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Quantum Computer Breakthrough: The Crosstalk Problem in Silicon Qubits · The Quark Side - Quantum Physics Podcast

• 0:01
  Speaker 1: Welcome to the quarre Side Quantum Physics Podcast, an exploration
• 0:05
  Speaker 1: of the fundamental structure of reality. We're quantum laws govern matter, energy,
• 0:11
  Speaker 1: and information. Here, uncertainty is a feature, not a flaw,
• 0:16
  Speaker 1: and understanding begins at the smallest scales.
• 0:27
  Speaker 2: Welcome back today. We are going to try to wrap
• 0:29
  Speaker 2: our heads around something that is simultaneously incredibly small and
• 0:34
  Speaker 2: absolutely massive in terms of its implications.
• 0:37
  Speaker 3: That's a good way to put it.
• 0:38
  Speaker 2: We're talking about the future of computing, but you know,
• 0:40
  Speaker 2: not the shiny marketing brochure version of it.
• 0:43
  Speaker 3: No, we are getting into the messy reality, the plumbing
• 0:47
  Speaker 3: behind the palace, so to speak.
• 0:49
  Speaker 2: We always hear the grand promise, right We hear that
• 0:52
  Speaker 2: quantum computers are going to solve these impossible problems revolutionized medicine,
• 0:56
  Speaker 2: crack encryption codes that would take a normal computer a
• 0:59
  Speaker 2: billion years to solve. It's painted is this sci fi
• 1:03
  Speaker 2: utopia where the machine just knows the answer.
• 1:07
  Speaker 3: It is the ultimate promise of computational power. It's elaked
• 1:10
  Speaker 3: not just a step. But the gap between that promise
• 1:12
  Speaker 3: and where we are today is well, it's filled with
• 1:15
  Speaker 3: some of the most difficult engineering problems in human history.
• 1:18
  Speaker 2: And today, we are going to zoom in on one
• 1:20
  Speaker 2: of those problems.
• 1:21
  Speaker 3: Way way in, right down to the component.
• 1:23
  Speaker 2: Level, past the headlines, past the hype, and right down
• 1:27
  Speaker 2: to the microscopic reality of what it actually takes to
• 1:30
  Speaker 2: build one of these machines. Because it turns out building
• 1:32
  Speaker 2: the future isn't just about big ideas. Wow, It's about
• 1:36
  Speaker 2: controlling individual electrons in spaces that are only tens of
• 1:41
  Speaker 2: nanometers wide.
• 1:42
  Speaker 3: It is an engineering challenge as much as it is
• 1:44
  Speaker 3: a physics challenge. You are you're literally wrestling with nature
• 1:48
  Speaker 3: at its most fundamental level. Howso well, you're trying to
• 1:52
  Speaker 3: force sub atomic particles to behave against their natural chaotic instincts,
• 1:57
  Speaker 3: to hold them still and get them to do useful
• 1:59
  Speaker 3: work for you.
• 2:00
  Speaker 2: And we have a specific tension to look at today.
• 2:02
  Speaker 2: We want to pack these things called quantum dots closer
• 2:05
  Speaker 2: and closer together to build powerful computers, because a computer
• 2:09
  Speaker 2: with two bits isn't very useful, not at all. But
• 2:11
  Speaker 2: new research shows that these tiny neighbors are starting to
• 2:14
  Speaker 2: well shout over each other.
• 2:15
  Speaker 3: Shouting is a very vivid way to put it. In
• 2:18
  Speaker 3: the quantum world, shouting is more about energy exchange and
• 2:20
  Speaker 3: frequency shows but the effect is the same distraction, interference noise.
• 2:26
  Speaker 2: And we're basing today's discussion on a brand news study
• 2:30
  Speaker 2: who was published just yesterday February twelve, twenty twenty six,
• 2:33
  Speaker 2: and physical review applied.
• 2:35
  Speaker 3: Yes, this is coming out of the Reichen Center for
• 2:37
  Speaker 3: Quantum Computing. The research was led by Takashi Kobyashi and
• 2:40
  Speaker 3: his team.
• 2:40
  Speaker 2: And Reichen is a big deal in this space.
• 2:42
  Speaker 3: Oh absolutely. Reraichen is a heavyweight in this field. So
• 2:45
  Speaker 3: when they publish a paper on noise mechanisms, the community,
• 2:49
  Speaker 3: you know it, sits up and listens.
• 2:50
  Speaker 2: So what is our mission here? We aren't looking at
• 2:53
  Speaker 2: a breakthrough where everything works perfectly. We are looking at
• 2:56
  Speaker 2: a hurdle that was just discovered or at least just
• 3:00
  Speaker 2: precisely measured.
• 3:01
  Speaker 3: That's the key.
• 3:02
  Speaker 2: The goal is to understand what happens when quantum bits
• 3:05
  Speaker 2: or quibbits get too close for comfort.
• 3:08
  Speaker 3: It's a story about the growing pains of technology. To
• 3:11
  Speaker 3: solve a problem, you first have to measure it precisely.
• 3:14
  Speaker 3: That is what Reichen has done. They haven't just found
• 3:17
  Speaker 3: a problem, They've put a number on it, They've quantified it.
• 3:20
  Speaker 2: So we're building the supercomputer of the future. But the
• 3:23
  Speaker 2: parts are arguing with each other.
• 3:25
  Speaker 3: In a manner of speaking. Yes, but understanding that argument
• 3:30
  Speaker 3: is the key to silencing it, or perhaps, as we'll
• 3:33
  Speaker 3: see later, even using it.
• 3:35
  Speaker 2: Okay, let's get into the nuts and bolts stand Before
• 3:37
  Speaker 2: we can really talk about the noise, we need to
• 3:39
  Speaker 2: understand the instrument. The study focuses on something called quantum dots.
• 3:44
  Speaker 2: Now I see that term, and my first thought is
• 3:47
  Speaker 2: those high end TV screens with the really vibrant colors.
• 3:50
  Speaker 2: Are we talking about the same thing.
• 3:52
  Speaker 3: That's a great question, and it's a common point of confusion.
• 3:55
  Speaker 3: It's the same underlying physics, yes, but for a completely
• 3:58
  Speaker 3: different application.
• 3:59
  Speaker 2: Okay.
