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