterday February twelve, twenty twenty six,
<v Speaker 2>and physical review applied.
<v Speaker 3>Yes, this is coming out of the Reichen Center for
<v Speaker 3>Quantum Computing. The research was led by Takashi Kobyashi and
<v Speaker 3>his team.
<v Speaker 2>And Reichen is a big deal in this space.
<v Speaker 3>Oh absolutely. Reraichen is a heavyweight in this field. So
<v Speaker 3>when they publish a paper on noise mechanisms, the community,
<v Speaker 3>you know it, sits up and listens.
<v Speaker 2>So what is our mission here? We aren't looking at
<v Speaker 2>a breakthrough where everything works perfectly. We are looking at
<v Speaker 2>a hurdle that was just discovered or at least just
<v Speaker 2>precisely measured.
<v Speaker 3>That's the key.
<v Speaker 2>The goal is to understand what happens when quantum bits
<v Speaker 2>or quibbits get too close for comfort.
<v Speaker 3>It's a story about the growing pains of technology. To
<v Speaker 3>solve a problem, you first have to measure it precisely.
<v Speaker 3>That is what Reichen has done. They haven't just found
<v Speaker 3>a problem, They've put a number on it, They've quantified it.
<v Speaker 2>So we're building the supercomputer of the future. But the
<v Speaker 2>parts are arguing with each other.
<v Speaker 3>In a manner of speaking. Yes, but understanding that argument
<v Speaker 3>is the key to silencing it, or perhaps, as we'll
<v Speaker 3>see later, even using it.
<v Speaker 2>Okay, let's get into the nuts and bolts stand Before
<v Speaker 2>we can really talk about the noise, we need to
<v Speaker 2>understand the instrument. The study focuses on something called quantum dots.
<v Speaker 2>Now I see that term, and my first thought is
<v Speaker 2>those high end TV screens with the really vibrant colors.
<v Speaker 2>Are we talking about the same thing.
<v Speaker 3>That's a great question, and it's a common point of confusion.
<v Speaker 3>It's the same underlying physics, yes, but for a completely
<v Speaker 3>different application.
<v Speaker 2>Okay.
<v Speaker 3>In a TV a quantum dot is a semiconductor nanocrystal
<v Speaker 3>that's designed to glow a very specific pure color when
<v Speaker 3>you hit it with light or electricity. It's an emitter.
<v Speaker 2>It makes pretty colors, it makes very.
<v Speaker 3>Pretty, very precise colors. But in a quantum computer, a
<v Speaker 3>quantum dot is a trap.
<v Speaker 2>A trap.
<v Speaker 3>Think of it as a potential, well, a tiny, tiny
<v Speaker 3>region in a semiconductor material where we can confine a
<v Speaker 3>single charged particle, in this case a single electron.
<v Speaker 2>So you're catching one electron and holding it in place.
<v Speaker 3>That's the entire goal.
<v Speaker 2>Okay, so why silicon. I feel like when people see
<v Speaker 2>pictures of quantum computers, they see those beautiful, big, golden chandeliers.
<v Speaker 2>The superconducting quantum computers from Google or IBM, those don't
<v Speaker 2>look like chips. But Reichen is using silicon.
<v Speaker 3>That is a crucial distinction, and it really defines the
<v Speaker 3>different competing approaches. The chandelier computers use superconducting circuits. Those
<v Speaker 3>are in electronic terms, quite lo large. They're macroscopic loops
<v Speaker 3>of metal cooled to extreme temperatures.
<v Speaker 2>And what Ryichan is doing well, Reichen.
<v Speaker 3>Is working on what's known as silicon's spin quibits. Looks
<v Speaker 3>much more like the chips you'd find in your laptop.
<v Speaker 2>Or your phone. And the advantage of that is the.
<v Speaker 3>Hope is that because we already have, you know, fifty
<v Speaker 3>years of experience and trillions of dollars of infrastructure for
<v Speaker 3>manufacturing billions of transistors on a silicon wafer, right the
<v Speaker 3>whole semiconductor industry exactly, the hope is we can eventually
<v Speaker 3>leverage all of that to manufacture billions of kribbits the
<v Speaker 3>same way.
<v Speaker 2>So it's a bed on scalability. If we can make
<v Speaker 2>it work in silicon, we can make millions of them
<v Speaker 2>and maybe make them cheap.
<v Speaker 3>That is the long term vision. But making a standard
<v Speaker 3>transistor is very very different from making a quantum dot
<v Speaker 3>for a quibit. How so, well, in a transistor, you're
<v Speaker 3>essentially just creating a switch for a current. You're controlling
<v Speaker 3>a river of electrons flowing through a channel.
<v Speaker 4>On or off a whole crowd of them, a whole crowd.
<v Speaker 3>But in a quantum dot, you need to isolate one
<v Speaker 3>single electron from that river, just one, just one, and
<v Speaker 3>you need to hold it there very gently without disturbing
<v Speaker 3>its fragile quantum state.
