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Quantum Chaos Isn’t Completely Chaotic: Scientists Discover Hidden Patterns · The Quark Side - Quantum Physics Podcast

• 0:01
  Speaker 1: Welcome to the Quarkside Quantum Physics Podcast, an exploration of
• 0:05
  Speaker 1: the fundamental structure of reality, where quantum laws govern matter, energy,
• 0:11
  Speaker 1: and information. Here, uncertainty is a feature, not a flaw,
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  Speaker 1: and understanding begins at the smallest scales.
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  Speaker 2: I want you to imagine, just for a second, that
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  Speaker 2: you're holding a glass of perfectly clear water.
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  Speaker 1: Okay.
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  Speaker 3: I'm picturing it.
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  Speaker 2: Right. Now, take a tiny dropper and let a single
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  Speaker 2: drop of dark blue ink fall right into the center
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  Speaker 2: of that glass. We kind of all know the outcome there. Oh, yeah.
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  Speaker 3: It hits the water, billows out, and just spreads totally chaotically.
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  Speaker 2: Exactly. In a matter of seconds, that distinct, perfect drop
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  Speaker 2: is just gone forever, leaving you with this cloudy, messy
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  Speaker 2: glass of blue water.
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  Speaker 3: Which is basically entropy in action. The universe has this
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  Speaker 3: relentless tendency toward disorder.
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  Speaker 2: Right. And under normal circumstances, you just cannot unmix that ink.
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  Speaker 2: Reversing that process seems physically impossible.
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  Speaker 1: Yeah.
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  Speaker 3: The information about that original pristine drop becomes hopelessly scrambled
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  Speaker 3: across the entire volume of water.
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  Speaker 2: You would never expect that cloudy water to just spontaneously
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  Speaker 2: gather all the ink and perfectly reform it into a
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  Speaker 2: crisp drop. But when we shift our focus to the
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  Speaker 2: microscopic realm.
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  Speaker 3: Specifically to complex quantum many-body systems.
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  Speaker 2: Yes. Scientists have actually managed to locate the equivalent of
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  Speaker 2: that reversing ink drop. They've mapped out these hidden repeating
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  Speaker 2: orders inside systems that should be entirely overwhelmed by chaos.
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  Speaker 3: It's incredible. It really challenges some deeply held assumptions about
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  Speaker 3: how quantum systems thermalize or, you know, reach equilibrium.
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  Speaker 2: So our mission here today is to explore this massive
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  Speaker 2: recent breakthrough. Researchers basically mapped out literal islands of stability
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  Speaker 2: residing within this swirling sea of quantum chaos.
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  Speaker 3: Yeah, we're looking at a scenario where a highly entangled
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  Speaker 3: system somehow manages to sustain stable, recognizable, repeating patterns.
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  Speaker 2: And we really want to dig into the mechanisms behind
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  Speaker 2: this mapping, why these states exist, and honestly how this
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  Speaker 2: could completely change the future of quantum technology.
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  Speaker 3: To really appreciate the weight of this discovery, we have
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  Speaker 3: to understand why the quantum world is normally defined by
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  Speaker 3: just pure, unadulterated chaos.
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  Speaker 2: Right, because before we can appreciate the order, we have
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  Speaker 2: to look at the mess.
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  Speaker 3: Exactly. In classical mechanics, the physics we experience daily, we
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  Speaker 3: use something called phase space.
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  Speaker 2: Which is like a mathematical map, right?
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  Speaker 1: Right.
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  Speaker 3: It's a mathematical space where every possible state of a
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  Speaker 3: system corresponds to a unique point. And in that classical
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  Speaker 3: phase space, order and chaos have very clearly defined separate territories.
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  Speaker 2: Okay, let's unpack this with a really big example. Think
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  Speaker 2: about the solar system. The planets move in these seemingly perfect,
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  Speaker 2: Periodic orbits. That's the order.
