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  • SERGIO BOIXO: Hi.

  • I am Sergio Boixo from the Google AI Quantum team,

  • and today, we're going to talk about an experiment we're

  • working on, which is known as quantum supremacy.

  • The latest experimental quantum processor

  • produced at Google, Bristlecone, has 72 qubits or quantum bits.

  • We're testing quantum circuits in Bristlecone

  • with the goal of reducing errors.

  • By their nature, quantum gates have a probability of errors,

  • and errors can cross quantum circuits.

  • As we calibrate quantum circuits,

  • we bring down the probability of error.

  • We simulate quantum circuits with traditional computers

  • to benchmark and calibrate quantum circuits.

  • As we work to reduce the probability of an error,

  • simulations gets exponentially harder.

  • This means that it gets too computationally intensive even

  • for a supercomputer to keep up.

  • From this, we get the name quantum supremacy

  • for this experiment.

  • This has to do with something called a strong Church-Turing

  • thesis in computer science.

  • Traditional computers from the abacus to your laptop

  • implement equivalent operations or classical gates,

  • although a modern computer is, of course, much, much faster.

  • The strong Church-Turing thesis says

  • that all universal computers are equivalent in this way,

  • and can simulate each other efficiently.

  • But according to quantum computing,

  • the strong Church-Turing thesis is false,

  • and quantum computers can solve some problems exponentially

  • faster than other universal computers.

  • So what we're trying to do is kind

  • of breaking the strong Church-Turing thesis.

  • You can think of a qubit as an arrow pointing

  • to some direction on a sphere.

  • Quantum gates are operations on qubits.

  • Similar to classical gates, we often depict quantum gates

  • as boxes, with the input on one side

  • and the output on the opposite side.

  • In a quantum circuit, we apply layers

  • of gates, one per clock cycle.

  • A measurement at the end of the quantum circuit

  • produces a string of beats.

  • For the quantum supremacy experiment,

  • we choose the quantum gates at random.

  • This is a Hello World program for quantum computers.

  • Crucially, in this case, we have the strongest critical evidence

  • against the strong Church-Turing thesis.

  • It takes exponential time to simulate a random quantum

  • circuit with a classical computer.

  • According to quantum mechanics, every particle

  • can also act as a wave, and this applies to qubits.

  • The quantum state of a quantum computer

  • contains an exponential number of waves

  • or computational paths.

  • This is the property that we are testing.

  • The output state of a random quantum circuit

  • looks like the speckles of a laser.

  • This is a fingerprint of the quantum circuit.

  • For some bit strings, the computational paths

  • interfere constructively, and the intensity of the output

  • probability grows.

  • For others, the computational paths interfere destructively,

  • and the output probability decreases.

  • Simulating interference of the exponential number

  • of computational paths in the quantum circuit

  • takes exponential time.

  • We can check if we obtain the correct fingerprint

  • in the experiment, and measure the probability of error.

  • First, we get around a million bit strings

  • from the quantum computer.

  • This takes a few seconds.

  • Then we use an expensive classical simulation

  • to check if those bit strings have high probability.

  • If this is the case, the error rate is low,

  • and the experiment has succeeded.

  • The implication will be that quantum computers

  • seem to be breaking the strong Church-Turing thesis.

  • As we reduce errors farther, we expect

  • to see a similar exponential speed up

  • for a practical problem.

  • So what's next?

  • Visit the other videos in these series

  • to learn more about how a quantum computer works

  • and how to program it.

  • You can also visit OpenFermion to learn more about how

  • quantum computers can be used to solve problems

  • in chemistry and material science,

  • or check out the links included below.

  • Thank you.

SERGIO BOIXO: Hi.

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量子至上的解釋(QuantumCasts)。 (Quantum supremacy explained (QuantumCasts))

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    林宜悉 發佈於 2021 年 01 月 14 日
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