<v Speaker 2>Okay, so how do you build this trap? You can't
<v Speaker 2>exactly use tiny tweezers.
<v Speaker 3>No, you use electric fields. It's actually quite clever. You
<v Speaker 3>fabricate tiny metal gates electrodes on top of the silicon.
<v Speaker 2>Wafer, like the gates in a normal transistor.
<v Speaker 3>Very similar, but the arrangement is different. When you apply
<v Speaker 3>a negative voltage to these metal gates, they create an
<v Speaker 3>electric field that repels the electrons and the silicon underneath.
<v Speaker 2>So you're pushing them away.
<v Speaker 3>You're pushing them away and by arranging these gates in
<v Speaker 3>a very specific pattern, like a little corral, you can
<v Speaker 3>create a small island, a box of low energy surrounded
<v Speaker 3>by high energy walls. An electron can get trapped in
<v Speaker 3>that box.
<v Speaker 2>So it's like using invisible force fields to build a
<v Speaker 2>jail cell for an electron.
<v Speaker 3>A very very small jail cell. We are talking about
<v Speaker 3>structures that are just tens of nanometers across.
<v Speaker 2>And once the electron is in there, what are we
<v Speaker 2>actually using? As the bit in my laptop, it's voltage
<v Speaker 2>being high or low that represents a one or a zero.
<v Speaker 2>What is it here?
<v Speaker 3>Here? We are using a fundamental quantum property of the electron.
<v Speaker 3>We're using its spin spin.
<v Speaker 2>This is one of those chronum terms that everyone uses,
<v Speaker 2>but it's always a bit fuzzy. Is the electron actually
<v Speaker 2>spinning like a tiny top?
<v Speaker 3>Not literally? No, that's just a helpful classical analogy. And
<v Speaker 3>electron is a point particle. It has no physical size
<v Speaker 3>to spin around.
<v Speaker 2>So what is it? Then?
<v Speaker 3>It's an intrinsic property like its charge or its mass.
<v Speaker 3>It has an intrinsic angular momentum and an associated magnetic
<v Speaker 3>moment It behaves as if it were a tiny spinning bar.
<v Speaker 4>Magnet a tiny magnet eject and.
<v Speaker 3>That tiny magnet can point in different directions. For our purposes,
<v Speaker 3>we care about whether it's pointing up or down relative
<v Speaker 3>to an external magnetic field we apply.
<v Speaker 2>Okay, so spin up is a one and spin down
<v Speaker 2>is a zero. Is that the basic idea.
<v Speaker 3>That is the basis of it. Yes, that's how you
<v Speaker 3>define your computationational lysis states. But because it's a quantum system,
<v Speaker 3>it's not that simple. It's not that simple. It can
<v Speaker 3>also exist in a superposition of both up and down
<v Speaker 3>at the same time until you measure it. That's where
<v Speaker 3>the real power comes from, right.
<v Speaker 2>The classic quantum weirdness. So, just to recap, we have
<v Speaker 2>a silicon chip. We have tiny electric gates creating a trap.
<v Speaker 2>We catch one single electron. We use its magnetic spin
<v Speaker 2>as our data storage.
<v Speaker 3>Correct. But here's the problem, the core vulnerability of this
<v Speaker 3>whole approach. That spin state is incredibly fragile. Well canness
<v Speaker 3>it up almost anything. The biggest enemy is thermal energy
<v Speaker 3>just heat. Heat is just atoms and electrons jiggling around.
<v Speaker 3>If the environment is too hot, the electron shakes, it
<v Speaker 3>interacts with the lattice of silicon atoms around it, and
<v Speaker 3>the spin state just flips randomly. Your data is lost instantly,
<v Speaker 3>your one becomes a zero, or your superposition just collapses.
<v Speaker 3>The information is gone. This is called decoherence.
<v Speaker 2>So you have to freeze it.
<v Speaker 3>You have to freeze it to near absolute zero. These
<v Speaker 3>chips don't just sit on a lab bench. They live
<v Speaker 3>inside a complex machine called a dilution refrigerator, operating at
<v Speaker 3>temperatures in the millikelvin range.
<v Speaker 2>How cold is that?
<v Speaker 3>It is significantly colder there than in deep space.
<v Speaker 2>So I should picture this a giant humming silver cylinder
<v Speaker 2>in a lab. Inside it, there are layers and layers
<v Speaker 2>of gold and copper plates that get progressively colder and colder,
<v Speaker 2>and at the very bottom, in the deepest cold sits
<v Speaker 2>this tiny silicon chip with these nanometer scale traps.
<v Speaker 3>That is the exact picture, and that is where the
<v Speaker 3>racing team is working. They aren't just looking at one dot.
<v Speaker 3>Though one quibbit is a science experiment, a computer needs many, and.