• 4:00
  Speaker 3: In a TV a quantum dot is a semiconductor nanocrystal
• 4:04
  Speaker 3: that's designed to glow a very specific pure color when
• 4:08
  Speaker 3: you hit it with light or electricity. It's an emitter.
• 4:11
  Speaker 2: It makes pretty colors, it makes very.
• 4:12
  Speaker 3: Pretty, very precise colors. But in a quantum computer, a
• 4:16
  Speaker 3: quantum dot is a trap.
• 4:18
  Speaker 2: A trap.
• 4:18
  Speaker 3: Think of it as a potential, well, a tiny, tiny
• 4:22
  Speaker 3: region in a semiconductor material where we can confine a
• 4:25
  Speaker 3: single charged particle, in this case a single electron.
• 4:29
  Speaker 2: So you're catching one electron and holding it in place.
• 4:31
  Speaker 3: That's the entire goal.
• 4:32
  Speaker 2: Okay, so why silicon. I feel like when people see
• 4:37
  Speaker 2: pictures of quantum computers, they see those beautiful, big, golden chandeliers.
• 4:42
  Speaker 2: The superconducting quantum computers from Google or IBM, those don't
• 4:46
  Speaker 2: look like chips. But Reichen is using silicon.
• 4:50
  Speaker 3: That is a crucial distinction, and it really defines the
• 4:52
  Speaker 3: different competing approaches. The chandelier computers use superconducting circuits. Those
• 4:58
  Speaker 3: are in electronic terms, quite lo large. They're macroscopic loops
• 5:02
  Speaker 3: of metal cooled to extreme temperatures.
• 5:05
  Speaker 2: And what Ryichan is doing well, Reichen.
• 5:07
  Speaker 3: Is working on what's known as silicon's spin quibits. Looks
• 5:10
  Speaker 3: much more like the chips you'd find in your laptop.
• 5:12
  Speaker 2: Or your phone. And the advantage of that is the.
• 5:15
  Speaker 3: Hope is that because we already have, you know, fifty
• 5:17
  Speaker 3: years of experience and trillions of dollars of infrastructure for
• 5:21
  Speaker 3: manufacturing billions of transistors on a silicon wafer, right the
• 5:24
  Speaker 3: whole semiconductor industry exactly, the hope is we can eventually
• 5:28
  Speaker 3: leverage all of that to manufacture billions of kribbits the
• 5:31
  Speaker 3: same way.
• 5:32
  Speaker 2: So it's a bed on scalability. If we can make
• 5:34
  Speaker 2: it work in silicon, we can make millions of them
• 5:37
  Speaker 2: and maybe make them cheap.
• 5:38
  Speaker 3: That is the long term vision. But making a standard
• 5:42
  Speaker 3: transistor is very very different from making a quantum dot
• 5:45
  Speaker 3: for a quibit. How so, well, in a transistor, you're
• 5:49
  Speaker 3: essentially just creating a switch for a current. You're controlling
• 5:52
  Speaker 3: a river of electrons flowing through a channel.
• 5:55
  Speaker 4: On or off a whole crowd of them, a whole crowd.
• 5:58
  Speaker 3: But in a quantum dot, you need to isolate one
• 6:02
  Speaker 3: single electron from that river, just one, just one, and
• 6:05
  Speaker 3: you need to hold it there very gently without disturbing
• 6:09
  Speaker 3: its fragile quantum state.
• 6:11
  Speaker 2: Okay, so how do you build this trap? You can't
• 6:13
  Speaker 2: exactly use tiny tweezers.
• 6:15
  Speaker 3: No, you use electric fields. It's actually quite clever. You
• 6:17
  Speaker 3: fabricate tiny metal gates electrodes on top of the silicon.
• 6:21
  Speaker 2: Wafer, like the gates in a normal transistor.
• 6:23
  Speaker 3: Very similar, but the arrangement is different. When you apply
• 6:26
  Speaker 3: a negative voltage to these metal gates, they create an
• 6:30
  Speaker 3: electric field that repels the electrons and the silicon underneath.
• 6:34
  Speaker 2: So you're pushing them away.
• 6:35
  Speaker 3: You're pushing them away and by arranging these gates in
• 6:38
  Speaker 3: a very specific pattern, like a little corral, you can
• 6:41
  Speaker 3: create a small island, a box of low energy surrounded
• 6:45
  Speaker 3: by high energy walls. An electron can get trapped in
• 6:48
  Speaker 3: that box.
• 6:49
  Speaker 2: So it's like using invisible force fields to build a
• 6:52
  Speaker 2: jail cell for an electron.
• 6:53
  Speaker 3: A very very small jail cell. We are talking about
• 6:56
  Speaker 3: structures that are just tens of nanometers across.
• 6:59
  Speaker 2: And once the electron is in there, what are we
• 7:01
  Speaker 2: actually using? As the bit in my laptop, it's voltage
• 7:04
  Speaker 2: being high or low that represents a one or a zero.
• 7:07
  Speaker 2: What is it here?
• 7:08
  Speaker 3: Here? We are using a fundamental quantum property of the electron.
• 7:12
  Speaker 3: We're using its spin spin.
• 7:15
  Speaker 2: This is one of those chronum terms that everyone uses,
• 7:17
  Speaker 2: but it's always a bit fuzzy. Is the electron actually
• 7:20
  Speaker 2: spinning like a tiny top?
• 7:22
  Speaker 3: Not literally? No, that's just a helpful classical analogy. And
• 7:25
  Speaker 3: electron is a point particle. It has no physical size
• 7:28
  Speaker 3: to spin around.
• 7:28
  Speaker 2: So what is it? Then?
• 7:29
  Speaker 3: It's an intrinsic property like its charge or its mass.
• 7:33
  Speaker 3: It has an intrinsic angular momentum and an associated magnetic
• 7:37
  Speaker 3: moment It behaves as if it were a tiny spinning bar.
• 7:40
  Speaker 4: Magnet a tiny magnet eject and.
• 7:42
  Speaker 3: That tiny magnet can point in different directions. For our purposes,
• 7:46
  Speaker 3: we care about whether it's pointing up or down relative
• 7:50
  Speaker 3: to an external magnetic field we apply.
• 7:52
  Speaker 2: Okay, so spin up is a one and spin down
• 7:54
  Speaker 2: is a zero. Is that the basic idea.