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  Speaker 3: Yeah, very predictable.
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  Speaker 2: But the overall late-time motion of the whole system is
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  Speaker 2: actually chaotic and impossible to predict far into the future.
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  Speaker 2: But the order and the chaos exist in the same universe.
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  Speaker 2: They just occupy different regions. They don't overlap.
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  Speaker 3: Right, which ties into the KAM theorem, which basically says
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  Speaker 3: that even when a system is mostly chaotic, you can
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  Speaker 3: have distinct surviving pockets of regular motion.
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  Speaker 2: But translating that concept into quantum mechanics is where things
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  Speaker 2: get incredibly messy.
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  Speaker 3: Messy is an understatement. The rules just don't map cleanly.
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  Speaker 3: In classical chaos, a tiny change in starting conditions leads
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  Speaker 3: to wildly different outcomes. Think of the butterfly effect.
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  Speaker 2: But quantum chaos is different.
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  Speaker 3: Very different. It's fundamentally driven by entanglement spreading. When the
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  Speaker 3: parts of a quantum many-body system interact, their states become
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  Speaker 3: deeply entwined.
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  Speaker 2: They can't be treated independently anymore.
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  Speaker 3: Exactly. The system starts exploring its massive Hilbert space, and
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  Speaker 3: the local information gets completely smeared out globally.
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  Speaker 2: It essentially thermalizes. So if you take local measurements, it
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  Speaker 2: just looks like random noise.
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  Speaker 3: Yeah. And this leaves physicists with a huge theoretical headache.
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  Speaker 3: They've been wondering for a long time, if quantum chaos
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  Speaker 3: is just this relentless spread of entanglement, can a quantum
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  Speaker 3: system even have a mixed phase space like a classical system?
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  Speaker 2: Like, can regular orderly motion actually survive embedded within all
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  Speaker 2: that thermalizing entanglement?
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  Speaker 3: That is the core mystery. But proving it has been
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  Speaker 3: notoriously difficult.
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  Speaker 2: Because quantum states are so incredibly delicate, right?
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  Speaker 3: Oh, absolutely. The moment you try to probe a highly
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  Speaker 3: entangled system to see if it's doing something regular, your
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  Speaker 3: measurement forces it to collapse.
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  Speaker 2: You essentially ruin the very dynamics you're trying to observe
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  Speaker 2: just by.
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  Speaker 3: Looking at it.
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  Speaker 2: Right.
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  Speaker 3: So researchers had to invent a completely new, gentle way
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  Speaker 3: to search for this hidden order without destroying the coherence.
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  Speaker 2: Which brings us to this incredible collaborative team. We've got
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  Speaker 2: researchers at Zhejiang University, specifically Hangdong, and a team at
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  Speaker 2: the University of Leeds. Right.
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  Speaker 3: Led by senior author Zlatko Popic, along with postdocs Ji
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  Speaker 3: Ren and Andrew Hallam.
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  Speaker 2: And what they did is just brilliant. They ran their
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  Speaker 2: experiment on a superconducting quantum processor. Specifically, they isolated a
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  Speaker 2: 24-qubit ladder system.
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  Speaker 3: And that processor is actually part of a much larger
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  Speaker 3: chip that has over 100 qubits.
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  Speaker 2: Yeah, but I want to pause on that setup for
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  Speaker 2: a second. Why use a ladder topology? I mean, why
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  Speaker 2: not just a standard 1D chain of qubits or a
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  Speaker 2: full 2D grid?
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  Speaker 3: That's a great question. A ladder, which is basically two
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  Speaker 3: parallel 1D chains coupled together, is this crucial middle ground.
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  Speaker 2: Okay, how so?
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  Speaker 3: Well, a pure 1D chain often has these built-in mathematical
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  Speaker 3: symmetries that naturally prevent chaos. It's too simple. But a
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  Speaker 3: fully 2D grid thermalizes incredibly fast. It's too chaotic.
