Artur Ekert explains how Albert Einstein and John Bell inspired quantum cryptography

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Hello, and welcome to the Physics World Weekly

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Podcast.

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I'm Hamish Johnston.

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This episode is part of our ongoing

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celebration

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of the International Year of Quantum Science and

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Technology,

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and I'm in conversation

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with the quantum cryptography

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pioneer and theoretical physicist,

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Arthur Eckert.

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Arthur has academic appointments at the University of

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Oxford and the National University of Singapore,

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where he founded the Centre for Quantum Technologies.

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I caught up with him at Oxford's

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Merton College, where he is professorial

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fellow

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in quantum physics and cryptography.

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Hi, Arthur. Welcome to the podcast. Yeah. Thank

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you for having me.

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So in Arthur, in 1991,

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you created a quantum cryptography protocol called e

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91.

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And that has its roots in a paradox

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of quantum physics

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that was first identified back in 1935

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by Albert Einstein

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and his colleagues Boris Podolsky

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and Nathan Rosen.

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So I thought we we could start off

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by maybe talking a bit about this EPR

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paradox and how it important it was for

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the development of quantum mechanics.

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So what was Einstein what what what upset

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Einstein about quantum mechanics, and how did he

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express it in this paradox?

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Yeah. Yes. I guess it it is a

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good starting point, for this, fascinating narrative.

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Well it's, you know, it's really difficult

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today to say what Einstein really had in

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mind,

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and volumes are written about it really, what

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Einstein

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could possibly

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have in mind. But but one thing we

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know is that he was, unhappy with, quantum

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theory simply because

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he thought that it was not complete.

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And by that,

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we mean that,

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it

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you

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like, in quantum theory, you had to use

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probabilities.

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It looked like,

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randomness was inherent part of it.

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And to Einstein, that was not a

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good

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theory to describe the real world. Right? So

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so it should be predictable. Once you know,

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initial conditions, you should be able

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to predict what's going to happen

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and not just only with certain probability. And

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if you do it with certain probabilities,

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that that can only reflect the lack of

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knowledge on your side. But the laws of

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physics should be such that, you should have

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deterministic

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predictions.

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I'm simplifying the whole story. There was a

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little bit of,

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about

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two systems,

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that when separated, they shouldn't be able to

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communicate instantaneously

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and so on and so forth. So Einstein

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was combining,

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his

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understanding

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that he developed working on,

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the special theory of relativity,

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with,

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his,

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with sort of belief how how the world

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should be,

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described.

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And, we call this kind of description,

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local,

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and

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we refer to,

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his program as a local hidden

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variable model by which we mean that

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according to Einstein, the quantum physics didn't provide

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complete description. The reason why we see

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this,

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probabilistic

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behavior

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is due to the fact that

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what

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people in quantum physics would say, oh, this

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is a the description of quantum state. To

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Einstein, it was just a description of quantum

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state. It was not actually

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complete that if you add some extra parameters,

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maybe then you should be able to make

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those precise predictions.

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So so it was about,

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predictability

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and about,

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what some people case say, spooky action at

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the distance. So

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so so to understand, everything should be

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described

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locally. You should be able

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by looking at the state of a given

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object here at a given location, you should

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be able to make predictions,

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and be able to say for sure what's

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going to happen. And if you don't have

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this, so that means that your theory is

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not complete. It's not quite right. So Einstein

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didn't really argue against quantum physics. He just

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simply thought it was just the job is

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not done, that you have to work harder

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and and work out details.

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I see. And I suppose like a lot

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of things in the early,

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formulations

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or approaches to to quantum physics, this

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remained just an idea

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for for a long time and a sort

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of a quirky

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way of thinking about things. But then,

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later on, John Bell, the Northern Irish physicist,

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I think it was in 1964,

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built a mathematical

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way of expressing

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this EPR,

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paradox.

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Yeah. And and that turned out to be

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a very profound thing, didn't it?

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That's right. Even though, you know, when we

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talk about it today, right, so, with the

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benefit of hindsight and and looking

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what happened so many years ago, we could

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see that those were very important things, but

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at the time, they were hardly noticed really.

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So,

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Einstein

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just for, you know, the EPR paper,

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the Einstein Podolsky and Rosen paper that you

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cited from 1935,

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the starting point for our story today. Right?

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That that one, when we look at it

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today, we can see that this was actually

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a very, very important paper.

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But at the time, I don't think people

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really

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were too much bothered about it. Most physicists

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have this,

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very instrumental approach.

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Take a theory, make predictions. If it works,

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fine. That's good enough. Einstein went one step

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further, and he wanted he was seeking

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good explanations, not only good predictions, but also

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good explanation, how it really, really works.

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And,

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so after that,

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there was silence, really. Nobody

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bothered too much. It was

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the number of beautiful results that people,

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worked out in quantum theory, mostly looking at

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atomic spectrum,

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atomic physics.

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So John Bell was one of those,

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again,

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an outlier, I would say, who,

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apart from having a real job, right, working

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at CERN designing accelerators,

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after hours, he was interested in the foundations.

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And

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and almost thirty years later,

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what he did,

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looking at this philosophical

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paper and trying to understand

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whether there are additional parameters that

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that we may need if we want to

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have a very precise description,

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whether he can translate the whole idea into

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something that is testable, something that's at least

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falsifiable

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so that you can set up an experiment.

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And you run this experiment,

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and it can

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refute the local hidden variable model,

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of Einstein.

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So that was a a great achievement of,

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John Bell. So he was able to take

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those what looked like a philosophical

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idea,

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Einstein paper, and translate it into something that

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you can show to experimentalists

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and say, go to your lab,

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do this experiment, and

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and then you can,

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refute one

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possible worldview that, Einstein was

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subscribing to.

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So Bell's theorem, which you've referred to, has

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been described by some as the most profound

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discovery in science. I mean that's that's a

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pretty strong statement. Yeah. Yeah. But so

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can you

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I mean why would somebody say that? Obviously

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it's very important.

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It's a very important concept or a set

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of ideas. What why is it so

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important in the sort of history of quantum

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mechanics and and how we understand quantum theory

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today?

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Well, it's

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it is it is certainly very important,

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result.

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The whole quantum theory is a very important

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result. Right? It's a new way of looking

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at nature. It's not even

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something that tells us about interactions. It's unlike

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gravity, electrodynamics,

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strong, or weak interaction.

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Quantum physics is a or quantum theory is

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a is a a meta statement. Right? It

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says no matter what the interaction are,

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then you have to describe them in a

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certain way.

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So so it gives you a,

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a framework

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in which you have to formulate any physical

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theory. So so changing this framework from from

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classical to quantum,

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even though people maybe didn't quite realize it

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at the very beginning, but it it is

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like a changing of world view.

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And then there's Einstein there, right, who is

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saying, look, not so fast. Right? So maybe

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maybe it's not

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exactly that you have to live with inherent

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randomness.

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Perhaps it's not complete.

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So Bell,

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in a way,

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is coming with a with a mathematical

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proposition which says, okay. Go to the lab,

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test it, and if you see a violation

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of this the so called Bell inequalities,

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then that's it. That you have to refute,

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that you have to drop the the the

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whole idea of, hidden variables. So there's no

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hope.

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There's no, like,

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you know,

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a a screenplay of some sort that tells

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exactly what's going to happen. The nature is

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inherently unpredictable.

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And and so that that is you know,

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when you think about it, that that is

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quite a strong statement. That's almost like changing

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a world view, and and I think,

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it's it's perfectly justified to refer to it

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as, as one of the

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greatest thing that that happened in in in

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our understanding

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of of the of the nature of reality.

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I should add perhaps that,

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that there there there's more to this story.

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I mean, just we don't have to go

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in because it may be part of the

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of of of of what we want to

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talk about, but, but nonetheless, it's connected that

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we make certain assumptions when when we look

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at,

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Bell theorem. And,

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so there are other ways to think about,

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00:11:09,455 --> 00:11:12,035
quantum physics. And and, for example,

284
00:11:12,480 --> 00:11:15,839
it may well be that, our perception of

285
00:11:15,839 --> 00:11:18,720
the world is simplistic and maybe that there

286
00:11:18,720 --> 00:11:19,940
is a room for

287
00:11:20,879 --> 00:11:21,379
multiverse,

288
00:11:21,919 --> 00:11:23,539
which is the Everett approach.

289
00:11:24,534 --> 00:11:26,615
And every approach is different in a sense

290
00:11:26,615 --> 00:11:28,774
that in in Bell's theorem, for example, we

291
00:11:28,774 --> 00:11:32,134
assume that, each each experiment has a outcome

292
00:11:32,134 --> 00:11:33,355
that we see and perceive,

293
00:11:33,815 --> 00:11:36,455
that in the Everett interpretation of all possible

294
00:11:36,455 --> 00:11:38,679
outcomes can happen. So so what I'm saying

295
00:11:38,679 --> 00:11:40,519
that there's more to this story. Right? So

296
00:11:40,519 --> 00:11:43,179
I may just say, okay. Bell, great. Wonderful

297
00:11:43,240 --> 00:11:45,419
idea. But then we also have Everett,

298
00:11:45,959 --> 00:11:48,759
great insight into the nature of reality and

299
00:11:48,759 --> 00:11:50,794
many others. So so I'm

300
00:11:51,735 --> 00:11:54,534
I'm I'm I agree with the statement that

301
00:11:54,534 --> 00:11:55,995
it was a a profound

302
00:11:57,735 --> 00:12:00,774
discovery or statement that was made about our

303
00:12:00,774 --> 00:12:03,355
ability to distinguish between different world views,

304
00:12:04,559 --> 00:12:06,500
but but there's more to it.

