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  • DANIEL SANK: Information is physical.

  • Written letters are carbon grains on paper.

  • Spoken words are vibrations of air molecules.

  • Computer bits are electric charge.

  • Each of these examples shares a common limitation--

  • they work under physics that was understood in the 1800s, known

  • as classical physics.

  • Science has progressed since then.

  • We've discovered a new set of laws called quantum mechanics.

  • Here's one of our chips designed to leverage

  • the rules of quantum mechanics to process information

  • in ways impossible on a computer based in classical physics.

  • You may have heard that quantum mechanics only

  • applies to microscopic objects, like atoms.

  • So how does this chip bring out quantum behavior?

  • I'm Daniel Sank, a research scientist working in the Google

  • AI Quantum Computing Lab.

  • In this video, we'll look at how our quantum

  • bits are made physically.

  • I want to explain the fundamental differences

  • between classical and quantum information

  • at the physical level so that you

  • can understand why our quantum bits are made how they are.

  • Physicists and computer scientists

  • both think in terms of states.

  • A physical state could be my position--

  • I can be on the left or on the right.

  • And physical laws determine how nature goes from one state

  • to another.

  • Observe.

  • If Sergio pushes me, my state changes.

  • A computer's state is the value of its memory bits

  • and computer programs determine how the computer goes

  • from one state to the next.

  • For example, when you hit the Play button,

  • YouTube's program started manipulating your computer's

  • memory to show this video.

  • Where physics has physical states and natural laws,

  • computer science has memory states and programs.

  • Think of the state of computer memory as a string of bits.

  • For end bits, there are two to the end possible strings.

  • But because we're based in classical physics,

  • the state of the computer is just one

  • of these states at each point in time.

  • On each step of a classical algorithm, we go from one state

  • to the next.

  • For example, the logic operation shown here

  • takes the state 0-0-0 to 1-1-0.

  • If we were to apply the same operation again,

  • we'd go from 1-1-0 to 0-1-0.

  • Compared to classical states, quantum states are more rich.

  • They can have weight in all possible classical states,

  • a situation physicists call superposition.

  • Each step of a quantum algorithm mixes the states

  • into complex superpositions.

  • For example, starting in 0-0-0, we

  • go to a superposition of 1-0-0, 1-0-1, and 1-1-1.

  • Then each of those three parts of the super post

  • state branches out to even more states.

  • The extra complexity of quantum computers

  • allows them to solve some problems

  • faster than a classical computer ever could.

  • We've discussed the computational difference

  • between classical and quantum, but how

  • do classical and quantum differ physically?

  • How do we bring out quantum mechanics

  • in our chip, which is so much bigger

  • than the tiny atoms in which quantum mechanics was

  • first discovered?

  • Let's take a detailed look at classical bits

  • at the physical level so that we can

  • understand the physical difference between classical

  • and quantum.

  • Classical computer bits are stored

  • in the presence or absence of charge

  • on a capacitor in a circuit called

  • dynamic RAM, or DRAM for short.

  • If there's charge, it's a logical 1,

  • and if there's no charge, it's a logical 0.

  • But there's more going on here.

  • Our logical 0 and 1 are actually made up

  • of the presence or absence of 300,000 electrons.

  • Why use so many?

  • In principle, we could just use the presence or absence of one

  • electron as our logical bid.

  • Well, physical bits are noisy.

  • Electrons are tiny and light, so they jiggle around and leak out

  • of the DRAM.

  • If we had only one electron and it were to leak out,

  • our bit would change value, which is an error.

  • By using lots of electrons, we're OK if a few leak out.

  • DRAM circuits periodically check the logical level

  • and replenish missing electrons.

  • Encoding one logical bit in the state of so many physical bits

  • gives classical information a level of reliability

  • that we take for granted.

  • We don't have to think about all those electrons bumping around

  • when we write our programs.

  • OK, so why can't we just put our DRAM

  • into a quantum superposition of 0 and 1?

  • Well, suppose we did have that superposition.

  • It wouldn't last long.

  • As soon as we do the first check to protect against a DRAM

  • error, we've forced the bit into either 0 or 1,

  • removing the quantum superposition state.

  • In fact, that collapse happens even

  • without us checking for errors.

  • A single photon interacting with just one of our electrons

  • can carry off information.

  • When that happens, it's as if the photon

  • observed the quantum state and the state collapses.

  • You can think of this as nature observing and thus destroying

  • our quantum states.

  • Errors like this are unique to quantum information.

  • In classical computing, you might be upset

  • if somebody peeks at your bits, but that peek doesn't

  • completely destroy them.