<v Speaker 2>This is where we hit the scaling problem, the crowded house,
<v Speaker 2>as we called it. Right.
<v Speaker 3>To perform calculations, to run in algorithm, quibits need to
<v Speaker 3>interact with each other in a controlled way. You need
<v Speaker 3>to be able to entangle them.
<v Speaker 2>Which means they can't be miles apart.
<v Speaker 3>They need to be close, very close.
<v Speaker 2>How close were we're talking.
<v Speaker 3>In the device and this study, the which is the
<v Speaker 3>distance from the center of one quantum dot to the
<v Speaker 3>center of the next, is about one hundred to one
<v Speaker 3>hundred and fifty nanometers.
<v Speaker 2>That is incredibly tight.
<v Speaker 3>It is extremely tight. And when you pack charged particles
<v Speaker 3>that close together, you run into a fundamental force of nature,
<v Speaker 3>the kulum interaction.
<v Speaker 2>Remind us what that is, It's.
<v Speaker 3>The most basic electric force. Like charges repel, opposite charges attract.
<v Speaker 3>So two electrons, two electrons are both negatively charged. They
<v Speaker 3>hate each other. They are constantly trying to push each
<v Speaker 3>other away.
<v Speaker 2>So i have two of these quantum dots right next
<v Speaker 2>to each other, and I've got one electron in each one.
<v Speaker 2>They're constantly pushing against the walls of their little jail cells,
<v Speaker 2>trying to get away from their neighbor.
<v Speaker 3>Ideally, the electric field walls of the trap are strong
<v Speaker 3>enough to hold them in place, but they still feel
<v Speaker 3>each other's presence. The force is still there. Okay, So
<v Speaker 3>if the electron in Dot A shifts its position slightly,
<v Speaker 3>and it can. It has a little bit of wiggle room,
<v Speaker 3>maybe just by a nanometer.
<v Speaker 4>The electron in dot beef that the electron.
<v Speaker 3>In dot B feels a change in the repulsive force.
<v Speaker 3>The push gets a little stronger or a little weaker.
<v Speaker 2>It's like if you are standing in a small room
<v Speaker 2>with a powerful magnet strapped to your chest and someone
<v Speaker 2>in the next room had one too, even if there's
<v Speaker 2>a wall, if they take a step towards that wall,
<v Speaker 2>you feel a push.
<v Speaker 3>That is a perfect analogy, and this brings us right
<v Speaker 3>to the core of the Reichen discovery. Takashi Kobayashi and
<v Speaker 3>his team, we're investigating this exact scenario. They wanted to know,
<v Speaker 3>does that push from the neighbor mess up the information
<v Speaker 3>stored in the spin of our kubit.
<v Speaker 2>And the answer I'm going to assume is yes, otherwise
<v Speaker 2>we wouldn't be doing this whole conversation.
<v Speaker 3>The answer is a definitive yes. But the mechanism, the
<v Speaker 3>how is the really fascinating part because it's not a
<v Speaker 3>direct effect. It involves a component we haven't even mentioned yet. Oh,
<v Speaker 3>the micro magnet.
<v Speaker 2>The micromagnet. Okay, wait, let's back up. We have electric
<v Speaker 2>gates to trap the electron. We have the electron itself
<v Speaker 2>acting like a tiny magnet with its spin, And why
<v Speaker 2>on earth do we need another magnet.
<v Speaker 3>This is a great question, and it gets right to
<v Speaker 3>the heart of why silicon spin quibets are so hard
<v Speaker 3>to build and control. Remember, what we want to do
<v Speaker 3>is control the spin of the electron.
<v Speaker 2>Right flip it from up to down, or put it
<v Speaker 2>in this superposition exactly.
<v Speaker 3>Now, spin responds to magnetic fields, so the obvious way
<v Speaker 3>to control it would be to zap it with a
<v Speaker 3>little magnetic pulse.
<v Speaker 2>Seems simple enough.
<v Speaker 3>It is if you only have one quibbit. But remember
<v Speaker 3>we're in a crowded house. If you want to build
<v Speaker 3>a real computer, you have thousands, maybe millions, of these
<v Speaker 3>dots packed together, and if I.
<v Speaker 2>Turn on a magnet, everything nearby feels it. It's hard
<v Speaker 2>to target just one dot without disturbing all its neighbors.
<v Speaker 3>Precisely, it's called addressability, and it's a huge problem. You'd
<v Speaker 3>have magnetic cross stock everywhere. So the engineer's thought, is
<v Speaker 3>there a better way. We are very good at generating
<v Speaker 3>localized electric fields. We can switch voltages on those tiny
<v Speaker 3>gates incredibly fast and with pinpoint precision.
<v Speaker 2>So they wanted to control the spin using electricity instead
<v Speaker 2>of magnetism.
<v Speaker 3>That was the goal.
<v Speaker 2>But can you even do that? I thought you said
<v Speaker 2>spin doesn't talk to electricity.