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  Speaker 2: Ah, Goldilocks zone.
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  Speaker 3: Exactly. The latter provides just enough connectivity to create deep,
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  Speaker 3: complex entanglement and chaotic dynamics, but it restricts things just
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  Speaker 3: enough so they can actually look for localized behaviors.
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  Speaker 2: Okay. Okay. So they've got this 24-qubit ladder, and they
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  Speaker 2: want to coax it into revealing hidden order. To do this,
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  Speaker 2: they built a feedback loop.
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  Speaker 1: Right.
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  Speaker 3: A loop between the superconducting quantum chip and an ordinary
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  Speaker 3: classical computer.
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  Speaker 2: And here's where it gets really interesting. But wait. Why
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  Speaker 2: do we need an ordinary computer at all? Aren't quantum
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  Speaker 2: computers supposed to be incredibly powerful on their own?
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  Speaker 3: You'd think so. But the reality is that current quantum
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  Speaker 3: processors struggle to preserve that delicate behavior for long periods.
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  Speaker 3: They suffer from decoherence.
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  Speaker 2: So the environment interferes and the system just descends into
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  Speaker 2: a messy noise.
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  Speaker 3: Yeah, exactly. If you just initialize those 24 qubits and
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  Speaker 3: let them run autonomous, they thermalize way before you can
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  Speaker 3: extract meaningful data about their long-term paths.
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  Speaker 2: Okay, but if the classical computer is stepping in, aren't
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  Speaker 2: we just doing a classical simulation of a quantum system?
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  Speaker 2: What's the point of the quantum chip?
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  Speaker 3: Well, simulating the forward time evolution of a highly entangled
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  Speaker 3: 24 qubit system on a normal computer is is computationally agonizing.
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  Speaker 3: It's just too much data.
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  Speaker 2: Oh, because the matrix of possible states grows exponentially.
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  Speaker 3: Exactly. The quantum processor is absolutely necessary because it naturally
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  Speaker 3: and instantly explores that complex space. It acts as the
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  Speaker 3: physical engine.
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  Speaker 2: It's generating the actual physics running through the chaotic landscape.
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  Speaker 3: Yes, but since it can't stop itself from thermalizing, the
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  Speaker 3: classical computer has to step in as a guide or
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  Speaker 3: a shepherd.
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  Speaker 2: What's fascinating here is how this Short evolution tag team
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  Speaker 2: approach bypasses the fragility of modern chips. Tell me how
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  Speaker 2: this feedback loop actually works in practice.
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  Speaker 3: Okay, so they prepare the quantum system in a specific
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  Speaker 3: starting configuration. Then they let it evolve for just a
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  Speaker 3: very brief moment.
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  Speaker 2: Long enough to explore its dynamics, but short enough that
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  Speaker 2: it hasn't completely lost coherence.
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  Speaker 1: Exactly.
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  Speaker 3: Then they hit pause and take simple measurements. They grab
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  Speaker 3: a snapshot of the quantum state.
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  Speaker 2: And this is where the classical computer tags in.
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  Speaker 1: Right.
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  Speaker 3: The ordinary computer takes that data and applies these complex
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  Speaker 3: mathematical tools called tensor networks.
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  Speaker 1: Okay.
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  Speaker 2: I want to dig into the how here because tensor
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  Speaker 2: networks sound super abstract. How does that actively map chaos
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  Speaker 2: into order?
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  Speaker 3: Think of a tensor network as like an entanglement filter.
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  Speaker 3: A fully chaotic thermalized quantum state is just mathematically bloated
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  Speaker 3: with entanglement entropy.
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  Speaker 2: It's too noisy. Right.
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  Speaker 3: So the tensor network looks at the measured snapshot and
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  Speaker 3: finds the closest matching state that possesses less entanglement. It
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  Speaker 3: artificially trims away the messy noise.