• 7:57
  Speaker 3: That is the basis of it. Yes, that's how you
• 7:59
  Speaker 3: define your computationational lysis states. But because it's a quantum system,
• 8:02
  Speaker 3: it's not that simple. It's not that simple. It can
• 8:05
  Speaker 3: also exist in a superposition of both up and down
• 8:08
  Speaker 3: at the same time until you measure it. That's where
• 8:11
  Speaker 3: the real power comes from, right.
• 8:12
  Speaker 2: The classic quantum weirdness. So, just to recap, we have
• 8:16
  Speaker 2: a silicon chip. We have tiny electric gates creating a trap.
• 8:20
  Speaker 2: We catch one single electron. We use its magnetic spin
• 8:24
  Speaker 2: as our data storage.
• 8:25
  Speaker 3: Correct. But here's the problem, the core vulnerability of this
• 8:28
  Speaker 3: whole approach. That spin state is incredibly fragile. Well canness
• 8:32
  Speaker 3: it up almost anything. The biggest enemy is thermal energy
• 8:35
  Speaker 3: just heat. Heat is just atoms and electrons jiggling around.
• 8:39
  Speaker 3: If the environment is too hot, the electron shakes, it
• 8:42
  Speaker 3: interacts with the lattice of silicon atoms around it, and
• 8:44
  Speaker 3: the spin state just flips randomly. Your data is lost instantly,
• 8:49
  Speaker 3: your one becomes a zero, or your superposition just collapses.
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  Speaker 3: The information is gone. This is called decoherence.
• 8:56
  Speaker 2: So you have to freeze it.
• 8:57
  Speaker 3: You have to freeze it to near absolute zero. These
• 9:00
  Speaker 3: chips don't just sit on a lab bench. They live
• 9:03
  Speaker 3: inside a complex machine called a dilution refrigerator, operating at
• 9:06
  Speaker 3: temperatures in the millikelvin range.
• 9:08
  Speaker 2: How cold is that?
• 9:09
  Speaker 3: It is significantly colder there than in deep space.
• 9:13
  Speaker 2: So I should picture this a giant humming silver cylinder
• 9:17
  Speaker 2: in a lab. Inside it, there are layers and layers
• 9:20
  Speaker 2: of gold and copper plates that get progressively colder and colder,
• 9:25
  Speaker 2: and at the very bottom, in the deepest cold sits
• 9:28
  Speaker 2: this tiny silicon chip with these nanometer scale traps.
• 9:32
  Speaker 3: That is the exact picture, and that is where the
• 9:34
  Speaker 3: racing team is working. They aren't just looking at one dot.
• 9:37
  Speaker 3: Though one quibbit is a science experiment, a computer needs many, and.
• 9:42
  Speaker 2: This is where we hit the scaling problem, the crowded house,
• 9:44
  Speaker 2: as we called it. Right.
• 9:45
  Speaker 3: To perform calculations, to run in algorithm, quibits need to
• 9:49
  Speaker 3: interact with each other in a controlled way. You need
• 9:51
  Speaker 3: to be able to entangle them.
• 9:53
  Speaker 2: Which means they can't be miles apart.
• 9:55
  Speaker 3: They need to be close, very close.
• 9:56
  Speaker 2: How close were we're talking.
• 9:57
  Speaker 3: In the device and this study, the which is the
• 10:01
  Speaker 3: distance from the center of one quantum dot to the
• 10:03
  Speaker 3: center of the next, is about one hundred to one
• 10:05
  Speaker 3: hundred and fifty nanometers.
• 10:07
  Speaker 2: That is incredibly tight.
• 10:08
  Speaker 3: It is extremely tight. And when you pack charged particles
• 10:11
  Speaker 3: that close together, you run into a fundamental force of nature,
• 10:15
  Speaker 3: the kulum interaction.
• 10:17
  Speaker 2: Remind us what that is, It's.
• 10:18
  Speaker 3: The most basic electric force. Like charges repel, opposite charges attract.
• 10:23
  Speaker 3: So two electrons, two electrons are both negatively charged. They
• 10:27
  Speaker 3: hate each other. They are constantly trying to push each
• 10:30
  Speaker 3: other away.
• 10:31
  Speaker 2: So i have two of these quantum dots right next
• 10:33
  Speaker 2: to each other, and I've got one electron in each one.
• 10:36
  Speaker 2: They're constantly pushing against the walls of their little jail cells,
• 10:40
  Speaker 2: trying to get away from their neighbor.
• 10:42
  Speaker 3: Ideally, the electric field walls of the trap are strong
• 10:45
  Speaker 3: enough to hold them in place, but they still feel
• 10:48
  Speaker 3: each other's presence. The force is still there. Okay, So
• 10:51
  Speaker 3: if the electron in Dot A shifts its position slightly,
• 10:54
  Speaker 3: and it can. It has a little bit of wiggle room,
• 10:57
  Speaker 3: maybe just by a nanometer.
• 10:58
  Speaker 4: The electron in dot beef that the electron.
• 11:01
  Speaker 3: In dot B feels a change in the repulsive force.
• 11:04
  Speaker 3: The push gets a little stronger or a little weaker.
• 11:06
  Speaker 2: It's like if you are standing in a small room
• 11:09
  Speaker 2: with a powerful magnet strapped to your chest and someone
• 11:11
  Speaker 2: in the next room had one too, even if there's
• 11:14
  Speaker 2: a wall, if they take a step towards that wall,
• 11:16
  Speaker 2: you feel a push.
• 11:17
  Speaker 3: That is a perfect analogy, and this brings us right
• 11:21
  Speaker 3: to the core of the Reichen discovery. Takashi Kobayashi and
• 11:25
  Speaker 3: his team, we're investigating this exact scenario. They wanted to know,
• 11:29
  Speaker 3: does that push from the neighbor mess up the information
• 11:33
  Speaker 3: stored in the spin of our kubit.
• 11:35
  Speaker 2: And the answer I'm going to assume is yes, otherwise
• 11:38
  Speaker 2: we wouldn't be doing this whole conversation.
• 11:39
  Speaker 3: The answer is a definitive yes. But the mechanism, the
• 11:43
  Speaker 3: how is the really fascinating part because it's not a
• 11:46
  Speaker 3: direct effect. It involves a component we haven't even mentioned yet. Oh,
• 11:49
  Speaker 3: the micro magnet.