<v Speaker 3>It doesn't, not directly. You need a translator. You need
<v Speaker 3>a physical mechanism that couples the electrons position which electricity controls,
<v Speaker 3>to its spin state. This is a real physical effect
<v Speaker 3>called spin orbit coupling. Okay, in some materials like gallium arsenide.
<v Speaker 3>This effect happens naturally and is quite strong. In silicon,
<v Speaker 3>which we want to use for manufacturing reasons, it's naturally
<v Speaker 3>very very weak.
<v Speaker 2>So we have to fake it.
<v Speaker 3>We have to engineer it. We create something called synthetic
<v Speaker 3>spin orbit coupling.
<v Speaker 2>Synthetic spin orbit coupling.
<v Speaker 3>That sounds expensive, it's clever engineering. This is why they
<v Speaker 3>install the micromagnet. They physically place a tiny strip of
<v Speaker 3>magnetic material like cobalt, right on top of the quantum dots.
<v Speaker 3>But this magnet isn't designed to create a uniform field.
<v Speaker 3>That's the key. It's designed to create a gradient.
<v Speaker 2>A gradient, what do you mean by that? So the
<v Speaker 2>magnetic field is stronger on one side of the day the.
<v Speaker 3>Other exactly that. Imagine the quantum dot is a small room.
<v Speaker 3>The magnetic field at the left wall is let's say,
<v Speaker 3>one hundred units strong. The magnetic field at the right
<v Speaker 3>wall is one hundred and five units strong. It's a slope.
<v Speaker 3>It's not flat.
<v Speaker 2>Okay, I have a magnetic slope across my tiny room.
<v Speaker 2>Why no?
<v Speaker 3>Remember the frequency at which the electrons spin processes, the
<v Speaker 3>speed at which it wobbles like a top, depends directly
<v Speaker 3>on the strength of the magnetic field it is sitting in.
<v Speaker 3>This is called the Larmer frequency.
<v Speaker 2>So if the electron is sitting on the left side
<v Speaker 2>of the room, it spin wobbles it's speed one hundred.
<v Speaker 2>If a moving to the right side, it wobbles its
<v Speaker 2>speed one oh five.
<v Speaker 3>Precisely. Now you see the trick. We can use our
<v Speaker 3>electric gates to gently nudge the electrons position. If we
<v Speaker 3>apply a little voltage, we can push it from the
<v Speaker 3>left side of the dot to the right.
<v Speaker 2>Huh. And by pushing it we force it to move
<v Speaker 2>into a stronger part of the magnetic.
<v Speaker 3>Field, which changes its spin speed.
<v Speaker 2>Uh huh. So we are using electricity to move it,
<v Speaker 2>but it's the motion through that pre engineered magnetic gradient
<v Speaker 2>that actually changes the spin.
<v Speaker 3>You've got it. The micromagnet acts as a translator. It
<v Speaker 3>converts an electric signal, which is a change in position,
<v Speaker 3>into a magnetic control, which is a change in spin rotation.
<v Speaker 4>Skeeed.
<v Speaker 3>It acts as a lever. It makes the electrons spin
<v Speaker 3>highly sensitive to its physical location.
<v Speaker 2>That sounds brilliant. It's a really clever workaround problem solved right.
<v Speaker 3>It is brilliant. It's a technique called electric dipole spin
<v Speaker 3>resonance or EDSR, and it allows for very fast, localized
<v Speaker 3>control of individual quibbits.
<v Speaker 2>But there's always a butt.
<v Speaker 3>There is no free lunch in physics.
<v Speaker 2>I knew it. Here comes the bill.
<v Speaker 3>By making the spin exquisitely sensitive to its own position,
<v Speaker 3>you have also made it exquisitely sensitive to anything else
<v Speaker 3>that might shift.
<v Speaker 2>Its position, like the neighbor pushing on the wall.
<v Speaker 3>Like the neighbor. Let's go back to our crowded house.
<v Speaker 3>We have two dots Dot A and Dot B side
<v Speaker 3>by side. Each one is sitting under this micromagnet gradient. Okay,
<v Speaker 3>Now suppose the electron in the neighboring dot Dot A
<v Speaker 3>move Maybe we're performing an operation on it, or maybe
<v Speaker 3>it just jiggles because of some background charge noise.
<v Speaker 2>The neighbor moves the sofa.
<v Speaker 3>The neighbor moves the sofa. When that electron in Dot
<v Speaker 3>A moves, the kolum repulsion it exerts on our electron
<v Speaker 3>in Dot B changes.
<v Speaker 2>The push gets stronger or weaker.
<v Speaker 3>And because our electron in Dot B is sitting in
<v Speaker 3>this carefully balanced electric trap, that tiny change in push
<v Speaker 3>from the neighbor is enough to shift its positions slightly.
<v Speaker 2>And because of the micromagnet ingredient, when our electron.
