#77 - John Preskill

**Craig Cannon** (0:00)
Hey, how's it going? This is Craig Cannon, and you're listening to Y Combinator's podcast.
Today's episode is with John Preskill. John's a theoretical physicist, and the Richard P. Feynman Professor of Theoretical Physics at Caltech. He once won a bet with Steven Hawking, and he writes that it made him briefly almost famous. So basically what happened is John and Kip Thorne bet that singularities could exist outside of black holes, and after six years, Hawking conceded. He said that they were possible in very special, quote, non-generic conditions. I'll link up some more details to that in the description.
And in this episode, we cover what John's been focusing on for years, which is quantum information, quantum computing and quantum error correction. All right, here we go. And what was the revelation that made scientists and physicists think that a quantum computer could exist?

**John Preskill** (0:54)
It's not obvious, you know, a lot of people thought it couldn't.
The idea that a quantum computer would be powerful was emphasized over 30 years ago by Richard Feynman, the Caltech physicist.
And it was interesting how he came to that realization. Feynman was interested in computation his whole life. You know, he had been involved during the war in Los Alamos. He was the head of the computation group. He was the guy who fixed the little mechanical calculators, and he had a whole crew of people who were calculating, and he figured out how to flow, you know, the work from one computer to another, all that kind of stuff. And as computing technology started to evolve, you know, he followed that. And in the 1970s, particle physicists like Feynman, that's my background too, got really interested in using computers to study the properties of elementary particles, like the quarks inside a nucleus. You know, we know a proton isn't really a fundamental object. It's got little beans rattling around inside, but they're quantum beans. And Galman, who's good at names, called them quarks.

**Craig Cannon** (2:16)
Yeah.

**John Preskill** (2:17)
And now we've had a theory since the 1970s of how quarks behave. And so in principle, you know everything about the theory, you can compute everything, but you can't because it's just too hard. And people started to simulate that physics with digital computers in the 70s, and there were some things that they could successfully compute, and some things they couldn't because it was just too hard. The resources required, you know, the memory, the time were out of reach. And so Feynman in the early 80s said, you know, nature is quantum mechanical, damn it. So if you want a simulation of nature, it should be quantum mechanical. You should use a quantum system to behave like another quantum system. At the time, he called it a universal quantum simulator.
And now we call it a quantum computer.
And the idea caught on about 10 years later when Peter Shor made the suggestion that we could solve problems which don't seem to have anything to do with physics, which are really things about numbers, like finding the prime factors of a big integer. And that caused a lot of excitement in part because the implications for cryptography are a bit disturbing.
But then physicists, good physicists, started to consider, can we really build this thing?
And some concluded and argued fairly cogently that no, you couldn't. Because of this difficulty, that it's so hard to isolate systems from the environment well enough for them to behave quantumly. And so it took a few years for that to sort out sort of at the theoretical level. In the mid-90s, we developed a theory called quantum error correction. It's about how to encode the quantum state that you'd like to protect in such a clever way that even if there are some interactions with the environment that you can't control, it still stays robust. But at first, that was just kind of a theorist fantasy. It was a little too far ahead of the technology. But 20 years later, the technology is catching up. So now this idea of quantum error correction has become something you can do in the lab.

**Craig Cannon** (4:31)
Yeah, and how does quantum error correction work? I've seen a bunch of diagrams, so maybe this is difficult to explain, but how would you explain it?

**John Preskill** (4:38)
Well, I would explain it this way. I don't think I've said the word entanglement yet.

**Craig Cannon** (4:42)
No, I have been checking off all the bingo words yet.

**John Preskill** (4:45)
Okay, so let's talk about entanglement, because it's part of the answer to your question, which I'm still not done answering. What is quantum physics?
So what do we mean by entanglement? It's really the characteristic way, maybe the most important way that we know, in which quantum is different from ordinary stuff, from classical. And what does it mean, entanglement? It means that you can have a physical system, which has many parts, which have interacted with one another, so it's in kind of a complex, correlated state of all those parts.