• 11:50
  Speaker 2: The micromagnet. Okay, wait, let's back up. We have electric
• 11:54
  Speaker 2: gates to trap the electron. We have the electron itself
• 11:57
  Speaker 2: acting like a tiny magnet with its spin, And why
• 12:01
  Speaker 2: on earth do we need another magnet.
• 12:03
  Speaker 3: This is a great question, and it gets right to
• 12:05
  Speaker 3: the heart of why silicon spin quibets are so hard
• 12:08
  Speaker 3: to build and control. Remember, what we want to do
• 12:11
  Speaker 3: is control the spin of the electron.
• 12:14
  Speaker 2: Right flip it from up to down, or put it
• 12:16
  Speaker 2: in this superposition exactly.
• 12:18
  Speaker 3: Now, spin responds to magnetic fields, so the obvious way
• 12:22
  Speaker 3: to control it would be to zap it with a
• 12:24
  Speaker 3: little magnetic pulse.
• 12:25
  Speaker 2: Seems simple enough.
• 12:26
  Speaker 3: It is if you only have one quibbit. But remember
• 12:29
  Speaker 3: we're in a crowded house. If you want to build
• 12:31
  Speaker 3: a real computer, you have thousands, maybe millions, of these
• 12:34
  Speaker 3: dots packed together, and if I.
• 12:36
  Speaker 2: Turn on a magnet, everything nearby feels it. It's hard
• 12:39
  Speaker 2: to target just one dot without disturbing all its neighbors.
• 12:42
  Speaker 3: Precisely, it's called addressability, and it's a huge problem. You'd
• 12:46
  Speaker 3: have magnetic cross stock everywhere. So the engineer's thought, is
• 12:50
  Speaker 3: there a better way. We are very good at generating
• 12:53
  Speaker 3: localized electric fields. We can switch voltages on those tiny
• 12:56
  Speaker 3: gates incredibly fast and with pinpoint precision.
• 13:00
  Speaker 2: So they wanted to control the spin using electricity instead
• 13:03
  Speaker 2: of magnetism.
• 13:04
  Speaker 3: That was the goal.
• 13:05
  Speaker 2: But can you even do that? I thought you said
• 13:06
  Speaker 2: spin doesn't talk to electricity.
• 13:08
  Speaker 3: It doesn't, not directly. You need a translator. You need
• 13:12
  Speaker 3: a physical mechanism that couples the electrons position which electricity controls,
• 13:16
  Speaker 3: to its spin state. This is a real physical effect
• 13:20
  Speaker 3: called spin orbit coupling. Okay, in some materials like gallium arsenide.
• 13:24
  Speaker 3: This effect happens naturally and is quite strong. In silicon,
• 13:29
  Speaker 3: which we want to use for manufacturing reasons, it's naturally
• 13:32
  Speaker 3: very very weak.
• 13:33
  Speaker 2: So we have to fake it.
• 13:34
  Speaker 3: We have to engineer it. We create something called synthetic
• 13:36
  Speaker 3: spin orbit coupling.
• 13:37
  Speaker 2: Synthetic spin orbit coupling.
• 13:39
  Speaker 3: That sounds expensive, it's clever engineering. This is why they
• 13:42
  Speaker 3: install the micromagnet. They physically place a tiny strip of
• 13:46
  Speaker 3: magnetic material like cobalt, right on top of the quantum dots.
• 13:50
  Speaker 3: But this magnet isn't designed to create a uniform field.
• 13:53
  Speaker 3: That's the key. It's designed to create a gradient.
• 13:56
  Speaker 2: A gradient, what do you mean by that? So the
• 13:58
  Speaker 2: magnetic field is stronger on one side of the day the.
• 14:00
  Speaker 3: Other exactly that. Imagine the quantum dot is a small room.
• 14:04
  Speaker 3: The magnetic field at the left wall is let's say,
• 14:06
  Speaker 3: one hundred units strong. The magnetic field at the right
• 14:09
  Speaker 3: wall is one hundred and five units strong. It's a slope.
• 14:12
  Speaker 3: It's not flat.
• 14:14
  Speaker 2: Okay, I have a magnetic slope across my tiny room.
• 14:17
  Speaker 2: Why no?
• 14:18
  Speaker 3: Remember the frequency at which the electrons spin processes, the
• 14:22
  Speaker 3: speed at which it wobbles like a top, depends directly
• 14:26
  Speaker 3: on the strength of the magnetic field it is sitting in.
• 14:28
  Speaker 3: This is called the Larmer frequency.
• 14:30
  Speaker 2: So if the electron is sitting on the left side
• 14:32
  Speaker 2: of the room, it spin wobbles it's speed one hundred.
• 14:35
  Speaker 2: If a moving to the right side, it wobbles its
• 14:37
  Speaker 2: speed one oh five.
• 14:38
  Speaker 3: Precisely. Now you see the trick. We can use our
• 14:40
  Speaker 3: electric gates to gently nudge the electrons position. If we
• 14:44
  Speaker 3: apply a little voltage, we can push it from the
• 14:46
  Speaker 3: left side of the dot to the right.
• 14:48
  Speaker 2: Huh. And by pushing it we force it to move
• 14:50
  Speaker 2: into a stronger part of the magnetic.
• 14:52
  Speaker 3: Field, which changes its spin speed.
• 14:54
  Speaker 2: Uh huh. So we are using electricity to move it,
• 14:56
  Speaker 2: but it's the motion through that pre engineered magnetic gradient
• 15:00
  Speaker 2: that actually changes the spin.
• 15:02
  Speaker 3: You've got it. The micromagnet acts as a translator. It
• 15:05
  Speaker 3: converts an electric signal, which is a change in position,
• 15:07
  Speaker 3: into a magnetic control, which is a change in spin rotation.
• 15:11
  Speaker 4: Skeeed.
• 15:12
  Speaker 3: It acts as a lever. It makes the electrons spin
• 15:15
  Speaker 3: highly sensitive to its physical location.
• 15:17
  Speaker 2: That sounds brilliant. It's a really clever workaround problem solved right.