<v Speaker 3>Moves, it moves into a different magnetic field strength, and
<v Speaker 3>therefore it spin frequency changes bingo.
<v Speaker 2>The neighbor's motion has changed the speed at which our
<v Speaker 2>quibod is spinning. This is the crosstock. This is what
<v Speaker 2>the Reichen paper calls a charge induced spin frequency shift.
<v Speaker 3>That's it in a nutshell.
<v Speaker 2>So the micromagnet is a complete double edged sword. It
<v Speaker 2>gives us the precise control we need, but at the
<v Speaker 2>same time it acts as an amplifier for the noise
<v Speaker 2>coming from the neighbors.
<v Speaker 3>That is the crux of the problem they investigated. You
<v Speaker 3>built a system that is hyper sensitive to motion, so
<v Speaker 3>you could control it with electricity, but now it's hyper
<v Speaker 3>sensitive to the unwonted motion of the guy next door.
<v Speaker 2>Okay, so this sounds bad in theory. But was it
<v Speaker 2>just a theoretical worry or is it actually messing things
<v Speaker 2>up in a real device.
<v Speaker 3>That is the billion dollar question, and it's what Kobayashi's
<v Speaker 3>team went to find out. They didn't just model this
<v Speaker 3>on a computer. They built the device, They cooled it
<v Speaker 3>down to one hundred miliatelvin, and they measured it directly.
<v Speaker 2>Wow.
<v Speaker 3>And the way they measured it is fascinating in itself.
<v Speaker 2>I was going to ask, how do you measure a
<v Speaker 2>tiny shift in the wobble frequency of a single electron
<v Speaker 2>trapped in a piece of silicon.
<v Speaker 3>You use a very sensitive technique called Ramsey interferometry.
<v Speaker 2>Ramseiator ferometry. Is that related to Ramsey kitchen nightmares?
<v Speaker 3>No relation, though it can be a nightmare if you
<v Speaker 3>don't get the calibration right. It works a bit like this.
<v Speaker 3>You start with the electron spin pointing up. Let's call
<v Speaker 3>that our starting line. Okay, You hit it with a
<v Speaker 3>precisely timed microwave pul ulse that puts it into a
<v Speaker 3>perfect superposition half up, half down. Now you can picture
<v Speaker 3>its state as a clockhand pointing, say at three o'clock.
<v Speaker 3>Got it, and then you just wait. You let it evolve.
<v Speaker 3>That clockhand starts sweeping around at its natural frequency, the
<v Speaker 3>Larmer frequency.
<v Speaker 2>Tick, tick tick.
<v Speaker 3>You let it tick for a very specific amount of time.
<v Speaker 3>Then you hit it with a second identical microwave pulse.
<v Speaker 3>If the frequency was exactly what you thought it was,
<v Speaker 3>this second pulse will push the clock hand all the
<v Speaker 3>way around to point down, so.
<v Speaker 2>You know exactly where it should end up. It's like
<v Speaker 2>checking your watch against a master clock exactly.
<v Speaker 3>But what if the frequency has shifted during that waiting time.
<v Speaker 3>What if the neighbor moved, pushed our electron into a
<v Speaker 3>stronger magnetic field and made the clock tick a little faster.
<v Speaker 2>Than the hand won't be where you expect it to be.
<v Speaker 3>It will have rotated too far. When you hit it
<v Speaker 3>with that second pulse. It won't end up pointing down.
<v Speaker 3>It'll be somewhere else. This difference between where it is
<v Speaker 3>and where it should be is called a phase error.
<v Speaker 2>And in quantum computing, phases everything.
<v Speaker 3>Phase is where the information is stored in many algorithms.
<v Speaker 3>If the phase drifts uncontrollably, the calculation is wrong. It's
<v Speaker 3>a fatal error.
<v Speaker 2>So what did the Reichen team do with this technique?
<v Speaker 3>They performed this Ramsey experiment over and over again, but
<v Speaker 3>while our quibbet's clock was ticking, they were deliberately moving
<v Speaker 3>the neighboring electron back and forth using its control gate.
<v Speaker 2>They poked the bear just to see if it would growl,
<v Speaker 2>and it did.
<v Speaker 3>They observed a clear, perfectly correlated shift in the quibits
<v Speaker 3>frequency that matched the movement of the neighbor electron. When
<v Speaker 3>the neighbor moved closer, the frequency went up by a
<v Speaker 3>specific amount. When it moved away, it went down. They
<v Speaker 3>mapped it out precisely, so.
<v Speaker 2>It's not theoretical anymore.
<v Speaker 3>It's a hard number, a quantified, measured effect. This is
<v Speaker 3>the first time this specific mechanism has been directly measured
<v Speaker 3>and isolated.
<v Speaker 2>How bad was it? Was it a tiny blip, something
<v Speaker 2>you could maybe ignore.
<v Speaker 3>According to the paper, the shift is large enough to
<v Speaker 3>cause a considerable quibit error rate.
