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The Quantum Speed Limit: Is There a Maximum Speed for Thermal Equilibrium? · 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,
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  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: Imagine you're trying to build, like, The most powerful machine
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  Speaker 2: in human history.
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  Speaker 1: Right.
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  Speaker 2: You're engineering this massive quantum computer that could, I don't know,
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  Speaker 2: theoretically fold complex proteins in seconds or model molecular structures
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  Speaker 2: for new medicines.
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  Speaker 3: Oh, yeah. The holy grail of computing.
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  Speaker 2: Exactly. And you've got the processing logic mapped out. You've
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  Speaker 2: got the top engineering talent. But your grand universe altering
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  Speaker 2: machine is being totally thwarted by the exact same mundane
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  Speaker 2: problem that dictates whether you can drink your morning coffee.
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  Speaker 3: You mean waiting for it to cool down.
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  Speaker 1: Right.
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  Speaker 2: You simply cannot get the parts to cool down fast enough. Okay,
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  Speaker 2: let's unpack this. We all know how to cool down
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  Speaker 2: a cup of coffee, right? You just blow on it.
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  Speaker 2: Or waiting for a car windshield to defrost in the winter.
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  Speaker 2: But what happens when we try to cool down the
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  Speaker 2: fundamental building blocks of the universe?
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  Speaker 3: Well, that's where things get really weird. Because the obstacle
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  Speaker 3: here isn't a lack of engineering ingenuity or like a
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  Speaker 3: failure of the refrigeration systems you're using. The barrier you
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  Speaker 3: hit is a fundamental, unbreakable law written into the fabric
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  Speaker 3: of the universe itself. The physics community has actually just
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  Speaker 3: mathematically proven that there is an absolute speed limit for
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  Speaker 3: how quickly a quantum system can thermalize.
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  Speaker 2: Thermalize meaning?
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  Speaker 3: Meaning how fast it can reach a stable, even temperature
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  Speaker 3: with its surrounding environment.
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  Speaker 2: Wow.
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  Speaker 1: Okay.
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  Speaker 2: Because in our everyday macroscopic world, temperature feels highly negotiable.
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  Speaker 2: You know, like you blow in a hot drink, you
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  Speaker 2: increase the surface area of your food to cool it,
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  Speaker 2: or you use liquid nitrogen on a computer processor.
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  Speaker 3: Yeah, we treat it like a pure engineering problem. Right.
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  Speaker 2: Like if we just build a better radiator or force
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  Speaker 2: more airflow, we can always pull the heat out faster.
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  Speaker 2: But when you zoom all the way down to that
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  Speaker 2: subatomic scale, like where qubits operate in quantum computers... Does
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  Speaker 2: the universe just suddenly revoke that negotiation?
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  Speaker 3: It completely revokes it, yeah. The macroscopic intuition we rely on,
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  Speaker 3: it totally collapses at the quantum level.
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  Speaker 2: Oh, wow.
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  Speaker 3: Because in our everyday world, thermalization depends heavily on physical factors,
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  Speaker 3: things like material density, surface area, the surrounding air or liquid.
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  Speaker 3: But when we look at qubits, an entirely different set
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  Speaker 3: of rules takes over. External conditions still matter a bit,
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  Speaker 3: but quantum mechanics enforces these strict absolute boundaries.
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  Speaker 2: Does this mean at the quantum level, you physically cannot
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  Speaker 2: force things to go faster, no matter how much energy
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  Speaker 2: you use? Like, let's say I'm trying to visualize this.
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  Speaker 2: Is it like traffic on a highway?
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  Speaker 3: How do you mean?
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  Speaker 2: Well, in the macro world, if traffic is moving too slow,
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  Speaker 2: you can just build wider lanes, right? That's like increasing
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  Speaker 2: surface area to move things faster. But at the quantum level,
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  Speaker 2: is it like being on a train track with a hard,
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  Speaker 2: unchangeable top speed?
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  Speaker 3: Yeah, that's actually a great way to look at it.
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  Speaker 3: What's so crucial here is that this finding is universal.
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  Speaker 3: No amount of manipulation, no amount of extra energy can
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  Speaker 3: break this boundary. It's less like a highway where a
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  Speaker 3: reckless driver could technically speed and more like, well, think
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  Speaker 3: of the frame rate of a video monitor.
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  Speaker 2: Oh, the refresh rate.
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  Speaker 3: Exactly. The universe simply does not tick fast enough for
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  Speaker 3: the change in energy state to happen any quicker. You
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  Speaker 3: can't force a quantum transition to outpace the fundamental frame
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  Speaker 3: rate of reality itself.
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  Speaker 4: The universe literally refusing to render the cooling process any faster.
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  Speaker 4: That is wild. But, I mean, physicists didn't just wake
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  Speaker 4: up yesterday and discover this out of nowhere, right?