• 15:21
  Speaker 3: It is brilliant. It's a technique called electric dipole spin
• 15:24
  Speaker 3: resonance or EDSR, and it allows for very fast, localized
• 15:29
  Speaker 3: control of individual quibbits.
• 15:31
  Speaker 2: But there's always a butt.
• 15:32
  Speaker 3: There is no free lunch in physics.
• 15:34
  Speaker 2: I knew it. Here comes the bill.
• 15:36
  Speaker 3: By making the spin exquisitely sensitive to its own position,
• 15:40
  Speaker 3: you have also made it exquisitely sensitive to anything else
• 15:44
  Speaker 3: that might shift.
• 15:45
  Speaker 2: Its position, like the neighbor pushing on the wall.
• 15:47
  Speaker 3: Like the neighbor. Let's go back to our crowded house.
• 15:50
  Speaker 3: We have two dots Dot A and Dot B side
• 15:52
  Speaker 3: by side. Each one is sitting under this micromagnet gradient. Okay,
• 15:56
  Speaker 3: Now suppose the electron in the neighboring dot Dot A
• 16:00
  Speaker 3: move Maybe we're performing an operation on it, or maybe
• 16:03
  Speaker 3: it just jiggles because of some background charge noise.
• 16:05
  Speaker 2: The neighbor moves the sofa.
• 16:07
  Speaker 3: The neighbor moves the sofa. When that electron in Dot
• 16:09
  Speaker 3: A moves, the kolum repulsion it exerts on our electron
• 16:13
  Speaker 3: in Dot B changes.
• 16:14
  Speaker 2: The push gets stronger or weaker.
• 16:16
  Speaker 3: And because our electron in Dot B is sitting in
• 16:20
  Speaker 3: this carefully balanced electric trap, that tiny change in push
• 16:24
  Speaker 3: from the neighbor is enough to shift its positions slightly.
• 16:28
  Speaker 2: And because of the micromagnet ingredient, when our electron.
• 16:30
  Speaker 3: Moves, it moves into a different magnetic field strength, and
• 16:34
  Speaker 3: therefore it spin frequency changes bingo.
• 16:37
  Speaker 2: The neighbor's motion has changed the speed at which our
• 16:40
  Speaker 2: quibod is spinning. This is the crosstock. This is what
• 16:44
  Speaker 2: the Reichen paper calls a charge induced spin frequency shift.
• 16:48
  Speaker 3: That's it in a nutshell.
• 16:49
  Speaker 2: So the micromagnet is a complete double edged sword. It
• 16:52
  Speaker 2: gives us the precise control we need, but at the
• 16:55
  Speaker 2: same time it acts as an amplifier for the noise
• 16:58
  Speaker 2: coming from the neighbors.
• 16:59
  Speaker 3: That is the crux of the problem they investigated. You
• 17:01
  Speaker 3: built a system that is hyper sensitive to motion, so
• 17:04
  Speaker 3: you could control it with electricity, but now it's hyper
• 17:07
  Speaker 3: sensitive to the unwonted motion of the guy next door.
• 17:10
  Speaker 2: Okay, so this sounds bad in theory. But was it
• 17:13
  Speaker 2: just a theoretical worry or is it actually messing things
• 17:16
  Speaker 2: up in a real device.
• 17:17
  Speaker 3: That is the billion dollar question, and it's what Kobayashi's
• 17:21
  Speaker 3: team went to find out. They didn't just model this
• 17:24
  Speaker 3: on a computer. They built the device, They cooled it
• 17:26
  Speaker 3: down to one hundred miliatelvin, and they measured it directly.
• 17:30
  Speaker 2: Wow.
• 17:30
  Speaker 3: And the way they measured it is fascinating in itself.
• 17:33
  Speaker 2: I was going to ask, how do you measure a
• 17:34
  Speaker 2: tiny shift in the wobble frequency of a single electron
• 17:38
  Speaker 2: trapped in a piece of silicon.
• 17:40
  Speaker 3: You use a very sensitive technique called Ramsey interferometry.
• 17:43
  Speaker 2: Ramseiator ferometry. Is that related to Ramsey kitchen nightmares?
• 17:47
  Speaker 3: No relation, though it can be a nightmare if you
• 17:49
  Speaker 3: don't get the calibration right. It works a bit like this.
• 17:53
  Speaker 3: You start with the electron spin pointing up. Let's call
• 17:55
  Speaker 3: that our starting line. Okay, You hit it with a
• 17:58
  Speaker 3: precisely timed microwave pul ulse that puts it into a
• 18:02
  Speaker 3: perfect superposition half up, half down. Now you can picture
• 18:06
  Speaker 3: its state as a clockhand pointing, say at three o'clock.
• 18:09
  Speaker 3: Got it, and then you just wait. You let it evolve.
• 18:12
  Speaker 3: That clockhand starts sweeping around at its natural frequency, the
• 18:15
  Speaker 3: Larmer frequency.
• 18:16
  Speaker 2: Tick, tick tick.
• 18:17
  Speaker 3: You let it tick for a very specific amount of time.
• 18:21
  Speaker 3: Then you hit it with a second identical microwave pulse.
• 18:25
  Speaker 3: If the frequency was exactly what you thought it was,
• 18:28
  Speaker 3: this second pulse will push the clock hand all the
• 18:30
  Speaker 3: way around to point down, so.
• 18:32
  Speaker 2: You know exactly where it should end up. It's like
• 18:34
  Speaker 2: checking your watch against a master clock exactly.
• 18:37
  Speaker 3: But what if the frequency has shifted during that waiting time.
• 18:41
  Speaker 3: What if the neighbor moved, pushed our electron into a
• 18:44
  Speaker 3: stronger magnetic field and made the clock tick a little faster.
• 18:48
  Speaker 2: Than the hand won't be where you expect it to be.
• 18:50
  Speaker 3: It will have rotated too far. When you hit it
• 18:52
  Speaker 3: with that second pulse. It won't end up pointing down.
• 18:55
  Speaker 3: It'll be somewhere else. This difference between where it is
• 18:58
  Speaker 3: and where it should be is called a phase error.
• 19:00
  Speaker 2: And in quantum computing, phases everything.
• 19:03
  Speaker 3: Phase is where the information is stored in many algorithms.
• 19:06
  Speaker 3: If the phase drifts uncontrollably, the calculation is wrong. It's
• 19:10
  Speaker 3: a fatal error.