<v Speaker 2>A considerable error rate. That doesn't sound good.
<v Speaker 3>It's not We measure the quality of our quantum operations
<v Speaker 3>in terms of fidelity, a perfect operation has one hundred
<v Speaker 3>percent fidelity. To make a fault tolerant quantum computer work,
<v Speaker 3>you need operation fidelities of ninety nine point nine percent
<v Speaker 3>or really ninety nine point nine nine percent or higher,
<v Speaker 3>so you.
<v Speaker 2>Can only have one error in ten thousand operations at most.
<v Speaker 3>This cross stalk mechanism introduces errors that can easily drop
<v Speaker 3>you well below that threshold.
<v Speaker 2>So this isn't just background here so we can filter out. Yeah,
<v Speaker 2>this is a potential showstopper. If you can't fix this,
<v Speaker 2>you can't scale up to a large number of quibits.
<v Speaker 3>Show stopper might be too strong, but it is definitely
<v Speaker 3>a gatekeeper. It's a problem you cannot pass until you
<v Speaker 3>solve it. If you try to build a chip with
<v Speaker 3>a million quibbits and they are all shifting each other's
<v Speaker 3>frequencies every time.
<v Speaker 2>They move, you just have a million sources of noise.
<v Speaker 3>You don't have a computer. You have a very expensive,
<v Speaker 3>very cold random number generator.
<v Speaker 2>Which is useful for casinos maybe, but not for curing
<v Speaker 2>cancer exactly.
<v Speaker 3>This is what the paper refers to when it says
<v Speaker 3>this problem prevents the silicon quantum computing community from realizing
<v Speaker 3>a large scale device. It's a roadblock.
<v Speaker 2>So we have identified the villain. It's this unlucky combination
<v Speaker 2>of the crowded neighborhood, the Kulam interaction, and the very
<v Speaker 2>sensitive amplifier that we put in for control, the micromagnet.
<v Speaker 3>That's the mechanism.
<v Speaker 2>Now. Usually when engineers find a source of noise, they
<v Speaker 2>try to eliminate it. They add shielding, or they redesign
<v Speaker 2>the parts to be further away. Can we do that here?
<v Speaker 3>Well, moving them further away as a non starter. You
<v Speaker 3>lose the ability to entangle them, which you need for computation.
<v Speaker 2>Okay, so that's out. What about shielding? Can you put
<v Speaker 2>a little wall between them?
<v Speaker 3>Shielding electric fields at the Mano scale inside a solid
<v Speaker 3>piece of silicon is incredibly difficult. It's not really a
<v Speaker 3>practical solution right now. So Kobayashi and his team suggest
<v Speaker 3>a different.
<v Speaker 4>Approach in the paper what's the plan?
<v Speaker 3>The plan has two parts. The first is mitigate. The
<v Speaker 3>second and more exciting part is exploitation.
<v Speaker 2>Mitigation and exploitation. Okay, let's start with mitigostion first.
<v Speaker 3>The key discovery here is that we have quantified it.
<v Speaker 3>We have measured it. We can predict it.
<v Speaker 2>Right. It's not random noise, it's correlated noise.
<v Speaker 3>That is the keyword. Random noise is a nightmare to fight.
<v Speaker 3>It's like trying to have a conversation in a room
<v Speaker 3>with a thousand people talking at once. But if the
<v Speaker 3>noise is correlated, if I know that every single time
<v Speaker 3>neighbor A moves to the left, my quibbit B is
<v Speaker 3>going to shift its frequency by exactly five megahertz, then
<v Speaker 3>I can do something about it.
<v Speaker 2>You can compensate for it exactly.
<v Speaker 3>This leads us to a strategy called feed forward correction.
<v Speaker 3>The classical computer that controls the quantum ship can be
<v Speaker 3>programmed with this knowledge.
<v Speaker 2>How would that work in practice?
<v Speaker 3>When the control system sends a command pulse to the
<v Speaker 3>gate of neighbor A telling it to move, it can,
<v Speaker 3>at the exact same time send a small corresponding correction
<v Speaker 3>pulse to the gate of quibbet B.
<v Speaker 2>It's like canceling headphones.
<v Speaker 3>That's a fantastic analogy noise. Canceling headphones have a microphone
<v Speaker 3>that listens to the outside sound, and then the electronics
<v Speaker 3>generate an inverse sound wave to cancel it out in
<v Speaker 3>your ear. Here we don't even need to listen. We
<v Speaker 3>know the sound the frequency shift is coming because we're
<v Speaker 3>the ones causing it by moving the neighbor. So we
<v Speaker 3>can preemptively adjust the dial the control voltage on quibit
<v Speaker 3>B to counteract the effect precisely as it happens.
<v Speaker 2>So you fix the error before it even has a
<v Speaker 2>chance to mess up the calculation.