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  Speaker 3: Oh, definitely not. No, they've had a hunch about this
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  Speaker 3: for years. They kept observing hints of what they call
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  Speaker 3: a Planckian limit in really complex systems, specifically superconductors.
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  Speaker 4: Okay, superconductors. Those are the materials that conduct electricity with
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  Speaker 4: zero resistance, right?
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  Speaker 3: Right, but usually only at extremely low temperatures. And when
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  Speaker 3: experimental physicists watched how these complex systems behaved as they
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  Speaker 3: cooled down, they kept seeing the process taper off at
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  Speaker 3: this very specific rate.
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  Speaker 4: It just kept hitting the same wall.
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  Speaker 3: Yeah, exactly. The data consistently pointed to a hard ceiling,
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  Speaker 3: which made them suspect there was a universal speed limit.
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  Speaker 3: But the general mathematical proof was just completely missing. That is,
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  Speaker 3: until Paolo Abuso at the Austrian Academy of Sciences and
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  Speaker 3: his colleagues stepped forward.
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  Speaker 4: Wait, so physicists were basically seeing the quantum speed limit
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  Speaker 4: signs out in the wild, like in superconductors, but they
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  Speaker 4: just couldn't prove who put them there or how they worked.
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  Speaker 3: What's fascinating here is the massive gap between experimental observation
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  Speaker 3: and a generalized mathematical proof. You can watch a specific system,
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  Speaker 3: like a superconductor, behave a certain way a thousand times
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  Speaker 3: in a lab. But without the underlying math, you can't
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  Speaker 3: say for sure if that speed limit is just some
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  Speaker 3: weird quirk of that specific material or an artifact of
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  Speaker 3: the magnetic fields in your lab, or if it's an
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  Speaker 3: inescapable law of nature. And Abioso's team finally cracked that code.
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  Speaker 3: They published their breakthrough in the journal Nature Physics.
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  Speaker 2: OK, here's where it gets really interesting, because how do
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  Speaker 2: you even approach the math on something like that? Traditional
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  Speaker 2: thermodynamics is all about steam engines and macroscopic heat transfer, right?
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  Speaker 3: Yeah, traditional thermodynamics completely falls apart here. It relies on
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  Speaker 3: the statistical averages of trillions of particles. You can't use
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  Speaker 3: steam engine math to analyze the behavior of a single qubit.
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  Speaker 2: Right, that makes sense. So what did they do?
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  Speaker 3: They had to abandon the classical concept of heat entirely.
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  Speaker 3: They built this universal mathematical framework treating thermalization as a
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  Speaker 3: process of, well, information scrambling.
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  Speaker 2: Wait. So instead of looking at physical thermometers, they looked
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  Speaker 2: at information moving between interacting particles to find this limit?
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  Speaker 4: Precisely.
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  Speaker 3: In the quantum realm, treating energy exchange as a flow
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  Speaker 3: of information is incredibly powerful. When a quantum system interacts
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  Speaker 3: with its environment, the unique information that defines its state
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  Speaker 3: starts to leak out.
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  Speaker 2: It gets entangled.
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  Speaker 3: Yes, it gets tangled up with the particles around it.
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  Speaker 3: So reaching thermal equilibrium just means that initial information is
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  Speaker 3: totally scrambled. The system is indistinguishable from the background bath.
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  Speaker 3: So cooling down is mathematically translated to how fast can
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  Speaker 3: this system spread its information.
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  Speaker 2: Oh, wow. So when they crunched the numbers on that,
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  Speaker 2: what did they find the limit was based on?
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  Speaker 3: Just three things. Temperature, the reduced Planck constant, and Boltzmann's constant.
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  Speaker 2: The Planck and Boltzmann's constants?
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  Speaker 1: Mm-hmm.
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  Speaker 2: That sounds incredibly fundamental. It is.
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  Speaker 3: The reduced Planck constant dictates the quantization of energy, and
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  Speaker 3: Boltzmann's constant bridges macroscopic temperature with microscopic energy. So the
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  Speaker 3: physical limits are absolute.
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  Speaker 2: That's crazy. But the study authors also talked about the
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  Speaker 2: Heisenberg limit, right? Like the uncertainty principle. How does that
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  Speaker 2: factor in?
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  Speaker 1: Right.
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  Speaker 3: So we usually think of Heisenberg as not knowing position
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  Speaker 3: and momentum at the same time. But there's a time
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  Speaker 3: and energy relationship, too. It dictates that a finite change
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  Speaker 3: in energy requires a finite amount of time to actually occur.
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  Speaker 2: Oh, because you can't have an instant shift.
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  Speaker 3: Exactly. A thermalization machine cannot violate that Heisenberg limit. The
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  Speaker 3: scrambling of information can never outpace that fundamental uncertainty.
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  Speaker 2: Okay, so let me get this straight. At high temperatures,
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  Speaker 2: it's like a car legally driving faster on a highway.