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  Speaker 2: Oh, wow. So it mathematically isolates the simpler, more ordered
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  Speaker 2: core of the state.
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  Speaker 3: Exactly. And once it calculates that cleaned-up version, it feeds
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  Speaker 3: those parameters back to the quantum processor.
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  Speaker 2: So they update the processor, reinitialize it to this cleaned-up state,
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  Speaker 2: and let it run for another brief fraction of a second.
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  Speaker 3: And repeat. Initialize. Evolve briefly, measure, trim the entanglement, re-upload, repeat.
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  Speaker 2: That is just a brilliant workaround. And over time, it
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  Speaker 2: homes in on repeating motion without anyone actually telling it
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  Speaker 2: what that motion should be.
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  Speaker 3: And the results were phenomenal. They published this in Nature Physics.
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  Speaker 3: This feedback loop successfully guided the irregular chaotic system into
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  Speaker 3: a repeating pattern.
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  Speaker 2: So what does this all mean?
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  Speaker 3: Well, they discovered literal islands of regular motion. residing completely
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  Speaker 3: within a sea of chaotic behavior.
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  Speaker 2: It makes me think of like finding perfectly calm, spinning
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  Speaker 2: eddies of water right in the middle of a raging
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  Speaker 2: chaotic river. The chaos is still there all around it,
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  Speaker 2: but the order is hiding within it.
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  Speaker 3: That's a perfect analogy. And they proved that a quantum
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  Speaker 3: many-body system can have a mixed-phase space. The regular motion
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  Speaker 3: and the chaos exist in the exact same physical system.
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  Speaker 2: And they found they could actually alter these islands, right?
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  Speaker 3: Yeah, by tweaking the parameters, like adjusting the interactions between
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  Speaker 3: the qubits. They can actually change the shape and boundaries
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  Speaker 3: of these regular paths.
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  Speaker 2: That level of control is just wild. Now, this connects
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  Speaker 2: to some previous work they did, right? On something called
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  Speaker 2: quantum scars?
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  Speaker 3: Yes. This entire project builds on their previous discovery of
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  Speaker 3: quantum many-body scars. Zlatko Popic and his team previously used
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  Speaker 3: a 30-qubit system that should have thermalized completely.
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  Speaker 2: But it didn't. Nope.
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  Speaker 3: Instead of settling into disorder, the system would evolve, become entangled...
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  Speaker 3: and then repeatedly return incredibly close to its unentangled starting configuration.
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  Speaker 2: It kept its memory. That is so weird. Let's visualize that.
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  Speaker 2: Imagine you're looking at a massive, densely packed crowd at
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  Speaker 2: a concert. Everyone is bumping into each other, totally random.
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  Speaker 3: A fully chaotic, thermalized system.
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  Speaker 2: Right. But then suddenly, a small group of people in
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  Speaker 2: the middle of this mosh pit start doing a perfectly
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  Speaker 2: synchronized choreographed dance. They split up and then return to
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  Speaker 2: their exact starting positions over and over.
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  Speaker 3: Completely unbothered by the chaos around them. That is exactly
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  Speaker 3: what a quantum scar is. It's a rare, low-entanglement trajectory
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  Speaker 3: shielded from thermalization.
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  Speaker 2: And the algorithm they used for this new experiment was
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  Speaker 2: actually inspired by that work.
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  Speaker 3: Yeah, an algorithm called Scarfinder from a paper they published
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  Speaker 3: in PRX Quantum. Scarfinder was basically a mathematical search party
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  Speaker 3: to find those rare trajectories.
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  Speaker 2: But this new physical feedback loop takes that to a
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  Speaker 2: whole new level. They went from finding isolated trajectories to
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  Speaker 2: mapping out entire continuous islands of stability.
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  Speaker 3: Exactly. It suggests these quantum scars aren't just bizarre anomalies.
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  Speaker 3: They might be the peaks of much larger structures of
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  Speaker 3: order hiding in the quantum realm.