  • Note that an error occurs whenever nature observes

  • any one of our physical bits, so while we normally

  • stack up more physical bits for redundancy,

  • that approach actually makes quantum errors worse.

  • That's the main difficulty in quantum computation--

  • the fundamental quantum constituents of matter

  • are small and easily subjected to noise,

  • but we can't brute force our way around that noise

  • with redundancy because bigger systems are

  • more subject to quantum errors.

  • At Google, we use a technique that

  • gets the best of both worlds.

  • We use circuits with a huge number of electrons,

  • but we prevent quantum errors with superconductivity.

  • In regular metals, like with a conventional DRAM circuit,

  • every individual electron does its own thing.

  • As electrons move around, they can bounce off

  • the positively charged ions of the metal,

  • radiating vibrational waves that carry off quantum information

  • about the electrons.

  • This hectic, bustling cauldron of physical interactions

  • generates a lot of quantum errors,

  • and the information gets lost before we can use it.

  • However, when certain metals are cooled down,

  • their electrons join together in a single unit.

  • The individual electrons no longer scatter

  • and the rate of quantum errors drops to almost 0.

  • Our quantum bits are, in fact, just electoral oscillators

  • built from aluminum, which become

  • superconducting when cooled to below 1 degree Kelvin.

  • The oscillators store tiny amounts of electrical energy.

  • When the oscillator is in the 0 state, it has 0 energy.

  • When it's in the 1 state, it has a single quantum of energy.

  • The two states of the oscillator with 0 or 1 quantum of energy

  • are the logical states of our quantum bit,

  • or qubit for short.

  • Here's a picture of a superconducting qubit

  • along with a circuit diagram.

  • The crosses indicate Josephson tunnel junctions,

  • which are nonlinear superconducting inductors.

  • We pick the resonance frequency of our oscillators

  • to be about 6 gigahertz, which sets the energy difference

  • between the 0 and 1 states.

  • That's a low enough frequency that we

  • can build control electronics from readily available

  • commercial parts, but also high enough

  • that the ambient thermal energy doesn't

  • scramble the oscillation and introduce errors.

  • 6 gigahertz corresponds to 300 millikelvin.

  • Fortunately, refrigerators that get to 15 millikelvin

  • are relatively standard commercial products.

  • For comparison, outer space is about 2.5 Kelvin.

  • I think it's cool that the cryostats in our lab

  • are colder than deep space.

  • Now let's take a minute to make a few comments on how

  • superconducting qubit architecture differs

  • from conventional computers.

  • In a conventional computer, memory and logic processing

  • are separated into the RAM and CPU.

  • When we want to do a computation,

  • we first move the data from the RAM to the CPU.

  • Then the circuits in the CPU do the computation,

  • and finally, the resulting data is written back to RAM.

  • In quantum computing, with superconducting qubits,

  • we can't afford the errors that would come

  • from moving the data around.

  • Instead, we build a grid of qubits, each one connected

  • to its neighbors.

  • The qubits stay put and we do logic operations

  • by sending control signals into individual qubits

  • or pairs of qubits.

  • Now that you have a basic picture of superconducting

  • qubits, let's take a look at one of the challenges

  • that we're still working on.

  • Superconductivity greatly reduces errors,

  • but there are still some.

  • For example, the electrons flowing in the oscillator

  • interact with charged particles in the surroundings,

  • leading to errors.

  • Suppose there were a charged ion inside the metal of our qubit.

  • The oscillating energy in the qubit

  • can transfer into that ion, causing the qubit's logic state

  • to flip, thus creating an error.

  • Improving the qubit fabrication process

  • to reduce these atomic imperfections

  • is a big part of our research.

  • Over the last several years, improvements

  • in microfabrication techniques have

  • decreased our qubit error rates a lot,

  • and we're still improving.

  • In this video, we focused on the idea

  • that information is physical.

  • We discussed the physical incarnation

  • of classical and quantum computer bits.

  • We introduced quantum errors and explained

  • why we need superconductivity to eliminate those errors.

  • If you'd like to know more, you can leave questions

  • in the comments section below.

  • It's important to me that you can understand

  • the physical aspects of quantum computation

  • as clearly as possible.

  • I'm also pretty active on Physics Stack Exchange.

  • You can find great questions and answers there, too.

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超伝導クビットを用いた量子コンピュータの構築 (QuantumCasts) (Building a quantum computer with superconducting qubits (QuantumCasts))

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    林宜悉 に公開 2021 年 01 月 14 日
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