305
00:12:07,039 --> 00:12:07,539
So

306
00:12:07,919 --> 00:12:09,379
Bell's theorem, I suppose,

307
00:12:10,320 --> 00:12:13,539
when he formulated it was still a theorem.

308
00:12:13,679 --> 00:12:16,660
And as you say, he he offered guidance

309
00:12:16,879 --> 00:12:17,700
to experimentalists,

310
00:12:19,315 --> 00:12:21,394
but it was still, I mean, it's still

311
00:12:21,394 --> 00:12:23,415
a very difficult experiment to do,

312
00:12:24,035 --> 00:12:25,735
in 1964.

313
00:12:26,035 --> 00:12:26,535
Yeah.

314
00:12:27,154 --> 00:12:27,654
So,

315
00:12:28,355 --> 00:12:29,975
in 2022,

316
00:12:30,754 --> 00:12:31,735
Alain Aspe,

317
00:12:32,115 --> 00:12:32,935
John Klauser,

318
00:12:33,539 --> 00:12:34,919
and Anton Zeilinger,

319
00:12:35,940 --> 00:12:37,799
won the Nobel Prize for physics

320
00:12:38,179 --> 00:12:41,720
for their pioneering work in actually doing

321
00:12:42,179 --> 00:12:43,080
Bell tests

322
00:12:43,459 --> 00:12:44,279
in the lab.

323
00:12:45,700 --> 00:12:47,059
And this was work, I think, that was

324
00:12:47,059 --> 00:12:49,000
done is it eighties, nineties,

325
00:12:50,315 --> 00:12:51,295
roughly then?

326
00:12:53,195 --> 00:12:56,394
Sadly, of course, John Bell died in 1990,

327
00:12:56,394 --> 00:12:57,295
so he couldn't

328
00:12:57,675 --> 00:12:59,055
share the Nobel Prize,

329
00:13:00,715 --> 00:13:01,710
for that work.

330
00:13:02,429 --> 00:13:04,750
Can you give us a flavor of what

331
00:13:04,750 --> 00:13:05,889
Aspe, Clauser,

332
00:13:06,269 --> 00:13:09,230
and Zollinger did in the lab? How did

333
00:13:09,230 --> 00:13:09,970
they translate

334
00:13:10,670 --> 00:13:12,450
John Bell's ideas into

335
00:13:12,910 --> 00:13:14,050
actual experiments?

336
00:13:15,695 --> 00:13:16,595
Right. So,

337
00:13:18,014 --> 00:13:20,254
so John Bell, as you pointed out, John

338
00:13:20,254 --> 00:13:22,834
Bell came up with a a mathematical statement.

339
00:13:23,294 --> 00:13:25,855
What was good about this mathematical statement was

340
00:13:25,855 --> 00:13:26,914
that, it

341
00:13:27,695 --> 00:13:29,475
it could be taken to,

342
00:13:31,100 --> 00:13:33,200
could be translated into an experiment.

343
00:13:33,820 --> 00:13:34,220
And,

344
00:13:34,779 --> 00:13:35,279
so

345
00:13:36,139 --> 00:13:39,179
so John Bell proposed what we refer today

346
00:13:39,179 --> 00:13:41,420
as Bell inequalities. In fact, there is just

347
00:13:41,420 --> 00:13:42,639
not, like, one

348
00:13:43,235 --> 00:13:45,475
Bell inequality, but there are many versions of

349
00:13:45,475 --> 00:13:47,554
it. So the most common one is known

350
00:13:47,554 --> 00:13:49,654
as a CHSH inequality, but,

351
00:13:50,514 --> 00:13:52,434
but let let's refer to them as Bell

352
00:13:52,434 --> 00:13:55,334
inequalities. So so Bell inequality is a statement

353
00:13:55,714 --> 00:13:58,195
which to an experimentalist, it means go to

354
00:13:58,195 --> 00:13:59,779
the lab, set up experiment,

355
00:14:00,559 --> 00:14:03,059
and observe certain correlations. So,

356
00:14:03,600 --> 00:14:05,600
you have to have two particles, like two

357
00:14:05,600 --> 00:14:06,100
photons,

358
00:14:08,000 --> 00:14:10,100
and you send them into two different locations,

359
00:14:10,159 --> 00:14:12,720
and you want to, say, measure polarization of

360
00:14:12,720 --> 00:14:15,195
those photons. And when you do that, you

361
00:14:15,195 --> 00:14:18,654
realize that, the outcomes are not completely independent

362
00:14:18,715 --> 00:14:20,875
from each other. Those photons, which we call

363
00:14:20,875 --> 00:14:22,014
entangled photons,

364
00:14:22,715 --> 00:14:25,754
they re they respond to the measurements in

365
00:14:25,754 --> 00:14:28,175
a in a very correlated way.

366
00:14:29,610 --> 00:14:31,710
And then you you study those correlations.

367
00:14:32,490 --> 00:14:34,649
You measure several different types, like the four

368
00:14:34,649 --> 00:14:36,970
different types of correlations, and you set up

369
00:14:36,970 --> 00:14:37,470
inequality.

370
00:14:38,649 --> 00:14:41,210
And if this inequality is satisfied, then you

371
00:14:41,210 --> 00:14:42,009
say fine.

372
00:14:42,889 --> 00:14:45,835
That is consistent with what Einstein wanted to

373
00:14:45,835 --> 00:14:47,934
see. But if it is violated,

374
00:14:48,715 --> 00:14:50,174
then you know that Einstein

375
00:14:50,715 --> 00:14:54,235
view of local hidden variable model is is

376
00:14:54,235 --> 00:14:55,855
not correct, is refuted.

377
00:14:57,529 --> 00:14:59,850
And, again, you know, John Bell, as you

378
00:14:59,850 --> 00:15:02,090
said, in the early sixties, John Bell comes

379
00:15:02,090 --> 00:15:02,750
with this,

380
00:15:03,929 --> 00:15:04,669
Bell inequality

381
00:15:05,049 --> 00:15:05,549
statement.

382
00:15:06,570 --> 00:15:07,470
One would say

383
00:15:08,355 --> 00:15:11,235
from today's perspective, great thing. You should rush

384
00:15:11,235 --> 00:15:13,235
to the lab immediately and do all those

385
00:15:13,235 --> 00:15:15,475
kind of experiments. That didn't happen for for

386
00:15:15,634 --> 00:15:17,495
maybe for two reasons. First of all,

387
00:15:17,875 --> 00:15:19,334
most people didn't actually

388
00:15:21,794 --> 00:15:24,309
look at it as or didn't consider this

389
00:15:24,309 --> 00:15:25,670
as an interesting result.

390
00:15:26,070 --> 00:15:28,230
It was still on the fringe. It was

391
00:15:28,230 --> 00:15:30,570
still mostly philosophical thing.

392
00:15:31,029 --> 00:15:33,110
The other reason was the technology to,

393
00:15:33,750 --> 00:15:36,950
implement this kind of experiment was not, so

394
00:15:36,950 --> 00:15:38,325
mature at the time.

395
00:15:39,524 --> 00:15:42,085
So to the, you know, another thirty years

396
00:15:42,085 --> 00:15:42,985
or so, right,

397
00:15:43,524 --> 00:15:45,304
from the sixties to,

398
00:15:47,605 --> 00:15:48,825
like, eighties and nineties

399
00:15:49,365 --> 00:15:51,605
for people to start playing with it. And,

400
00:15:51,605 --> 00:15:52,105
again,

401
00:15:52,644 --> 00:15:55,720
not not mainstream physics, really. If you look

402
00:15:55,720 --> 00:15:57,579
at, those early experiments,

403
00:15:58,519 --> 00:16:01,559
again, a bunch of outliers, a postdoc who,

404
00:16:01,879 --> 00:16:04,039
like John Clauser, who didn't really want to

405
00:16:04,039 --> 00:16:06,440
do what his supervisor was asking him to

406
00:16:06,440 --> 00:16:09,705
do and started playing with that, Alain Aspair,

407
00:16:10,644 --> 00:16:11,464
a colorful,

408
00:16:12,325 --> 00:16:13,945
character himself who

409
00:16:14,325 --> 00:16:16,644
with a passion to really understand how it

410
00:16:16,644 --> 00:16:17,384
all works,

411
00:16:18,004 --> 00:16:20,504
and many others, and and later Anton Zeilinger

412
00:16:20,565 --> 00:16:21,225
as well.

413
00:16:22,259 --> 00:16:24,839
So what happened was that the Klaus who

414
00:16:25,779 --> 00:16:27,000
set up this experiment,

415
00:16:28,820 --> 00:16:30,820
he was the first to see the violation

416
00:16:30,820 --> 00:16:32,659
of the Bell inequalities. But I want to

417
00:16:32,659 --> 00:16:34,580
also add that at the time, there were

418
00:16:34,580 --> 00:16:35,080
experiments

419
00:16:36,174 --> 00:16:38,174
which were pointing in the other direction saying

420
00:16:38,174 --> 00:16:38,995
that, you know,

421
00:16:39,454 --> 00:16:42,495
maybe Einstein is right. So it was not

422
00:16:42,495 --> 00:16:44,995
entirely obvious. It was one of those interesting

423
00:16:45,054 --> 00:16:46,355
experiments where

424
00:16:47,375 --> 00:16:49,554
it's it's I wouldn't call it a cliffhanger,

425
00:16:49,695 --> 00:16:51,929
but some or something of that kind. Right?