• 19:12
  Speaker 2: So what did the Reichen team do with this technique?
• 19:14
  Speaker 3: They performed this Ramsey experiment over and over again, but
• 19:18
  Speaker 3: while our quibbet's clock was ticking, they were deliberately moving
• 19:21
  Speaker 3: the neighboring electron back and forth using its control gate.
• 19:24
  Speaker 2: They poked the bear just to see if it would growl,
• 19:26
  Speaker 2: and it did.
• 19:27
  Speaker 3: They observed a clear, perfectly correlated shift in the quibits
• 19:31
  Speaker 3: frequency that matched the movement of the neighbor electron. When
• 19:35
  Speaker 3: the neighbor moved closer, the frequency went up by a
• 19:38
  Speaker 3: specific amount. When it moved away, it went down. They
• 19:41
  Speaker 3: mapped it out precisely, so.
• 19:43
  Speaker 2: It's not theoretical anymore.
• 19:44
  Speaker 3: It's a hard number, a quantified, measured effect. This is
• 19:47
  Speaker 3: the first time this specific mechanism has been directly measured
• 19:51
  Speaker 3: and isolated.
• 19:52
  Speaker 2: How bad was it? Was it a tiny blip, something
• 19:54
  Speaker 2: you could maybe ignore.
• 19:55
  Speaker 3: According to the paper, the shift is large enough to
• 19:58
  Speaker 3: cause a considerable quibit error rate.
• 20:01
  Speaker 2: A considerable error rate. That doesn't sound good.
• 20:03
  Speaker 3: It's not We measure the quality of our quantum operations
• 20:06
  Speaker 3: in terms of fidelity, a perfect operation has one hundred
• 20:10
  Speaker 3: percent fidelity. To make a fault tolerant quantum computer work,
• 20:13
  Speaker 3: you need operation fidelities of ninety nine point nine percent
• 20:17
  Speaker 3: or really ninety nine point nine nine percent or higher,
• 20:19
  Speaker 3: so you.
• 20:20
  Speaker 2: Can only have one error in ten thousand operations at most.
• 20:24
  Speaker 3: This cross stalk mechanism introduces errors that can easily drop
• 20:28
  Speaker 3: you well below that threshold.
• 20:29
  Speaker 2: So this isn't just background here so we can filter out. Yeah,
• 20:32
  Speaker 2: this is a potential showstopper. If you can't fix this,
• 20:36
  Speaker 2: you can't scale up to a large number of quibits.
• 20:39
  Speaker 3: Show stopper might be too strong, but it is definitely
• 20:42
  Speaker 3: a gatekeeper. It's a problem you cannot pass until you
• 20:46
  Speaker 3: solve it. If you try to build a chip with
• 20:48
  Speaker 3: a million quibbits and they are all shifting each other's
• 20:50
  Speaker 3: frequencies every time.
• 20:51
  Speaker 2: They move, you just have a million sources of noise.
• 20:54
  Speaker 3: You don't have a computer. You have a very expensive,
• 20:56
  Speaker 3: very cold random number generator.
• 20:58
  Speaker 2: Which is useful for casinos maybe, but not for curing
• 21:01
  Speaker 2: cancer exactly.
• 21:03
  Speaker 3: This is what the paper refers to when it says
• 21:05
  Speaker 3: this problem prevents the silicon quantum computing community from realizing
• 21:09
  Speaker 3: a large scale device. It's a roadblock.
• 21:11
  Speaker 2: So we have identified the villain. It's this unlucky combination
• 21:15
  Speaker 2: of the crowded neighborhood, the Kulam interaction, and the very
• 21:19
  Speaker 2: sensitive amplifier that we put in for control, the micromagnet.
• 21:22
  Speaker 3: That's the mechanism.
• 21:23
  Speaker 2: Now. Usually when engineers find a source of noise, they
• 21:27
  Speaker 2: try to eliminate it. They add shielding, or they redesign
• 21:31
  Speaker 2: the parts to be further away. Can we do that here?
• 21:33
  Speaker 3: Well, moving them further away as a non starter. You
• 21:36
  Speaker 3: lose the ability to entangle them, which you need for computation.
• 21:39
  Speaker 2: Okay, so that's out. What about shielding? Can you put
• 21:42
  Speaker 2: a little wall between them?
• 21:44
  Speaker 3: Shielding electric fields at the Mano scale inside a solid
• 21:47
  Speaker 3: piece of silicon is incredibly difficult. It's not really a
• 21:51
  Speaker 3: practical solution right now. So Kobayashi and his team suggest
• 21:55
  Speaker 3: a different.
• 21:55
  Speaker 4: Approach in the paper what's the plan?
• 21:57
  Speaker 3: The plan has two parts. The first is mitigate. The
• 22:00
  Speaker 3: second and more exciting part is exploitation.
• 22:03
  Speaker 2: Mitigation and exploitation. Okay, let's start with mitigostion first.
• 22:07
  Speaker 3: The key discovery here is that we have quantified it.
• 22:09
  Speaker 3: We have measured it. We can predict it.
• 22:11
  Speaker 2: Right. It's not random noise, it's correlated noise.
• 22:15
  Speaker 3: That is the keyword. Random noise is a nightmare to fight.
• 22:19
  Speaker 3: It's like trying to have a conversation in a room
• 22:21
  Speaker 3: with a thousand people talking at once. But if the
• 22:24
  Speaker 3: noise is correlated, if I know that every single time
• 22:27
  Speaker 3: neighbor A moves to the left, my quibbit B is
• 22:30
  Speaker 3: going to shift its frequency by exactly five megahertz, then
• 22:33
  Speaker 3: I can do something about it.
• 22:34
  Speaker 2: You can compensate for it exactly.
• 22:36
  Speaker 3: This leads us to a strategy called feed forward correction.
• 22:40
  Speaker 3: The classical computer that controls the quantum ship can be
• 22:43
  Speaker 3: programmed with this knowledge.
• 22:45
  Speaker 2: How would that work in practice?
• 22:47
  Speaker 3: When the control system sends a command pulse to the
• 22:49
  Speaker 3: gate of neighbor A telling it to move, it can,
• 22:52
  Speaker 3: at the exact same time send a small corresponding correction
• 22:56
  Speaker 3: pulse to the gate of quibbet B.