<v Speaker 3>Ideally, yes, you can program these corrections right into the
<v Speaker 3>gate operations. It adds complexity to the control software, but
<v Speaker 3>it's a viable path forward.
<v Speaker 2>Okay, that makes sense. That's the mitigation was the exploitation part.
<v Speaker 3>This is the part that I find most exciting. Kobayashi's
<v Speaker 3>team suggests that instead of just canceling this effect, we might.
<v Speaker 4>Be able to use it, use the noise.
<v Speaker 2>How well.
<v Speaker 3>Think about what this crosstalk effect actually represents. It is
<v Speaker 3>a form of coupling. It is a connection between two
<v Speaker 3>quibbits that were not supposed to be directly interacting.
<v Speaker 2>Quibot A is talking to quibt B, even if we
<v Speaker 2>didn't want it to initially, it's an accidental phone line.
<v Speaker 3>And in quantum logic, what do we need to do?
<v Speaker 3>We need to perform two quibit gits. These are operations
<v Speaker 3>where the state of one quibbot conditionally changes the state
<v Speaker 3>of another. That is how you build logic, That is
<v Speaker 3>how you build an algorithm like Shores algorithm for factoring.
<v Speaker 2>So normally you have to work really hard to force
<v Speaker 2>them to talk to each other in a controlled way.
<v Speaker 3>Yes, a common way is to physically lower the electric
<v Speaker 3>potential barrier between them, let their electron wave functions overlap,
<v Speaker 3>and let them interact via something called the exchange interaction.
<v Speaker 3>It's tricky and has to be managed very carefully. But here,
<v Speaker 3>but here, the micromagnet has accidentally given us a new
<v Speaker 3>long range interaction. They are talking to each other via
<v Speaker 3>the magnetic gradient and the electric field even when their
<v Speaker 3>wave functions aren't touching.
<v Speaker 2>So the researchers are basically saying, don't mute the crosstalk.
<v Speaker 2>Let's try to turn it into a high fidelity phone line.
<v Speaker 3>That's it. The paper suggests this could lead to a
<v Speaker 3>new class of quibit operations. If you can control this
<v Speaker 3>energy shift precisely, if you can turn it on and
<v Speaker 3>off by moving the neighbor electron, you could potentially use
<v Speaker 3>it to perform logic gates between quibots that are not
<v Speaker 3>immediate neighbors.
<v Speaker 2>So you could do a calculation between quibot A and
<v Speaker 2>QUIBTC using quibotb's position as the switch.
<v Speaker 3>That's the kind of new capability it might open up.
<v Speaker 3>You can turn this bug into a feature.
<v Speaker 2>That is the ultimate engineering judo move, use the force
<v Speaker 2>of the problem itself to solve the problem.
<v Speaker 3>It connects to the broader history of developing these silicon quibots.
<v Speaker 3>Time and again we're finding that things we initially thought
<v Speaker 3>were bugs, things like subtle variations in the silicon crystal,
<v Speaker 3>the valley splitting, or roughness at the silicon oxide interface.
<v Speaker 3>And now this micromagnet cross stock can sometimes be understood, controlled,
<v Speaker 3>and engineered into resources.
<v Speaker 2>If you understand the physics deep enough.
<v Speaker 3>If you understand the physics deep enough, it's a testament
<v Speaker 3>to how mature the field is getting. We're moving from
<v Speaker 3>just trying to get one quibot to work to understanding
<v Speaker 3>the complex multi quibot interactions in a large system.
<v Speaker 2>It seems like a constant race between the problems we
<v Speaker 2>discover and the solutions we invent.
<v Speaker 3>It is the silicon quantum computing community. As the text calls.
<v Speaker 3>It is in a fierce but friendly race against the
<v Speaker 3>superconducting folks and the trapped ion folk.
<v Speaker 2>Different horses in the same race.
<v Speaker 3>Right, and silicon has this massive theoretical advantage of manufacturability,
<v Speaker 3>but it has these complex material physics challenges that the
<v Speaker 3>other platforms don't. This study is a major step in
<v Speaker 3>clearing the fog around one of those challenges.
<v Speaker 2>So let's just zoom back out for a second. We
<v Speaker 2>started with single electron shivering in the cold. We built
<v Speaker 2>an electric jail cell to trap it. We gave it
<v Speaker 2>a special magnet to control it. We then realized that
<v Speaker 2>magnet made it listen to its neighbors too much. We
<v Speaker 2>then figured out exactly how it was listening, measured the volume,
<v Speaker 2>and now we are planning to turn that unwonted listening
<v Speaker 2>into a controlled conversation.
<v Speaker 3>That is a beautiful summary of the scientific process. Right there,
<v Speaker 3>we went from identifying noise to potentially creating a new
<v Speaker 3>kind of signal.
<v Speaker 2>Why does this matter to the person listening right now?
<v Speaker 2>They might be driving to work or doing the dishes.