426
00:16:51,929 --> 00:16:53,789
So so you are there as an experiment.

427
00:16:54,330 --> 00:16:57,370
Imagine Hamish, you go to this lab of

428
00:16:57,370 --> 00:16:59,149
Alain Aspair and look at this,

429
00:16:59,690 --> 00:17:01,929
set of correlations and thinking which way will

430
00:17:01,929 --> 00:17:04,214
it go. So there's the suspense. Right? Is

431
00:17:04,214 --> 00:17:06,534
it going to be local hidden variable or

432
00:17:06,534 --> 00:17:08,634
quantum rules the world and forget about,

433
00:17:09,414 --> 00:17:10,315
any determinism?

434
00:17:11,174 --> 00:17:11,674
So,

435
00:17:13,414 --> 00:17:16,289
so the the the sequence of those experiments

436
00:17:16,349 --> 00:17:19,009
starting from John Clauser, then Alain Aspair,

437
00:17:19,950 --> 00:17:21,650
and then Nicolas Gisen,

438
00:17:22,589 --> 00:17:23,150
and then,

439
00:17:23,950 --> 00:17:27,150
many other colleagues to mention, even, our British

440
00:17:27,150 --> 00:17:29,630
colleagues here, like John Rarity and and Paul

441
00:17:29,630 --> 00:17:30,130
Tapster

442
00:17:30,715 --> 00:17:32,494
at the Defense Research Agency

443
00:17:33,035 --> 00:17:34,815
that I had the pleasure to work with.

444
00:17:36,154 --> 00:17:36,894
So they

445
00:17:37,914 --> 00:17:40,955
they they implemented those experiments. And and then

446
00:17:40,955 --> 00:17:42,414
Anton Zeilinger who,

447
00:17:44,235 --> 00:17:46,015
really managed to close

448
00:17:46,640 --> 00:17:48,740
many loopholes in those experiments.

449
00:17:51,119 --> 00:17:53,279
So so the so the that that sequence

450
00:17:53,279 --> 00:17:55,619
of experiments over a period of time

451
00:17:57,039 --> 00:17:58,259
made it quite clear

452
00:17:58,694 --> 00:17:58,934
that,

453
00:18:00,375 --> 00:18:03,095
the local hidden variable model is not sustainable,

454
00:18:03,095 --> 00:18:04,934
so it doesn't really work. So it it

455
00:18:05,015 --> 00:18:07,894
you have to think about inherent randomness if

456
00:18:07,894 --> 00:18:08,454
you want to

457
00:18:09,174 --> 00:18:10,394
if you drop the multiverse,

458
00:18:11,015 --> 00:18:14,119
just then there's inherent randomness in nature.

459
00:18:14,900 --> 00:18:16,039
I see. And

460
00:18:16,819 --> 00:18:17,319
how

461
00:18:18,819 --> 00:18:20,599
how do you interpret

462
00:18:21,299 --> 00:18:23,539
or or or how should one interpret a

463
00:18:24,154 --> 00:18:25,534
well, let's say a positive

464
00:18:26,075 --> 00:18:28,474
bell result, positive in the sense that it

465
00:18:28,474 --> 00:18:29,294
points towards

466
00:18:29,994 --> 00:18:33,134
quantum physics. Do do you interpret that as,

467
00:18:33,595 --> 00:18:34,095
yes,

468
00:18:34,794 --> 00:18:37,054
entanglement as predicted by

469
00:18:37,380 --> 00:18:39,960
quantum physics is a real thing,

470
00:18:40,339 --> 00:18:42,420
and that the system that you are looking

471
00:18:42,420 --> 00:18:44,200
at must be quantum,

472
00:18:44,660 --> 00:18:47,859
it simply cannot be a classical system. Is

473
00:18:47,859 --> 00:18:50,099
that the the right way to interpret? Yes,

474
00:18:50,099 --> 00:18:51,734
of course. If you if you want to

475
00:18:51,974 --> 00:18:52,875
use this dichotomy,

476
00:18:53,335 --> 00:18:55,335
classical and quantum, so that's one way of

477
00:18:55,335 --> 00:18:57,414
looking at it. Usually, you just take the

478
00:18:57,414 --> 00:18:59,734
view that everything is quantum and classical. It's

479
00:18:59,734 --> 00:19:01,595
just an approximation to quantum.

480
00:19:02,214 --> 00:19:02,535
And,

481
00:19:03,255 --> 00:19:05,355
so having this, Bell test,

482
00:19:06,375 --> 00:19:08,909
it it shows that, there's just no way

483
00:19:08,909 --> 00:19:10,849
to explain what you see,

484
00:19:11,950 --> 00:19:15,169
using, kind of a classical tricks where things

485
00:19:15,309 --> 00:19:18,029
can be deterministic. You can you have to

486
00:19:18,029 --> 00:19:19,250
just drop your hope

487
00:19:19,630 --> 00:19:21,809
that you will be able in some experiments

488
00:19:22,345 --> 00:19:24,744
to say exactly that your photon will go

489
00:19:24,744 --> 00:19:27,065
into this photo detector and not into that

490
00:19:27,065 --> 00:19:28,924
one. So so that's that's not possible.

491
00:19:29,704 --> 00:19:30,105
And,

492
00:19:30,585 --> 00:19:32,984
so that's that's a big thing. Right. And

493
00:19:32,984 --> 00:19:33,484
and

494
00:19:33,944 --> 00:19:34,764
would it mean,

495
00:19:35,144 --> 00:19:36,000
you know, I know

496
00:19:36,400 --> 00:19:37,920
going back to 1935,

497
00:19:37,920 --> 00:19:38,420
Einstein

498
00:19:39,200 --> 00:19:41,059
and his colleagues had their concerns.

499
00:19:42,880 --> 00:19:45,539
If, you know, if Einstein were around when

500
00:19:45,920 --> 00:19:49,119
those experiments were finally done, and Podolsky and

501
00:19:49,119 --> 00:19:51,214
Rosen, Do you think that they would be

502
00:19:51,434 --> 00:19:52,674
satisfied that they would,

503
00:19:53,515 --> 00:19:55,994
Well, what are Their their their question would

504
00:19:55,994 --> 00:19:57,994
be answered by those experiments. Of course, a

505
00:19:57,994 --> 00:19:59,994
very interesting question. So I I would it

506
00:19:59,994 --> 00:20:01,375
would be fantastic to

507
00:20:01,835 --> 00:20:03,454
have Einstein at the time

508
00:20:04,019 --> 00:20:04,420
and,

509
00:20:04,820 --> 00:20:07,480
and see his reaction to to those experiments.

510
00:20:09,220 --> 00:20:11,299
I I think there are two ways around.

511
00:20:11,299 --> 00:20:13,700
So first of all, there's still still a

512
00:20:13,700 --> 00:20:16,340
possibility, but I don't think Einstein would subscribe

513
00:20:16,340 --> 00:20:16,845
to this.

514
00:20:17,404 --> 00:20:18,625
But there's still a possibility

515
00:20:19,005 --> 00:20:20,384
that everything is deterministic

516
00:20:20,765 --> 00:20:23,184
because when you run the Bell measurement,

517
00:20:25,404 --> 00:20:27,345
in the process of running this

518
00:20:28,684 --> 00:20:29,184
experiment,

519
00:20:29,859 --> 00:20:31,880
you have to make some random choices.

520
00:20:32,259 --> 00:20:34,740
It's very important that you randomly choose the

521
00:20:34,740 --> 00:20:35,240
setting,

522
00:20:36,019 --> 00:20:36,500
of,

523
00:20:36,819 --> 00:20:37,720
measuring devices

524
00:20:38,179 --> 00:20:39,880
which measure, say, polarization.

525
00:20:40,819 --> 00:20:41,319
Now

526
00:20:43,139 --> 00:20:44,440
if those choices

527
00:20:45,065 --> 00:20:45,724
are predetermined,

528
00:20:46,505 --> 00:20:47,244
not random,

529
00:20:47,944 --> 00:20:48,345
then,

530
00:20:48,744 --> 00:20:51,144
you can see the violation of the Bell

531
00:20:51,144 --> 00:20:51,644
inequalities

532
00:20:52,025 --> 00:20:54,904
even though you can still explain everything using

533
00:20:54,904 --> 00:20:57,625
this local hidden variable model. So what I'm

534
00:20:57,625 --> 00:20:58,605
saying is that

535
00:20:58,909 --> 00:21:01,169
if you don't have free will as experimenter

536
00:21:01,950 --> 00:21:04,509
to have this free freedom of choosing the

537
00:21:04,509 --> 00:21:05,009
setting

538
00:21:05,950 --> 00:21:08,750
so that everything can be then scripted, your

539
00:21:08,750 --> 00:21:09,069
action

540
00:21:09,950 --> 00:21:12,269
so if you if you involve absolutely everything,

541
00:21:12,269 --> 00:21:14,974
the whole universe, so call it superdeterminism,

542
00:21:16,154 --> 00:21:18,494
then, of course, we cannot exclude this possibility.

543
00:21:18,714 --> 00:21:21,855
So this particular experiment assumes that,

544
00:21:22,394 --> 00:21:25,535
the the settings of those devices are independent

545
00:21:25,755 --> 00:21:27,375
from devices itself.

546
00:21:28,619 --> 00:21:30,700
Not so many people would subscribe to this

547
00:21:30,700 --> 00:21:33,680
worldview that everything is absolutely super deterministic.