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  Speaker 2: But it still can't break the speed of light. But
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  Speaker 2: at freezing temperatures... It's like the car turns into a
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  Speaker 2: bicycle and its speed is suddenly limited by entirely different mechanics,
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  Speaker 2: specifically the gap between its lowest gears.
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  Speaker 3: I love that analogy. Yeah. If we connect this to
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  Speaker 3: the bigger picture, at high temperatures, the system is flooded
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  Speaker 3: with chaotic thermal energy and the broad constants cap the
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  Speaker 3: speed based on the macroscopic temperature.
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  Speaker 2: But when you drop down to near absolute zero.
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  Speaker 3: The chaos vanishes. Energy in quantum mechanics isn't a smooth ramp,
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  Speaker 3: it's discrete steps. At freezing temps, the system only has
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  Speaker 3: enough ambient energy to exist in its ground state, or
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  Speaker 3: the very first step above it.
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  Speaker 2: So you're at the bottom of a pitch black staircase.
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  Speaker 3: Yes, and the absolute limit on how fast it can
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  Speaker 3: thermalize is dictated purely by the energy difference between those
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  Speaker 3: bottom two steps. It becomes a highly localized speed limit
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  Speaker 3: based on that exact energy gap.
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  Speaker 2: That is just mind-blowing. The rules completely shift based on temperature. OK,
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  Speaker 2: so we've been deep in the theoretical physics here, talking
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  Speaker 2: about information scrambling and Heisenberg limits. So what does this
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  Speaker 2: all mean for us? Like, why does knowing the limits
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  Speaker 2: of how fast a qubit cools down actually help us
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  Speaker 2: build a better computer today?
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  Speaker 3: Well, knowledge is really most valuable when you apply it, right?
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  Speaker 3: And this framework is vital for quantum engineering. Qubits are
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  Speaker 3: extraordinarily fragile. If they interact too much with ambient heat,
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  Speaker 3: the information scrambles before your calculation is even finished.
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  Speaker 1: Right.
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  Speaker 2: Decoherence. That's why we put them in those massive dilution refrigerators.
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  Speaker 3: Exactly. We chill them to fractions of a degree above
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  Speaker 3: absolute zero. But computation involves changing states, which generates energy.
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  Speaker 3: So engineers have to constantly reset the qubits, cooling them
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  Speaker 3: back down to their ground state for the next calculation.
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  Speaker 2: Oh, I see. And if you don't know the universe's
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  Speaker 2: hard speed limit for that cooling process... You're flying blind.
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  Speaker 3: You might design a processor that demands a qubit reset
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  Speaker 3: in one nanosecond. Totally unaware that the universe physically requires
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  Speaker 3: two nanoseconds for that transition.
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  Speaker 2: And the whole system crashes.
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  Speaker 3: Total catastrophic failure. Massive error rates. You literally cannot build
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  Speaker 3: a stable quantum machine without knowing the fundamental speed limits
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  Speaker 3: of its parts. Abiyuzo's team just handed engineers the blueprints
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  Speaker 3: for the physical workings of the universe.
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  Speaker 2: That's incredible. It saved them from trying to engineer their
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  Speaker 2: way out.
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  Speaker 4: Of a law of physics.
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  Speaker 3: Exactly.
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  Speaker 2: Yeah.
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  Speaker 3: They can now design architectures that operate right up to
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  Speaker 3: the edge of the universe's speed limit without crossing it.
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  Speaker 1: Wow.
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  Speaker 2: Okay, so just to recap this whole journey. We started
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  Speaker 2: with the relatable frustration of a hot cup of coffee.
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  Speaker 2: and zoomed all the way down to the behavior of qubits.
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  Speaker 3: A bit of a trip, yeah. Totally.
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  Speaker 2: And we uncovered this universal quantum speed limit dictated by
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  Speaker 2: the reduced Planck and Boltzmann constants. We saw how this
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  Speaker 2: mathematical breakthrough by the Austrian Academy of Sciences really paved
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  Speaker 2: the way for the future of quantum computing.
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  Speaker 3: It's a huge milestone.
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  Speaker 2: It really is. But, you know, it leaves me with
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  Speaker 2: this really wild, mind-bending question for everyone listening to ponder
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  Speaker 2: on their own. We've established that there is a fundamental
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  Speaker 2: limit to how fast quantum systems can thermalize, right?
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  Speaker 3: Right.
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  Speaker 2: Well, if quantum computers are physically bound by these hard
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  Speaker 2: speed limits of cooling and heating, does this mean there
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  Speaker 2: is an ultimate unbreakable ceiling on how fast a quantum
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  Speaker 2: computer can ever process information before it simply cannot cool
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  Speaker 2: down fast enough?
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  Speaker 3: Hmm.
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  Speaker 2: Like, is the ultimate speed of artificial intelligence and quantum
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  Speaker 2: processing physically capped by the reduced Planck and Boltzmann constants?
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  Speaker 2: It's wild to think about. Keep wondering about that, everyone.
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  Speaker 2: And thanks for joining us on this exploration.