• 22:58
  Speaker 2: It's like canceling headphones.
• 23:01
  Speaker 3: That's a fantastic analogy noise. Canceling headphones have a microphone
• 23:04
  Speaker 3: that listens to the outside sound, and then the electronics
• 23:07
  Speaker 3: generate an inverse sound wave to cancel it out in
• 23:10
  Speaker 3: your ear. Here we don't even need to listen. We
• 23:13
  Speaker 3: know the sound the frequency shift is coming because we're
• 23:17
  Speaker 3: the ones causing it by moving the neighbor. So we
• 23:19
  Speaker 3: can preemptively adjust the dial the control voltage on quibit
• 23:23
  Speaker 3: B to counteract the effect precisely as it happens.
• 23:27
  Speaker 2: So you fix the error before it even has a
• 23:29
  Speaker 2: chance to mess up the calculation.
• 23:31
  Speaker 3: Ideally, yes, you can program these corrections right into the
• 23:34
  Speaker 3: gate operations. It adds complexity to the control software, but
• 23:38
  Speaker 3: it's a viable path forward.
• 23:39
  Speaker 2: Okay, that makes sense. That's the mitigation was the exploitation part.
• 23:43
  Speaker 3: This is the part that I find most exciting. Kobayashi's
• 23:45
  Speaker 3: team suggests that instead of just canceling this effect, we might.
• 23:48
  Speaker 4: Be able to use it, use the noise.
• 23:50
  Speaker 2: How well.
• 23:51
  Speaker 3: Think about what this crosstalk effect actually represents. It is
• 23:54
  Speaker 3: a form of coupling. It is a connection between two
• 23:57
  Speaker 3: quibbits that were not supposed to be directly interacting.
• 24:00
  Speaker 2: Quibot A is talking to quibt B, even if we
• 24:02
  Speaker 2: didn't want it to initially, it's an accidental phone line.
• 24:05
  Speaker 3: And in quantum logic, what do we need to do?
• 24:08
  Speaker 3: We need to perform two quibit gits. These are operations
• 24:11
  Speaker 3: where the state of one quibbot conditionally changes the state
• 24:15
  Speaker 3: of another. That is how you build logic, That is
• 24:18
  Speaker 3: how you build an algorithm like Shores algorithm for factoring.
• 24:22
  Speaker 2: So normally you have to work really hard to force
• 24:24
  Speaker 2: them to talk to each other in a controlled way.
• 24:27
  Speaker 3: Yes, a common way is to physically lower the electric
• 24:31
  Speaker 3: potential barrier between them, let their electron wave functions overlap,
• 24:36
  Speaker 3: and let them interact via something called the exchange interaction.
• 24:40
  Speaker 3: It's tricky and has to be managed very carefully. But here,
• 24:43
  Speaker 3: but here, the micromagnet has accidentally given us a new
• 24:47
  Speaker 3: long range interaction. They are talking to each other via
• 24:50
  Speaker 3: the magnetic gradient and the electric field even when their
• 24:52
  Speaker 3: wave functions aren't touching.
• 24:54
  Speaker 2: So the researchers are basically saying, don't mute the crosstalk.
• 24:57
  Speaker 2: Let's try to turn it into a high fidelity phone line.
• 25:00
  Speaker 3: That's it. The paper suggests this could lead to a
• 25:03
  Speaker 3: new class of quibit operations. If you can control this
• 25:08
  Speaker 3: energy shift precisely, if you can turn it on and
• 25:11
  Speaker 3: off by moving the neighbor electron, you could potentially use
• 25:14
  Speaker 3: it to perform logic gates between quibots that are not
• 25:17
  Speaker 3: immediate neighbors.
• 25:18
  Speaker 2: So you could do a calculation between quibot A and
• 25:20
  Speaker 2: QUIBTC using quibotb's position as the switch.
• 25:24
  Speaker 3: That's the kind of new capability it might open up.
• 25:26
  Speaker 3: You can turn this bug into a feature.
• 25:28
  Speaker 2: That is the ultimate engineering judo move, use the force
• 25:31
  Speaker 2: of the problem itself to solve the problem.
• 25:34
  Speaker 3: It connects to the broader history of developing these silicon quibots.
• 25:37
  Speaker 3: Time and again we're finding that things we initially thought
• 25:39
  Speaker 3: were bugs, things like subtle variations in the silicon crystal,
• 25:43
  Speaker 3: the valley splitting, or roughness at the silicon oxide interface.
• 25:47
  Speaker 3: And now this micromagnet cross stock can sometimes be understood, controlled,
• 25:51
  Speaker 3: and engineered into resources.
• 25:53
  Speaker 2: If you understand the physics deep enough.
• 25:56
  Speaker 3: If you understand the physics deep enough, it's a testament
• 25:59
  Speaker 3: to how mature the field is getting. We're moving from
• 26:02
  Speaker 3: just trying to get one quibot to work to understanding
• 26:05
  Speaker 3: the complex multi quibot interactions in a large system.
• 26:09
  Speaker 2: It seems like a constant race between the problems we
• 26:12
  Speaker 2: discover and the solutions we invent.
• 26:13
  Speaker 3: It is the silicon quantum computing community. As the text calls.
• 26:18
  Speaker 3: It is in a fierce but friendly race against the
• 26:21
  Speaker 3: superconducting folks and the trapped ion folk.
• 26:23
  Speaker 2: Different horses in the same race.
• 26:25
  Speaker 3: Right, and silicon has this massive theoretical advantage of manufacturability,
• 26:31
  Speaker 3: but it has these complex material physics challenges that the
• 26:34
  Speaker 3: other platforms don't. This study is a major step in
• 26:37
  Speaker 3: clearing the fog around one of those challenges.
• 26:39
  Speaker 2: So let's just zoom back out for a second. We
• 26:42
  Speaker 2: started with single electron shivering in the cold. We built
• 26:44
  Speaker 2: an electric jail cell to trap it. We gave it
• 26:47
  Speaker 2: a special magnet to control it. We then realized that
• 26:49
  Speaker 2: magnet made it listen to its neighbors too much. We
• 26:52
  Speaker 2: then figured out exactly how it was listening, measured the volume,
• 26:56
  Speaker 2: and now we are planning to turn that unwonted listening
• 26:59
  Speaker 2: into a controlled conversation.