548
00:21:34,619 --> 00:21:36,460
We would like to believe that we have

549
00:21:36,460 --> 00:21:39,019
free will, that the whole point of science

550
00:21:39,019 --> 00:21:41,660
is to make choices and and set up

551
00:21:41,660 --> 00:21:43,599
devices and and see what happens,

552
00:21:44,154 --> 00:21:45,914
but that cannot be excluded. So it has

553
00:21:45,914 --> 00:21:47,275
to be mentioned that there is still this

554
00:21:47,275 --> 00:21:49,515
possibility. Another possibility, of course, I don't know,

555
00:21:49,515 --> 00:21:52,075
again, whether Einstein will subscribe to it, would

556
00:21:52,075 --> 00:21:54,555
be the Everett interpretation, which has all kind

557
00:21:54,555 --> 00:21:56,414
of interesting features, which somehow

558
00:21:57,035 --> 00:21:58,174
tells you that

559
00:21:59,509 --> 00:22:01,130
that there are just

560
00:22:01,509 --> 00:22:03,769
that we live in this multiple of realities,

561
00:22:04,069 --> 00:22:04,470
and,

562
00:22:05,109 --> 00:22:07,029
and it's not as science fiction as it

563
00:22:07,029 --> 00:22:09,769
sounds. Right? I know people just, like Everett

564
00:22:09,829 --> 00:22:10,329
interpretation

565
00:22:10,630 --> 00:22:13,445
because it has all these strange connotations to

566
00:22:13,445 --> 00:22:15,525
many parallel unitises and so on and so

567
00:22:15,525 --> 00:22:18,244
forth. But, honestly, if you take quantum physics

568
00:22:18,244 --> 00:22:18,744
seriously

569
00:22:19,605 --> 00:22:19,924
and,

570
00:22:20,884 --> 00:22:21,785
say that there's

571
00:22:22,244 --> 00:22:22,744
no

572
00:22:23,605 --> 00:22:26,569
border between classical and quantum, that there's not

573
00:22:26,569 --> 00:22:28,089
that much you are left with. If you

574
00:22:28,170 --> 00:22:30,170
if you're serious about this description, then you

575
00:22:30,170 --> 00:22:31,069
have to somehow,

576
00:22:31,769 --> 00:22:34,329
lean towards the Everett interpretation. So I I

577
00:22:34,329 --> 00:22:34,829
guess

578
00:22:35,369 --> 00:22:37,710
my hope my hope is, you know,

579
00:22:38,009 --> 00:22:41,214
is that Einstein would endorse the Everett interpretation.

580
00:22:41,514 --> 00:22:42,734
That that's my guess.

581
00:22:43,434 --> 00:22:46,575
So, Archer, that brings us very nicely to,

582
00:22:47,274 --> 00:22:49,615
quantum cryptography and in particularly

583
00:22:51,194 --> 00:22:52,014
and in particular,

584
00:22:52,474 --> 00:22:54,414
the, e 91

585
00:22:54,690 --> 00:22:55,809
protocol that,

586
00:22:56,210 --> 00:22:57,509
that you came up with.

587
00:22:58,769 --> 00:23:01,169
Can you explain how it is or how

588
00:23:01,169 --> 00:23:04,470
it relies on Bell's theorem or is inspired

589
00:23:04,769 --> 00:23:05,829
by Bell's theorem?

590
00:23:06,609 --> 00:23:07,669
Yes. So that's,

591
00:23:08,529 --> 00:23:09,909
what happened was that

592
00:23:11,144 --> 00:23:11,884
when I

593
00:23:12,345 --> 00:23:13,965
was a PhD student in Oxford,

594
00:23:15,384 --> 00:23:17,485
I was supposed to be working on

595
00:23:18,904 --> 00:23:19,404
improving

596
00:23:20,345 --> 00:23:22,125
quantum communication with

597
00:23:23,019 --> 00:23:26,059
using what quantum opticians at that time called,

598
00:23:26,460 --> 00:23:28,079
the squeezed states of light.

599
00:23:28,619 --> 00:23:30,480
It was interesting topic even though,

600
00:23:31,660 --> 00:23:33,119
I have to say improve

601
00:23:33,660 --> 00:23:35,980
improving signal to noise ratio and the hopes

602
00:23:35,980 --> 00:23:36,880
we had with

603
00:23:37,615 --> 00:23:38,355
squeezed states were,

604
00:23:40,894 --> 00:23:43,554
well, never really materialized the way we wanted,

605
00:23:44,255 --> 00:23:46,095
them. That doesn't mean that we don't use

606
00:23:46,095 --> 00:23:47,774
those states of light. I mean, we use

607
00:23:47,774 --> 00:23:49,394
them in very precise interferometers.

608
00:23:49,774 --> 00:23:51,730
But but for me, it was a starting

609
00:23:51,730 --> 00:23:54,069
point to look into the foundations of,

610
00:23:54,930 --> 00:23:55,910
quantum physics.

611
00:23:56,769 --> 00:23:59,490
And, with great interest, I was reading some

612
00:23:59,490 --> 00:24:02,869
historical papers and in particular, this 1935

613
00:24:03,329 --> 00:24:05,430
paper by Einstein, Podolsky, and Rosen.

614
00:24:05,904 --> 00:24:08,224
And Einstein and I have to say that

615
00:24:08,224 --> 00:24:09,365
I really like,

616
00:24:09,984 --> 00:24:11,125
the way he writes.

617
00:24:12,464 --> 00:24:14,964
So I'm assuming here that the paper was,

618
00:24:15,105 --> 00:24:16,484
mostly or influenced

619
00:24:17,585 --> 00:24:19,845
in style by Einstein. So Einstein's style

620
00:24:20,865 --> 00:24:22,789
is is is very clear.

621
00:24:23,649 --> 00:24:24,149
And,

622
00:24:25,970 --> 00:24:26,789
so EPR

623
00:24:27,490 --> 00:24:27,990
paper

624
00:24:29,329 --> 00:24:32,289
is carefully structured so that the Einstein wants

625
00:24:32,289 --> 00:24:32,950
to define

626
00:24:33,904 --> 00:24:35,125
what does it mean for

627
00:24:35,664 --> 00:24:38,164
something to exist, for example, and,

628
00:24:38,865 --> 00:24:41,204
and by exist meaning prior to the measurement.

629
00:24:42,304 --> 00:24:44,484
Because, you know, once you measure something, it's

630
00:24:44,865 --> 00:24:47,505
usually you say, okay. There was something there.

631
00:24:47,505 --> 00:24:48,964
It had a certain property.

632
00:24:49,399 --> 00:24:52,380
I measured it, and I reveal this property.

633
00:24:53,240 --> 00:24:53,980
If there's

634
00:24:54,519 --> 00:24:55,339
a a box,

635
00:24:56,119 --> 00:24:57,179
I look at it,

636
00:24:57,480 --> 00:24:59,339
switch on the light, and it's red,

637
00:24:59,720 --> 00:25:01,419
I measure a property,

638
00:25:01,880 --> 00:25:04,154
redness of the box. Right? So and but

639
00:25:04,154 --> 00:25:06,075
but but I assume, of course, that me

640
00:25:06,075 --> 00:25:07,855
looking at it didn't make it red.

641
00:25:08,235 --> 00:25:10,154
It used to be red. Right? So it

642
00:25:10,154 --> 00:25:11,375
had to be red before

643
00:25:11,674 --> 00:25:14,174
I look at it. Einstein wants to

644
00:25:15,275 --> 00:25:18,015
define what it means for something to exist.

645
00:25:18,630 --> 00:25:21,109
And, he has this statement. There was just,

646
00:25:21,109 --> 00:25:23,750
like, one statement in that paper that fired

647
00:25:23,750 --> 00:25:26,549
my imagination, really, and that was my starting

648
00:25:26,549 --> 00:25:29,190
point. And that statement roughly says that the

649
00:25:29,190 --> 00:25:30,330
element of reality

650
00:25:30,710 --> 00:25:31,529
that's Ishan

651
00:25:31,990 --> 00:25:32,490
just

652
00:25:32,950 --> 00:25:34,545
called something that that,

653
00:25:34,924 --> 00:25:35,664
you know, exists.

654
00:25:36,845 --> 00:25:39,325
The element of reality according to Einstein is

655
00:25:39,325 --> 00:25:40,945
defined in such a way that

656
00:25:41,325 --> 00:25:42,065
if you can

657
00:25:42,525 --> 00:25:45,244
find out about the value of a certain

658
00:25:45,244 --> 00:25:46,945
physical property without

659
00:25:47,419 --> 00:25:50,140
disturbing this property, then there exists an element

660
00:25:50,140 --> 00:25:51,359
of reality associated

661
00:25:51,660 --> 00:25:52,480
with this property.

662
00:25:53,819 --> 00:25:56,160
So that means, like, if we can

663
00:25:57,019 --> 00:25:58,619
measure something and use

664
00:25:59,259 --> 00:26:01,019
without messing up this property

665
00:26:01,774 --> 00:26:03,534
So that that that means that there's a

666
00:26:03,534 --> 00:26:05,855
notion of passive measurement. Right? So that that

667
00:26:05,855 --> 00:26:08,335
really reveals the value of this property without

668
00:26:08,335 --> 00:26:09,315
disturbing it.

669
00:26:10,335 --> 00:26:12,335
And then to me and and I had

670
00:26:12,335 --> 00:26:12,835
some

671
00:26:14,250 --> 00:26:14,990
from my

672
00:26:15,289 --> 00:26:17,549
previous interest in in secure communication.