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  Speaker 2: Okay, so bringing this down to Earth for you, the listener,
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  Speaker 2: how does this impact us? Because mapping these islands isn't
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  Speaker 2: just a neat party trick for physicists, right?
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  Speaker 3: Not at all. It has massive implications for how we
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  Speaker 3: use quantum technology in the real world.
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  Speaker 2: Like, could finding and controlling these regular motions be the
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  Speaker 2: key to preserving useful quantum behavior for much longer? Basically
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  Speaker 2: making quantum computers viable?
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  Speaker 3: Yes. The biggest bottleneck in quantum computing right now is
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  Speaker 3: fighting decoherence. Usually we try brute force error correction, which
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  Speaker 3: takes tons of extra qubits and overhead.
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  Speaker 2: But this suggests a completely different strategy.
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  Speaker 3: If we connect this to the bigger picture, knowing these
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  Speaker 3: islands exist natively within the system means We could potentially
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  Speaker 3: design algorithms to intentionally steer computations into these non-thermalizing zones.
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  Speaker 2: You just park your quantum data on a stable island?
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  Speaker 2: You use the system's own inherent scarring behavior to shield
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  Speaker 2: it from the chaos?
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  Speaker 3: Exactly. You artificially extend the lifetime of the quantum state.
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  Speaker 2: That is huge. And Popovich pointed out another major takeaway here,
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  Speaker 2: that quantum processors can now serve as actual instruments of
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  Speaker 2: scientific discovery.
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  Speaker 1: Right.
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  Speaker 3: They aren't just future calculators anymore. The machine physically explored
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  Speaker 3: its own dynamics and revealed these hidden structures to the scientists.
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  Speaker 3: It's a laboratory in itself.
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  Speaker 2: So what's next? Where do they go from here?
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  Speaker 3: The team wants to find out how widespread this behavior
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  Speaker 3: really is. What determines the stability of these islands? How
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  Speaker 3: does changing the number or arrangement of qubits affect them?
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  Speaker 2: What happens if they use a full 2D grid instead
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  Speaker 2: of a ladder?
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  Speaker 3: Exactly. And they really want to pin down the exact
• 13:38
  Speaker 3: mathematical relationship between these newly mapped islands and the previously
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  Speaker 3: observed quantum scars.
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  Speaker 2: It's an entirely new geography to explore. To recap this
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  Speaker 2: incredible journey, we started with the idea of a drop
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  Speaker 2: of ink permanently dissolving in water, the rapid, messy chaos
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  Speaker 2: of quantum entanglement.
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  Speaker 3: And we saw how this brilliant quantum classical feedback loop
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  Speaker 3: allowed scientists to bypass that thermalization and map out these
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  Speaker 3: hidden islands of stability.
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  Speaker 2: They found the synchronized dance hiding inside the chaotic crowd.
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  Speaker 2: It proves that order isn't lost in the quantum realm,
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  Speaker 2: it just needs the right map to be found.
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  Speaker 3: Learning to stabilize that recurrent activity is a fundamental step
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  Speaker 3: toward really controlling quantum systems.
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  Speaker 2: Which leaves us with a pretty mind-expanding question to ponder
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  Speaker 2: on your own. If these resilient islands of perfect, repeating
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  Speaker 2: order naturally exist, hidden deep within the chaotic quantum realm,
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  Speaker 2: which is the very foundation of our entire universe, could
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  Speaker 2: this imply that the universe inherently resists total chaos? at
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  Speaker 2: its most microscopic level.
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  Speaker 3: It's a fascinating thought.
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  Speaker 2: What other hidden patterns might be dictating the physical laws
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  Speaker 2: we experience every day, just waiting for the right feedback
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  Speaker 2: loop to reveal them? Maybe the ink drop isn't permanently
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  Speaker 2: lost to the water after all. Maybe we just needed
• 14:55
  Speaker 2: a better map to find it.