<v Speaker 2>Why should they care about the Larmer frequency of a
<v Speaker 2>single electron spin?
<v Speaker 3>It matters because it peels back the curtain on how
<v Speaker 3>progress and technology actually happens. We often get this very clean,
<v Speaker 3>linear narrative of progress. We think of technology as just
<v Speaker 3>this inevitable march of better, faster, cheaper.
<v Speaker 2>But it's not like that at all.
<v Speaker 3>It's not. It's a messy, winding path. It's a series
<v Speaker 3>of roadblocks. It's discovering that your clever solution, the micromagnet,
<v Speaker 3>caused a brand new problem, the crosstalk. It's the painstaking,
<v Speaker 3>unglamorous work of measuring and characterizing that problem.
<v Speaker 2>It's less of a Eureka moment and more of a huh,
<v Speaker 2>that's weird moment, followed by a year of work.
<v Speaker 3>A year of very careful work in a very cold room.
<v Speaker 3>And it's the realization that you cannot just build it
<v Speaker 3>and hope it works. You have to understand the fundamental
<v Speaker 3>physics of the noise, of the imperfections.
<v Speaker 2>And it really grounds the hype around quantum computing. This
<v Speaker 2>isn't magic. It's plumbing. It's just extremely high tech, atomic
<v Speaker 2>scale plumbing with very leaky pipes.
<v Speaker 3>It is, and for anyone interested in the future of computing,
<v Speaker 3>watching how the community solves these very specific hurdles like
<v Speaker 3>the crosstalk challenge that Reyken just mapped out is a
<v Speaker 3>much better preview of the future than any marketing material.
<v Speaker 3>This is where the real work is happening.
<v Speaker 2>If they do solve this, if they can either cancel
<v Speaker 2>this cross talk perfectly or turn it into a reliable tool,
<v Speaker 2>does Silicon win the race?
<v Speaker 3>Win is a strong word. There might be room for
<v Speaker 3>multiple technologies, but if they solve this, Silicon becomes a
<v Speaker 3>terrifyingly strong competitor. Why terrifyingly because if you can handle
<v Speaker 3>this cross talk, and you can solve a few other
<v Speaker 3>known materials issues, and you can make these things using
<v Speaker 3>standard semiconductor foundaries, the scaling potential is just massive. You
<v Speaker 3>could genuinely have millions of high quality quibbets on a
<v Speaker 3>single chip the size of your.
<v Speaker 2>Thumbail, whereas a golden chandeliers, as beautiful as they are,
<v Speaker 2>take up a whole room for a few hundred or
<v Speaker 2>a thousand quibbits.
<v Speaker 3>Exactly. Density matters for building a truly large scale error
<v Speaker 3>corrected machine.
<v Speaker 2>So I want to leave the listener with the final
<v Speaker 2>thought something to chew on, and it's about this idea
<v Speaker 2>of interconnectedness. One we tend to think of isolation as
<v Speaker 2>the ideal state. Right in our daily lives. We want
<v Speaker 2>peace and quiet to focus. In quantum computing, the mantra
<v Speaker 2>is always isolate your quibit so it doesn't decohere and
<v Speaker 2>lose its information.
<v Speaker 3>Isolation preserves the quantum state. That's rule number one.
<v Speaker 2>But a computer that is perfectly isolated does nothing. It's
<v Speaker 2>just a perfect box of secrets. To compute. To do
<v Speaker 2>anything useful, you must connect. You must have interactions.
<v Speaker 3>Information is physical, and physics is at its heart the
<v Speaker 3>study of interactions.
<v Speaker 2>So this struggle that the Reichen team is documenting, this
<v Speaker 2>push and pull between keeping the quibit safe from its
<v Speaker 2>ne neighbors but also needing it to talk to its neighbors.
<v Speaker 2>It's not just an engineering problem. It feels like the
<v Speaker 2>fundamental tension of the universe, doesn't it in.
<v Speaker 3>A way it is for any information processing system. It
<v Speaker 3>is the balance between coherence and coupling. Too much isolation
<v Speaker 3>and your system is useless. It can't process anything. Too
<v Speaker 3>much connection and it just becomes noise. Everything washes out.
<v Speaker 3>Finding that sweet spot, that perfectly controlled interaction is the
<v Speaker 3>whole game.
<v Speaker 2>And Reichen just showed us exactly where that line is
<v Speaker 2>drawn for this type of device.
<v Speaker 3>They gave us the map of a new part of
<v Speaker 3>that territory. Now the community has to figure out how
<v Speaker 3>to navigate it.
<v Speaker 2>Well, my brain is suitably entangled. Thank you for walking
<v Speaker 2>us through the nanoverse today. This was fascinating.
<v Speaker 3>It was a pleasure. It's always good to look at
<v Speaker 3>the small things that make the big things possible.
<v Speaker 2>We'll see you in the next one.
<v Speaker 3>Keep learning, stay curious.