673
00:26:17,930 --> 00:26:18,990
I had some,

674
00:26:20,490 --> 00:26:22,349
interest in the notion of eavesdropping

675
00:26:22,730 --> 00:26:25,785
in in a secure communication system. And then

676
00:26:26,984 --> 00:26:27,805
an eavesdropper

677
00:26:29,705 --> 00:26:31,005
wants to do exactly

678
00:26:31,945 --> 00:26:35,465
this, to measure something without disturbing this. So

679
00:26:35,465 --> 00:26:37,625
if there's a carrier of information that has

680
00:26:37,625 --> 00:26:40,045
certain properties and those properties carry

681
00:26:40,519 --> 00:26:42,380
data that you want to read.

682
00:26:42,839 --> 00:26:44,380
An eavesdropper wants to

683
00:26:45,079 --> 00:26:47,480
measure it without disturbing it, find out the

684
00:26:47,480 --> 00:26:50,359
value of this physical property without disturbing it.

685
00:26:50,359 --> 00:26:52,359
So then I made this mental connection. I

686
00:26:52,359 --> 00:26:53,384
think, gosh. You know?

687
00:26:53,865 --> 00:26:56,204
So Einstein is talking about eavesdropping

688
00:26:56,585 --> 00:26:58,684
even though it's not called eavesdropping,

689
00:27:00,105 --> 00:27:01,625
and maybe I can put it in in

690
00:27:01,625 --> 00:27:02,605
a different context.

691
00:27:03,384 --> 00:27:05,644
And then I knew about the Bell

692
00:27:06,105 --> 00:27:06,569
work,

693
00:27:07,769 --> 00:27:10,250
and I was, at the time, happily reading

694
00:27:10,250 --> 00:27:11,230
Alan Aspert,

695
00:27:12,649 --> 00:27:13,149
thesis

696
00:27:14,089 --> 00:27:16,589
with with description of his experiments.

697
00:27:17,609 --> 00:27:20,009
Then I thought, okay. We know that there

698
00:27:20,009 --> 00:27:20,909
are physical

699
00:27:21,505 --> 00:27:22,884
systems or experiments

700
00:27:23,825 --> 00:27:25,744
where you can show that there is no

701
00:27:25,744 --> 00:27:28,224
element of reality, that the whole thing, if

702
00:27:28,224 --> 00:27:30,085
the Bell inequality is violated,

703
00:27:30,865 --> 00:27:33,605
you cannot say that you have this physical

704
00:27:33,825 --> 00:27:35,365
the element of physical reality.

705
00:27:35,980 --> 00:27:36,720
It simply

706
00:27:37,500 --> 00:27:39,899
can be then translated into the statement that

707
00:27:39,899 --> 00:27:42,000
there's just no way there was an eavesdropping.

708
00:27:44,139 --> 00:27:46,059
And so that was the connection. So from

709
00:27:46,059 --> 00:27:48,559
Einstein definition of the element of reality,

710
00:27:49,294 --> 00:27:50,595
knowing about the Bell

711
00:27:51,134 --> 00:27:54,194
theorem that that shows that Einstein approach,

712
00:27:54,654 --> 00:27:55,554
can be refuted,

713
00:27:56,174 --> 00:27:58,035
knowing that it can be implemented,

714
00:27:59,375 --> 00:28:00,275
by people,

715
00:28:00,974 --> 00:28:03,400
you know, people who implemented it to to

716
00:28:03,400 --> 00:28:04,839
mention Clauser and,

717
00:28:05,640 --> 00:28:09,420
Aspe and Nicolas Giza and Anton Zeilinger and

718
00:28:09,759 --> 00:28:10,259
and,

719
00:28:10,839 --> 00:28:13,640
Jan Wei Pan in China and, late and

720
00:28:13,640 --> 00:28:15,815
that was later, by the way, and

721
00:28:16,115 --> 00:28:17,414
and our colleagues here

722
00:28:17,954 --> 00:28:18,774
in in Malvern.

723
00:28:19,554 --> 00:28:19,875
So that's

724
00:28:20,674 --> 00:28:21,734
knowing all this,

725
00:28:22,674 --> 00:28:25,095
you know, I just went on and and

726
00:28:25,154 --> 00:28:27,554
and and said, well, I can construct a

727
00:28:27,554 --> 00:28:28,054
system

728
00:28:28,930 --> 00:28:29,430
where

729
00:28:30,130 --> 00:28:31,990
the violation of Belling Equality

730
00:28:32,769 --> 00:28:34,549
certifies the lack of eavesdropping.

731
00:28:35,090 --> 00:28:37,170
And so that that that is sort of

732
00:28:37,170 --> 00:28:39,430
like a vagues a very very vague explanation.

733
00:28:39,650 --> 00:28:41,410
This is where the idea comes. And, of

734
00:28:41,410 --> 00:28:42,950
course, you know, you have the idea,

735
00:28:43,494 --> 00:28:45,734
and there's a long way to go because

736
00:28:45,734 --> 00:28:47,835
you have to do all kind of mathematical

737
00:28:47,974 --> 00:28:49,835
gymnastics to make sure that,

738
00:28:51,174 --> 00:28:52,794
you look at it as a cryptographer,

739
00:28:53,174 --> 00:28:55,894
that you look at the different property, you

740
00:28:55,894 --> 00:28:57,970
look at the secret key rate, and you

741
00:28:57,970 --> 00:28:59,269
introduce new things.

742
00:29:00,049 --> 00:29:02,150
So that's, that's I had to,

743
00:29:02,609 --> 00:29:04,710
work a little bit on that.

744
00:29:05,569 --> 00:29:06,789
And and that was,

745
00:29:07,250 --> 00:29:08,630
yeah, that was it, actually.

746
00:29:09,075 --> 00:29:10,615
So so so the idea,

747
00:29:10,994 --> 00:29:13,174
if I understand it correctly, is that

748
00:29:14,755 --> 00:29:17,335
information is sent, and it is,

749
00:29:18,674 --> 00:29:21,075
I suppose in in in a quantum form.

750
00:29:21,075 --> 00:29:22,755
Is that the right the right way to

751
00:29:22,755 --> 00:29:24,500
explain it? If an eavesdropper

752
00:29:24,799 --> 00:29:27,220
makes a measurement on it, it loses

753
00:29:27,839 --> 00:29:30,400
some or all of its quantumness. Correct. And

754
00:29:30,400 --> 00:29:33,140
then when you make the Bell test,

755
00:29:33,839 --> 00:29:35,440
if if if if you don't get a

756
00:29:35,440 --> 00:29:37,519
thumbs up on quantum from the Bell test,

757
00:29:37,519 --> 00:29:38,180
you know

758
00:29:38,535 --> 00:29:39,755
that somebody has

759
00:29:40,055 --> 00:29:42,215
fiddled around with that information. Could have in

760
00:29:42,215 --> 00:29:44,455
principle fiddled with that. Yeah. Yeah. That's right.

761
00:29:44,455 --> 00:29:44,955
So

762
00:29:45,894 --> 00:29:48,315
you're absolutely right. So the the the quantumness,

763
00:29:48,695 --> 00:29:51,815
the that is indicated by the violation of

764
00:29:51,815 --> 00:29:52,715
the Bell inequality

765
00:29:53,380 --> 00:29:55,720
is lost when there is an eavesdropper

766
00:29:56,259 --> 00:29:58,279
because that eavesdropper would make

767
00:29:59,140 --> 00:30:00,359
would introduce this

768
00:30:00,900 --> 00:30:01,960
element of reality.

769
00:30:02,740 --> 00:30:04,039
And and so,

770
00:30:05,059 --> 00:30:06,819
so, yeah, so that you can translate it

771
00:30:06,819 --> 00:30:10,234
into something that, is called key distribution cryptography.

772
00:30:10,534 --> 00:30:11,434
So that means

773
00:30:11,815 --> 00:30:12,794
that two individuals

774
00:30:13,095 --> 00:30:16,454
using exactly the experiment that was, set up

775
00:30:16,454 --> 00:30:16,954
by,

776
00:30:17,815 --> 00:30:20,294
those pioneers who started playing with the Bell

777
00:30:20,294 --> 00:30:20,794
inequalities,

778
00:30:22,100 --> 00:30:24,200
then you can you can simply just,

779
00:30:24,820 --> 00:30:26,759
use this experiment with some

780
00:30:27,059 --> 00:30:28,840
modification and minor variation

781
00:30:29,460 --> 00:30:30,279
of those experiments,

782
00:30:31,619 --> 00:30:34,119
to allow two individuals in different locations

783
00:30:34,924 --> 00:30:38,204
to share private random sequences that later on

784
00:30:38,204 --> 00:30:40,144
can be used for cryptographic purposes.

785
00:30:40,845 --> 00:30:42,704
So you you've come up with this idea,

786
00:30:43,005 --> 00:30:45,005
and, I suppose the next step is to

787
00:30:45,005 --> 00:30:46,545
to show that it's possible,

788
00:30:47,130 --> 00:30:49,609
And, which I'm guessing was probably not not

789
00:30:49,609 --> 00:30:52,170
a very easy thing. No. No. Yeah. You're

790
00:30:52,170 --> 00:30:52,670
right.

791
00:30:53,210 --> 00:30:54,809
So you joined forces with,

792
00:30:55,289 --> 00:30:56,109
John Rarity

793
00:30:56,570 --> 00:30:57,869
and Paul Tapster.

