A beginner's guide to quantum computing | Shohini Ghose

2,342,186 views ・ 2019-02-01

TED


Please double-click on the English subtitles below to play the video.

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Let's play a game.
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Imagine that you are in Las Vegas,
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in a casino,
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and you decide to play a game on one of the casino's computers,
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just like you might play solitaire or chess.
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The computer can make moves in the game, just like a human player.
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This is a coin game.
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It starts with a coin showing heads,
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and the computer will play first.
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It can choose to flip the coin or not,
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but you don't get to see the outcome.
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Next, it's your turn.
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You can also choose to flip the coin or not,
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and your move will not be revealed to your opponent, the computer.
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Finally, the computer plays again, and can flip the coin or not,
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and after these three rounds,
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the coin is revealed,
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and if it is heads, the computer wins,
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if it's tails, you win.
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So it's a pretty simple game,
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and if everybody plays honestly, and the coin is fair,
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then you have a 50 percent chance of winning this game.
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And to confirm that,
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I asked my students to play this game on our computers,
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and after many, many tries,
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their winning rate ended up being 50 percent, or close to 50 percent,
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as expected.
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Sounds like a boring game, right?
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But what if you could play this game on a quantum computer?
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Now, Las Vegas casinos do not have quantum computers,
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as far as I know,
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but IBM has built a working quantum computer.
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Here it is.
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But what is a quantum computer?
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Well, quantum physics describes
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the behavior of atoms and fundamental particles,
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like electrons and photons.
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So a quantum computer operates
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by controlling the behavior of these particles,
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but in a way that is completely different from our regular computers.
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So a quantum computer is not just a more powerful version
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of our current computers,
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just like a light bulb is not a more powerful candle.
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You cannot build a light bulb by building better and better candles.
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A light bulb is a different technology,
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based on deeper scientific understanding.
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Similarly, a quantum computer is a new kind of device,
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based on the science of quantum physics,
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and just like a light bulb transformed society,
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quantum computers have the potential to impact
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so many aspects of our lives,
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including our security needs, our health care and even the internet.
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So companies all around the world are working to build these devices,
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and to see what the excitement is all about,
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let's play our game on a quantum computer.
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So I can log into IBM's quantum computer from right here,
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which means I can play the game remotely,
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and so can you.
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To make this happen, you may remember getting an email ahead of time, from TED,
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asking you whether you would choose to flip the coin or not,
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if you played the game.
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Well, actually, we asked you to choose between a circle or a square.
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You didn't know it, but your choice of circle meant "flip the coin,"
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and your choice of square was "don't flip."
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We received 372 responses.
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Thank you.
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That means we can play 372 games against the quantum computer
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using your choices.
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And it's a pretty fast game to play,
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so I can show you the results right here.
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Unfortunately, you didn't do very well.
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(Laughter)
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The quantum computer won almost every game.
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It lost a few only because of operational errors in the computer.
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(Laughter)
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So how did it achieve this amazing winning streak?
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It seems like magic or cheating,
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but actually, it's just quantum physics in action.
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Here's how it works.
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A regular computer simulates heads or tails of a coin as a bit,
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a zero or a one,
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or a current flipping on and off inside your computer chip.
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A quantum computer is completely different.
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A quantum bit has a more fluid, nonbinary identity.
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It can exist in a superposition, or a combination of zero and one,
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with some probability of being zero and some probability of being one.
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In other words, its identity is on a spectrum.
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For example, it could have a 70 percent chance of being zero
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and a 30 percent chance of being one
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or 80-20 or 60-40.
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The possibilities are endless.
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The key idea here
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is that we have to give up on precise values of zero and one
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and allow for some uncertainty.
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So during the game,
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the quantum computer creates this fluid combination of heads and tails,
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zero and one,
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so that no matter what the player does,
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flip or no flip,
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the superposition remains intact.
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It's kind of like stirring a mixture of two fluids.
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Whether or not you stir, the fluids remain in a mixture,
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but in its final move,
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the quantum computer can unmix the zero and one,
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perfectly recovering heads so that you lose every time.
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(Laughter)
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If you think this is all a bit weird, you are absolutely right.
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Regular coins do not exist in combinations of heads and tails.
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We do not experience this fluid quantum reality
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in our everyday lives.
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So if you are confused by quantum,
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don't worry, you're getting it.
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(Laughter)
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But even though we don't experience quantum strangeness,
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we can see its very real effects in action.
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You've seen the data for yourself.
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The quantum computer won
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because it harnessed superposition and uncertainty,
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and these quantum properties are powerful,
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not just to win coin games,
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but also to build future quantum technologies.
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So let me give you three examples of potential applications
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that could change our lives.
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First of all, quantum uncertainty could be used to create private keys
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for encrypting messages sent from one location to another
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so that hackers could not secretly copy the key perfectly,
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because of quantum uncertainty.
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They would have to break the laws of quantum physics
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to hack the key.
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So this kind of unbreakable encryption is already being tested by banks
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and other institutions worldwide.
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Today, we use more than 17 billion connected devices globally.
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Just imagine the impact quantum encryption could have in the future.
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Secondly, quantum technologies could also transform health care and medicine.
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For example, the design and analysis of molecules for drug development
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is a challenging problem today,
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and that's because exactly describing and calculating
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all of the quantum properties of all the atoms in the molecule
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is a computationally difficult task, even for our supercomputers.
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But a quantum computer could do better,
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because it operates using the same quantum properties
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as the molecule it's trying to simulate.
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So future large-scale quantum simulations for drug development
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could perhaps lead to treatments for diseases like Alzheimer's,
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which affects thousands of lives.
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And thirdly, my favorite quantum application
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is teleportation of information from one location to another
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without physically transmitting the information.
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Sounds like sci-fi, but it is possible,
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because these fluid identities of the quantum particles
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can get entangled across space and time
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in such a way that when you change something about one particle,
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it can impact the other,
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and that creates a channel for teleportation.
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It's already been demonstrated in research labs
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and could be part of a future quantum internet.
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We don't have such a network as yet,
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but my team is working on these possibilities,
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by simulating a quantum network on a quantum computer.
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So we have designed and implemented some interesting new protocols
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such as teleportation among different users in the network
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and efficient data transmission
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and even secure voting.
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So it's a lot of fun for me, being a quantum physicist.
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I highly recommend it.
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(Laughter)
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We get to be explorers in a quantum wonderland.
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Who knows what applications we will discover next.
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We must tread carefully and responsibly
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as we build our quantum future.
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And for me, personally,
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I don't see quantum physics as a tool just to build quantum computers.
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I see quantum computers as a way for us to probe the mysteries of nature
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and reveal more about this hidden world outside of our experiences.
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How amazing that we humans,
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with our relatively limited access to the universe,
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can still see far beyond our horizons
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just using our imagination and our ingenuity.
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And the universe rewards us
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by showing us how incredibly interesting and surprising it is.
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The future is fundamentally uncertain,
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and to me, that is certainly exciting.
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Thank you.
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(Applause)
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