• 27:01
  Speaker 3: That is a beautiful summary of the scientific process. Right there,
• 27:04
  Speaker 3: we went from identifying noise to potentially creating a new
• 27:08
  Speaker 3: kind of signal.
• 27:09
  Speaker 2: Why does this matter to the person listening right now?
• 27:11
  Speaker 2: They might be driving to work or doing the dishes.
• 27:14
  Speaker 2: Why should they care about the Larmer frequency of a
• 27:17
  Speaker 2: single electron spin?
• 27:18
  Speaker 3: It matters because it peels back the curtain on how
• 27:21
  Speaker 3: progress and technology actually happens. We often get this very clean,
• 27:26
  Speaker 3: linear narrative of progress. We think of technology as just
• 27:29
  Speaker 3: this inevitable march of better, faster, cheaper.
• 27:32
  Speaker 2: But it's not like that at all.
• 27:34
  Speaker 3: It's not. It's a messy, winding path. It's a series
• 27:37
  Speaker 3: of roadblocks. It's discovering that your clever solution, the micromagnet,
• 27:42
  Speaker 3: caused a brand new problem, the crosstalk. It's the painstaking,
• 27:45
  Speaker 3: unglamorous work of measuring and characterizing that problem.
• 27:49
  Speaker 2: It's less of a Eureka moment and more of a huh,
• 27:52
  Speaker 2: that's weird moment, followed by a year of work.
• 27:55
  Speaker 3: A year of very careful work in a very cold room.
• 27:57
  Speaker 3: And it's the realization that you cannot just build it
• 28:00
  Speaker 3: and hope it works. You have to understand the fundamental
• 28:03
  Speaker 3: physics of the noise, of the imperfections.
• 28:05
  Speaker 2: And it really grounds the hype around quantum computing. This
• 28:10
  Speaker 2: isn't magic. It's plumbing. It's just extremely high tech, atomic
• 28:15
  Speaker 2: scale plumbing with very leaky pipes.
• 28:17
  Speaker 3: It is, and for anyone interested in the future of computing,
• 28:20
  Speaker 3: watching how the community solves these very specific hurdles like
• 28:24
  Speaker 3: the crosstalk challenge that Reyken just mapped out is a
• 28:27
  Speaker 3: much better preview of the future than any marketing material.
• 28:31
  Speaker 3: This is where the real work is happening.
• 28:33
  Speaker 2: If they do solve this, if they can either cancel
• 28:36
  Speaker 2: this cross talk perfectly or turn it into a reliable tool,
• 28:39
  Speaker 2: does Silicon win the race?
• 28:41
  Speaker 3: Win is a strong word. There might be room for
• 28:43
  Speaker 3: multiple technologies, but if they solve this, Silicon becomes a
• 28:47
  Speaker 3: terrifyingly strong competitor. Why terrifyingly because if you can handle
• 28:50
  Speaker 3: this cross talk, and you can solve a few other
• 28:52
  Speaker 3: known materials issues, and you can make these things using
• 28:55
  Speaker 3: standard semiconductor foundaries, the scaling potential is just massive. You
• 29:00
  Speaker 3: could genuinely have millions of high quality quibbets on a
• 29:03
  Speaker 3: single chip the size of your.
• 29:04
  Speaker 2: Thumbail, whereas a golden chandeliers, as beautiful as they are,
• 29:07
  Speaker 2: take up a whole room for a few hundred or
• 29:09
  Speaker 2: a thousand quibbits.
• 29:10
  Speaker 3: Exactly. Density matters for building a truly large scale error
• 29:14
  Speaker 3: corrected machine.
• 29:16
  Speaker 2: So I want to leave the listener with the final
• 29:17
  Speaker 2: thought something to chew on, and it's about this idea
• 29:20
  Speaker 2: of interconnectedness. One we tend to think of isolation as
• 29:24
  Speaker 2: the ideal state. Right in our daily lives. We want
• 29:27
  Speaker 2: peace and quiet to focus. In quantum computing, the mantra
• 29:31
  Speaker 2: is always isolate your quibit so it doesn't decohere and
• 29:35
  Speaker 2: lose its information.
• 29:37
  Speaker 3: Isolation preserves the quantum state. That's rule number one.
• 29:40
  Speaker 2: But a computer that is perfectly isolated does nothing. It's
• 29:42
  Speaker 2: just a perfect box of secrets. To compute. To do
• 29:45
  Speaker 2: anything useful, you must connect. You must have interactions.
• 29:49
  Speaker 3: Information is physical, and physics is at its heart the
• 29:53
  Speaker 3: study of interactions.
• 29:55
  Speaker 2: So this struggle that the Reichen team is documenting, this
• 29:57
  Speaker 2: push and pull between keeping the quibit safe from its
• 29:59
  Speaker 2: ne neighbors but also needing it to talk to its neighbors.
• 30:03
  Speaker 2: It's not just an engineering problem. It feels like the
• 30:05
  Speaker 2: fundamental tension of the universe, doesn't it in.
• 30:08
  Speaker 3: A way it is for any information processing system. It
• 30:11
  Speaker 3: is the balance between coherence and coupling. Too much isolation
• 30:16
  Speaker 3: and your system is useless. It can't process anything. Too
• 30:19
  Speaker 3: much connection and it just becomes noise. Everything washes out.
• 30:23
  Speaker 3: Finding that sweet spot, that perfectly controlled interaction is the
• 30:26
  Speaker 3: whole game.
• 30:27
  Speaker 2: And Reichen just showed us exactly where that line is
• 30:30
  Speaker 2: drawn for this type of device.
• 30:32
  Speaker 3: They gave us the map of a new part of
• 30:34
  Speaker 3: that territory. Now the community has to figure out how
• 30:36
  Speaker 3: to navigate it.
• 30:37
  Speaker 2: Well, my brain is suitably entangled. Thank you for walking
• 30:41
  Speaker 2: us through the nanoverse today. This was fascinating.
• 30:43
  Speaker 3: It was a pleasure. It's always good to look at
• 30:45
  Speaker 3: the small things that make the big things possible.
• 30:47
  Speaker 2: We'll see you in the next one.
• 30:49
  Speaker 3: Keep learning, stay curious.