794
00:30:58,730 --> 00:31:00,490
And they were I'm right in thinking they

795
00:31:00,490 --> 00:31:01,470
were at The UK's

796
00:31:01,954 --> 00:31:02,454
Defense

797
00:31:02,755 --> 00:31:03,974
Research Agency

798
00:31:04,515 --> 00:31:06,115
in Malvern? That's correct.

799
00:31:06,835 --> 00:31:09,954
And, which is in, I suppose in, Southwest

800
00:31:09,954 --> 00:31:11,974
England, or is it the Midlands, or

801
00:31:12,434 --> 00:31:15,190
on the border between, the West Country and

802
00:31:15,190 --> 00:31:15,769
the Midlands.

803
00:31:16,789 --> 00:31:18,970
And this was in 1991.

804
00:31:20,710 --> 00:31:22,070
Can you give us an idea of how

805
00:31:22,070 --> 00:31:24,390
you did that experiment or how you Yeah.

806
00:31:24,470 --> 00:31:26,070
And John and Paul did that ex So

807
00:31:26,070 --> 00:31:27,910
it's interesting, you know, because I I had

808
00:31:27,910 --> 00:31:28,214
this,

809
00:31:29,015 --> 00:31:30,634
I wrote this paper and

810
00:31:32,855 --> 00:31:34,714
and the the two things happened.

811
00:31:35,494 --> 00:31:36,855
First of all, you know, as a PhD

812
00:31:36,855 --> 00:31:37,355
student,

813
00:31:38,615 --> 00:31:40,934
you you work on something that that is

814
00:31:40,934 --> 00:31:42,794
a bit bizarre. It was another mainstream.

815
00:31:45,390 --> 00:31:48,349
And I had very low confidence in myself

816
00:31:48,349 --> 00:31:50,049
at the time, so I didn't think

817
00:31:50,429 --> 00:31:50,910
that,

818
00:31:53,069 --> 00:31:54,669
well, at least I was not so sure

819
00:31:54,669 --> 00:31:57,575
that that that that really makes sense. And

820
00:31:57,575 --> 00:31:59,494
then two things happened. The one thing was

821
00:31:59,494 --> 00:32:02,055
that I learned from, a colleague of mine

822
00:32:02,055 --> 00:32:02,555
that,

823
00:32:04,134 --> 00:32:05,994
Charlie Bennett and Gilles Brassard,

824
00:32:08,295 --> 00:32:10,934
my colleagues from, one from US, One from

825
00:32:10,934 --> 00:32:11,434
Canada,

826
00:32:12,869 --> 00:32:15,289
actually worked on something very similar,

827
00:32:16,630 --> 00:32:19,210
namely using quantum phenomena to

828
00:32:20,069 --> 00:32:22,170
set up a key distribution in cryptography.

829
00:32:22,710 --> 00:32:24,549
So that was to me even though I

830
00:32:24,549 --> 00:32:26,490
didn't know this work. There was no Internet

831
00:32:26,630 --> 00:32:28,855
at the time, and, and they published

832
00:32:29,634 --> 00:32:31,255
the work in a very obscure

833
00:32:31,794 --> 00:32:33,894
conference proceedings from Bangalore,

834
00:32:34,994 --> 00:32:35,894
which, you know,

835
00:32:36,274 --> 00:32:38,274
didn't exist in even in in Oxford, you

836
00:32:38,274 --> 00:32:39,875
couldn't find such a thing. And and, you

837
00:32:39,875 --> 00:32:41,394
know, to find it, you would have to

838
00:32:41,394 --> 00:32:44,070
know what to look for, but but I

839
00:32:44,070 --> 00:32:46,390
didn't. I didn't think that anyone was working

840
00:32:46,390 --> 00:32:47,210
on this. And

841
00:32:47,830 --> 00:32:48,309
so,

842
00:32:48,789 --> 00:32:49,910
but with the help of,

843
00:32:50,309 --> 00:32:51,509
David Deutsch, my,

844
00:32:52,070 --> 00:32:53,609
mentor here in Oxford,

845
00:32:55,625 --> 00:32:58,764
and, and Peter Knight, who was my supervisor

846
00:32:59,464 --> 00:33:01,325
at the time, Imperial College.

847
00:33:02,345 --> 00:33:04,105
I I had a chance to meet and

848
00:33:04,105 --> 00:33:06,845
talk to Charlie Bennett and and Gilles Brassard.

849
00:33:07,909 --> 00:33:10,390
And, I was both happy and unhappy, I

850
00:33:10,390 --> 00:33:11,929
have to say, at the same time.

851
00:33:12,390 --> 00:33:13,450
Happy because,

852
00:33:14,309 --> 00:33:16,390
that gave me confidence that what I was

853
00:33:16,390 --> 00:33:17,289
working on,

854
00:33:18,470 --> 00:33:21,829
was relevant, was maybe important. And if those

855
00:33:21,829 --> 00:33:22,305
guys

856
00:33:22,865 --> 00:33:25,125
rather established an eminent,

857
00:33:26,305 --> 00:33:26,805
physicist

858
00:33:27,105 --> 00:33:29,345
and computer scientist, I think at the time

859
00:33:29,345 --> 00:33:31,505
Gilles would refer to himself as a computer

860
00:33:31,505 --> 00:33:32,005
scientist.

861
00:33:34,225 --> 00:33:34,805
I think,

862
00:33:35,250 --> 00:33:36,849
so that was great, you know, that that

863
00:33:36,849 --> 00:33:39,410
that that means that I was working on

864
00:33:39,410 --> 00:33:41,410
something important. So that that gave me sort

865
00:33:41,410 --> 00:33:42,630
of a boost of confidence.

866
00:33:43,170 --> 00:33:44,630
I'm happy because, you know,

867
00:33:45,089 --> 00:33:46,230
would be nice to

868
00:33:47,009 --> 00:33:49,809
know that nobody thought about this before even

869
00:33:49,809 --> 00:33:50,309
though

870
00:33:50,734 --> 00:33:53,375
what Gilles and Charlie did was completely different.

871
00:33:53,375 --> 00:33:54,994
So they were looking at,

872
00:33:56,255 --> 00:33:58,275
how disturbance can be detected

873
00:33:58,894 --> 00:34:01,055
using different phenomena. So it was not really

874
00:34:01,055 --> 00:34:02,755
that much of a fusion of fundamental,

875
00:34:03,855 --> 00:34:04,914
physics with

876
00:34:05,549 --> 00:34:08,610
cryptography. It was Heisenberg answers the principle

877
00:34:09,869 --> 00:34:10,530
based on

878
00:34:11,309 --> 00:34:14,190
previous ideas introduced by Steven Wiesner, by late

879
00:34:14,190 --> 00:34:15,650
Steven Wiesner. So

880
00:34:16,110 --> 00:34:17,789
so I learned about that work,

881
00:34:18,190 --> 00:34:20,565
after I already did mine, and and that

882
00:34:20,565 --> 00:34:23,284
was interesting. So so I was confident that

883
00:34:23,284 --> 00:34:25,525
at least it's it's worth trying. It's worth

884
00:34:25,525 --> 00:34:27,364
doing something about it, but not so many

885
00:34:27,364 --> 00:34:28,905
people were really interested in.

886
00:34:29,364 --> 00:34:31,925
And and then another interesting thing happened. I

887
00:34:31,925 --> 00:34:32,164
was,

888
00:34:33,125 --> 00:34:33,605
giving,

889
00:34:34,349 --> 00:34:36,349
I was showing a poster, in fact, in

890
00:34:36,349 --> 00:34:37,170
one of those

891
00:34:37,550 --> 00:34:40,690
skiing conferences in Cortina D'Ampezzo, in fact.

892
00:34:41,470 --> 00:34:41,970
And

893
00:34:43,550 --> 00:34:45,250
on the slopes, I met generality.

894
00:34:46,204 --> 00:34:48,364
And while we were skiing, we were talking

895
00:34:48,364 --> 00:34:49,244
science, and,

896
00:34:50,525 --> 00:34:51,344
and John,

897
00:34:51,724 --> 00:34:54,525
who work at the Defense Research Agency in

898
00:34:54,525 --> 00:34:55,664
Malvern at the time,

899
00:34:55,965 --> 00:34:58,545
got interested. He said, well, look. We essentially

900
00:34:58,605 --> 00:35:00,480
have all the setup that

901
00:35:01,359 --> 00:35:03,039
that you are talking about so we can

902
00:35:03,039 --> 00:35:04,260
implement that thing.

903
00:35:05,359 --> 00:35:07,139
And so I was invited to,

904
00:35:07,840 --> 00:35:10,420
see their labs later. I traveled to Malvern,

905
00:35:11,199 --> 00:35:13,039
and it was it was really interesting to

906
00:35:13,039 --> 00:35:14,719
work with a duo of John Rarity and

907
00:35:14,719 --> 00:35:15,460
Paul Tapster.

908
00:35:17,025 --> 00:35:18,704
I I, you know, I like them because

909
00:35:18,704 --> 00:35:20,704
they were so different and so complimentary in

910
00:35:20,704 --> 00:35:21,284
a way.

911
00:35:21,585 --> 00:35:22,704
So John was,

912
00:35:23,184 --> 00:35:23,684
rather

913
00:35:24,464 --> 00:35:25,284
an extrovert,

914
00:35:26,464 --> 00:35:28,005
and and very

915
00:35:28,464 --> 00:35:28,964
much

916
00:35:29,589 --> 00:35:31,909
willing to talk about everything that they're working

917
00:35:31,909 --> 00:35:32,650
on and

918
00:35:33,349 --> 00:35:36,069
and traveling places, going to conferences, giving good

919
00:35:36,069 --> 00:35:36,569
talks.

920
00:35:37,030 --> 00:35:39,989
Paul, in contrast, was an introvert. So he

921
00:35:39,989 --> 00:35:42,069
worked in the lab, and, I think,

922
00:35:43,025 --> 00:35:43,844
for Paul,

923
00:35:44,224 --> 00:35:45,364
most of the world

924
00:35:45,664 --> 00:35:48,065
was confined to a triangle that was defined

925
00:35:48,065 --> 00:35:49,125
by his house,

926
00:35:49,505 --> 00:35:51,525
his lap in DRA,

927
00:35:51,825 --> 00:35:54,164
and his bridge clap. So he played bridge.

928
00:35:54,945 --> 00:35:56,864
So that was Paul. But Paul was one

929
00:35:56,864 --> 00:35:58,244
of the most amazing

930
00:35:59,079 --> 00:36:01,279
experimental physicists that I came across,

931
00:36:01,719 --> 00:36:04,699
so he could just make magic happen. So

932
00:36:04,760 --> 00:36:06,359
working with them was a was a great

933
00:36:06,359 --> 00:36:07,719
pleasure. I have to say as a theorist,

934
00:36:07,719 --> 00:36:09,480
you know, I just I was I was

935
00:36:09,480 --> 00:36:10,920
quite happy if I could just go to

936
00:36:10,920 --> 00:36:13,079
the lab and identify all the components on

937
00:36:13,079 --> 00:36:16,155
the Octopi table. So so I wouldn't claim

938
00:36:16,214 --> 00:36:18,454
that I really contributed that much to this

939
00:36:18,454 --> 00:36:20,775
experiment, but we worked together on this. And,

940
00:36:21,094 --> 00:36:23,574
we managed to demonstrate that it really works

941
00:36:23,574 --> 00:36:25,034
in this different setting.

942
00:36:27,655 --> 00:36:29,114
Violation of Bell inequalities

943
00:36:30,019 --> 00:36:30,519
can

944
00:36:30,900 --> 00:36:31,719
be, understood,

945
00:36:32,900 --> 00:36:35,780
or can be used for for practical purposes.

946
00:36:35,780 --> 00:36:38,260
So so at this point, the interesting fusion

947
00:36:38,260 --> 00:36:38,760
happened.

948
00:36:39,859 --> 00:36:41,400
On one hand, you have cryptographers

949
00:36:42,425 --> 00:36:43,324
working on,

950
00:36:43,864 --> 00:36:46,744
secure communication, and then there's this big problem

951
00:36:46,744 --> 00:36:48,764
that they call the key distribution problem.

952
00:36:49,224 --> 00:36:50,905
And this is like a holy grail of

953
00:36:50,905 --> 00:36:53,405
cryptography. Find the solution to the key distribution

954
00:36:53,545 --> 00:36:54,045
problem.

955
00:36:54,659 --> 00:36:56,179
On the other hand, you have a bunch

956
00:36:56,179 --> 00:36:56,500
of,

957
00:36:57,299 --> 00:37:00,019
outliers in physics working on the foundations, doing

958
00:37:00,019 --> 00:37:03,079
experiments, and nobody cares about those experiments, honestly,

959
00:37:03,380 --> 00:37:06,339
because most working physicists say, okay. Fine. You

960
00:37:06,339 --> 00:37:07,755
guys are telling us that

961
00:37:08,315 --> 00:37:10,414
quantum is the way to go. But we

962
00:37:10,634 --> 00:37:12,315
we had known this for a long time.

963
00:37:12,315 --> 00:37:15,194
Right? So so so why why bother so

964
00:37:15,194 --> 00:37:16,574
much? It's purely philosophical.

965
00:37:17,114 --> 00:37:18,494
But here comes a moment,

966
00:37:21,034 --> 00:37:22,574
in in 1991

967
00:37:23,390 --> 00:37:24,829
and then in 1992

968
00:37:24,829 --> 00:37:27,170
when when we do these experiments

969
00:37:27,630 --> 00:37:29,730
that shows to the world that it's not

970
00:37:30,829 --> 00:37:33,010
completely irrelevant what those

971
00:37:33,309 --> 00:37:34,349
weirdos doing,

972
00:37:35,295 --> 00:37:36,194
quantum foundations,

973
00:37:37,295 --> 00:37:40,335
think. And their tools can be useful for

974
00:37:40,335 --> 00:37:42,675
something very practical for secure communication.

975
00:37:43,214 --> 00:37:44,755
So this fusion happens.

976
00:37:45,534 --> 00:37:46,514
Quantum foundations

977
00:37:47,214 --> 00:37:49,875
offers a solution to the key distribution problem,

978
00:37:50,539 --> 00:37:52,639
and the cryptographers at the same time,

979
00:37:53,659 --> 00:37:55,260
getting a new set of tools to play

980
00:37:55,260 --> 00:37:55,760
with.

981
00:37:56,780 --> 00:37:58,320
And from this moment onwards,

982
00:37:59,099 --> 00:38:01,039
another interesting thing happens, namely

983
00:38:02,699 --> 00:38:03,440
the experiments

984
00:38:03,820 --> 00:38:06,835
that show the violation of Bell inequalities were

985
00:38:06,835 --> 00:38:07,575
not perfect.

986
00:38:08,275 --> 00:38:10,434
They were not perfect because there were various

987
00:38:10,434 --> 00:38:10,934
loopholes,

988
00:38:11,555 --> 00:38:14,614
and those loopholes assumed something that

989
00:38:15,075 --> 00:38:16,855
most physicists would just dismiss.

990
00:38:17,394 --> 00:38:18,135
For example,

991
00:38:18,880 --> 00:38:22,500
they assume that nature may cheat you

992
00:38:23,679 --> 00:38:24,340
by implementing

993
00:38:25,280 --> 00:38:27,920
such and such local hidden variable model in

994
00:38:27,920 --> 00:38:28,659
one experiment

995
00:38:29,519 --> 00:38:30,179
and then

996
00:38:31,535 --> 00:38:34,175
experimenting, maybe implementing maybe another one in another

997
00:38:34,175 --> 00:38:34,675
experiment.

998
00:38:35,695 --> 00:38:36,434
Or that

999
00:38:36,735 --> 00:38:38,595
if you don't have a perfect detection

1000
00:38:39,215 --> 00:38:39,715
that

1001
00:38:40,015 --> 00:38:41,555
nature will conspire

1002
00:38:41,855 --> 00:38:43,289
against you. So you,

1003
00:38:43,769 --> 00:38:45,690
in the data that you see, you see

1004
00:38:45,690 --> 00:38:47,469
the violation of the Bell inequalities.

1005
00:38:47,769 --> 00:38:50,030
But if you were to include all possible

1006
00:38:50,090 --> 00:38:52,890
data, including those that you didn't register

1007
00:38:53,769 --> 00:38:55,769
so if you combine them together, you wouldn't

1008
00:38:55,769 --> 00:38:57,550
see the violation of the Bell inequalities.

1009
00:38:58,155 --> 00:39:00,255
And then, you know, physicists would say, come

1010
00:39:00,715 --> 00:39:01,295
on. Honestly,

1011
00:39:02,074 --> 00:39:04,735
nature conspiring against you? No way.

1012
00:39:05,114 --> 00:39:07,454
But if you put the whole thing into

1013
00:39:07,675 --> 00:39:08,894
a crypto setting,

1014
00:39:09,195 --> 00:39:10,894
it's not nature. It's your adversary

1015
00:39:11,275 --> 00:39:14,110
there, and that adversary can cheat you in

1016
00:39:14,110 --> 00:39:16,510
any way possible. Right? So you have to

1017
00:39:16,510 --> 00:39:17,570
close those loopholes.

1018
00:39:18,190 --> 00:39:20,670
So that gave the final push to the

1019
00:39:20,670 --> 00:39:23,949
foundations. Right? So so now that the closing

1020
00:39:23,949 --> 00:39:27,164
those loopholes was not just question of being

1021
00:39:27,164 --> 00:39:30,125
sort of, like, pedanting from a philosophical point

1022
00:39:30,125 --> 00:39:32,045
of view. It was just more to say,

1023
00:39:32,045 --> 00:39:33,965
if you want to show the security of

1024
00:39:33,965 --> 00:39:36,065
the system, you have to close the loopholes

1025
00:39:36,125 --> 00:39:38,625
full stop. And here comes, I I think,

1026
00:39:39,405 --> 00:39:42,900
people like Rasmus Hansen and Anton Zeilinger and

1027
00:39:42,900 --> 00:39:43,639
and other

1028
00:39:44,019 --> 00:39:46,359
who had technology at the time

1029
00:39:46,980 --> 00:39:49,000
to close the loopholes, and that happened.

1030
00:39:49,380 --> 00:39:51,139
So so you could see at this point

1031
00:39:51,139 --> 00:39:54,255
a beautiful synergy of the two. Right? Cryptographers

1032
00:39:54,554 --> 00:39:55,054
telling

1033
00:39:55,355 --> 00:39:57,195
people in the foundations, come on, guys. You

1034
00:39:57,195 --> 00:39:58,255
have to close the loopholes.

1035
00:39:58,715 --> 00:40:00,715
And people on the foundation side say, oh,

1036
00:40:00,715 --> 00:40:03,195
great. You know? So far, nobody cared about

1037
00:40:03,195 --> 00:40:05,594
this, and now someone does. So let's close

1038
00:40:05,594 --> 00:40:07,775
the loopholes, and let's have those experiments.

1039
00:40:08,269 --> 00:40:08,769
So

1040
00:40:09,070 --> 00:40:12,349
so the Nobel Prize that's in 2022

1041
00:40:12,349 --> 00:40:13,329
went to Closer

1042
00:40:14,269 --> 00:40:16,210
and Asper and Zeilinger,

1043
00:40:18,190 --> 00:40:19,250
that really shows

1044
00:40:19,869 --> 00:40:20,369
evolution

1045
00:40:20,829 --> 00:40:21,309
of,

1046
00:40:21,789 --> 00:40:23,409
people starting from

1047
00:40:23,934 --> 00:40:24,434
doing

1048
00:40:25,534 --> 00:40:26,034
really,

1049
00:40:26,734 --> 00:40:27,554
blue sky,

1050
00:40:29,054 --> 00:40:30,594
curiosity driven research

1051
00:40:31,775 --> 00:40:32,275
to

1052
00:40:32,815 --> 00:40:34,034
beautiful implementation,

1053
00:40:34,414 --> 00:40:36,949
full implementation of the Bell inequalities

1054
00:40:38,690 --> 00:40:39,190
that

1055
00:40:39,489 --> 00:40:42,150
already comes with understanding that it's relevant.

1056
00:40:42,849 --> 00:40:46,070
Clauser and Asper were mostly driven by curiosity.

1057
00:40:46,210 --> 00:40:48,449
I'm Anton, I I think, was driven both

1058
00:40:48,449 --> 00:40:49,109
by curiosity.

1059
00:40:50,454 --> 00:40:52,454
So working with him was always a pleasure,

1060
00:40:52,454 --> 00:40:54,454
and we talk a lot about the philosophy

1061
00:40:54,454 --> 00:40:56,534
and foundations, but he also had a benefit

1062
00:40:56,534 --> 00:40:58,934
of understanding that this is important from the

1063
00:40:58,934 --> 00:41:00,315
practical point of view.

1064
00:41:01,574 --> 00:41:04,954
So, Archer, today, they're they're actually commercially available

1065
00:41:05,369 --> 00:41:06,509
quantum cryptography

1066
00:41:06,809 --> 00:41:07,309
systems.

1067
00:41:08,009 --> 00:41:09,130
Although I'm guessing that,

1068
00:41:10,089 --> 00:41:13,069
there's probably many improvements that can be made.

1069
00:41:13,130 --> 00:41:15,049
Do do you think that at some point

1070
00:41:15,049 --> 00:41:16,509
in time, quantum cryptography

1071
00:41:17,049 --> 00:41:18,269
is going to replace

1072
00:41:18,945 --> 00:41:21,364
the the sort of classical or standard cryptography

1073
00:41:21,664 --> 00:41:23,204
systems that we use today.

1074
00:41:25,664 --> 00:41:27,744
Well, I don't think that there will be

1075
00:41:27,744 --> 00:41:28,965
a complete replacement.

1076
00:41:30,385 --> 00:41:31,605
But I think that

1077
00:41:33,369 --> 00:41:35,949
quantum crypto will certainly play a significant

1078
00:41:36,250 --> 00:41:38,429
or increasingly significant role

1079
00:41:38,809 --> 00:41:40,349
in in our secure,

1080
00:41:41,050 --> 00:41:42,269
communication environment.

1081
00:41:43,369 --> 00:41:44,269
To start with,

1082
00:41:45,690 --> 00:41:46,909
one thing that happened

1083
00:41:47,530 --> 00:41:48,554
after my work

1084
00:41:49,914 --> 00:41:50,414
was,

1085
00:41:52,554 --> 00:41:55,135
I wouldn't well, improvement in understanding

1086
00:41:55,914 --> 00:41:57,454
of the system that I proposed,

1087
00:41:58,074 --> 00:41:58,554
and,

1088
00:41:58,875 --> 00:42:01,054
that is known as a device independent

1089
00:42:02,394 --> 00:42:02,894
cryptography.

1090
00:42:03,755 --> 00:42:05,730
And that that is a

1091
00:42:06,449 --> 00:42:08,210
from the fundamental point of view, it's it's

1092
00:42:08,210 --> 00:42:08,710
simply

1093
00:42:09,090 --> 00:42:10,070
absolutely unbelievable.

1094
00:42:11,410 --> 00:42:12,150
It essentially

1095
00:42:13,329 --> 00:42:14,070
says that

1096
00:42:15,010 --> 00:42:16,869
you can purchase devices

1097
00:42:18,625 --> 00:42:19,924
from your adversary,

1098
00:42:20,464 --> 00:42:21,444
from your enemy,

1099
00:42:22,625 --> 00:42:24,404
run the test on those devices.

1100
00:42:24,944 --> 00:42:27,744
And if they pass CHSH or Bell test,

1101
00:42:27,744 --> 00:42:28,244
essentially,

1102
00:42:28,784 --> 00:42:30,784
then those devices are good. I mean, you

1103
00:42:30,784 --> 00:42:32,644
can use them. You don't have to

1104
00:42:33,230 --> 00:42:35,309
inspect devices. You don't have to open them.

1105
00:42:35,309 --> 00:42:37,730
You don't have to tinker with unknown technology.

1106
00:42:37,789 --> 00:42:39,549
You know, usually, you don't have expertise to

1107
00:42:39,549 --> 00:42:40,210
do that.

1108
00:42:40,909 --> 00:42:43,889
So unlike today where we have to certify

1109
00:42:43,949 --> 00:42:46,349
those devices, so you produce a a device

1110
00:42:46,349 --> 00:42:47,089
that say

1111
00:42:47,585 --> 00:42:49,924
generates random numbers for crypto purposes.

1112
00:42:50,545 --> 00:42:52,625
So how do you know that this is

1113
00:42:52,625 --> 00:42:54,484
really generating random numbers?

1114
00:42:55,025 --> 00:42:55,344
And,

1115
00:42:55,824 --> 00:42:58,005
the only way is to trust

1116
00:42:59,630 --> 00:43:01,969
some sort of authority. So there's a certifying

1117
00:43:02,190 --> 00:43:03,410
agency that will

1118
00:43:04,269 --> 00:43:04,769
say,

1119
00:43:05,070 --> 00:43:06,989
okay. You know, a bunch of wise guys

1120
00:43:06,989 --> 00:43:09,070
looked into this, and we can get you

1121
00:43:09,070 --> 00:43:11,230
trust us, and we guarantee that this is

1122
00:43:11,230 --> 00:43:13,625
this is fine. But we don't want this

1123
00:43:13,625 --> 00:43:16,744
necessarily in the future. Right? So having the

1124
00:43:16,744 --> 00:43:18,364
self testifying devices,

1125
00:43:19,625 --> 00:43:21,804
and it's not so much about necessarily

1126
00:43:22,264 --> 00:43:24,425
buying them from the enemy. Nobody would do

1127
00:43:24,425 --> 00:43:26,984
it. But but sometimes when you desire a

1128
00:43:26,984 --> 00:43:27,484
device,

1129
00:43:29,090 --> 00:43:31,269
you unwillingly, you may introduce some

1130
00:43:32,769 --> 00:43:34,449
bugs into the system and,

1131
00:43:34,930 --> 00:43:36,610
and then you want to avoid this. So

1132
00:43:36,610 --> 00:43:38,690
so this self testing is a wonderful thing

1133
00:43:38,690 --> 00:43:41,030
because you don't have to do anything

1134
00:43:41,329 --> 00:43:43,425
inside the device. You don't touch the device.

1135
00:43:43,425 --> 00:43:46,305
You just provide those devices with some inputs

1136
00:43:46,305 --> 00:43:48,965
that generate some outputs. You run a statistical

1137
00:43:49,105 --> 00:43:50,945
test, and you say, okay. Those devices are

1138
00:43:50,945 --> 00:43:52,805
fine. No matter where they come from,

1139
00:43:53,265 --> 00:43:55,184
they pass the test. They pass the test.

1140
00:43:55,184 --> 00:43:55,925
They're good.

1141
00:43:56,809 --> 00:43:59,389
And, so this device independent crypto

1142
00:44:00,409 --> 00:44:00,909
is,

1143
00:44:01,530 --> 00:44:02,750
is fantastic because

1144
00:44:03,210 --> 00:44:04,829
you can then put your devices

1145
00:44:05,130 --> 00:44:08,349
into location, so, like, on satellite, for example.

1146
00:44:08,489 --> 00:44:10,010
And you don't have to worry too much

1147
00:44:10,010 --> 00:44:10,510
about

1148
00:44:11,224 --> 00:44:11,724
someone

1149
00:44:14,025 --> 00:44:16,664
tinkering with your devices there because if that

1150
00:44:16,664 --> 00:44:18,344
happens, you can see it in the test.

1151
00:44:18,344 --> 00:44:20,125
Right? So so they self testify

1152
00:44:20,425 --> 00:44:21,065
they self,

1153
00:44:21,545 --> 00:44:22,605
self test themselves.

1154
00:44:23,930 --> 00:44:26,650
So that that is from the fundamental point

1155
00:44:26,650 --> 00:44:28,190
of view. That's, actually

1156
00:44:28,890 --> 00:44:31,050
a very interesting new development. I think it's

1157
00:44:31,050 --> 00:44:33,930
pushing the the the limits of privacy to

1158
00:44:33,930 --> 00:44:34,510
the limits.

1159
00:44:37,210 --> 00:44:39,985
So that that self self testing, is that

1160
00:44:40,204 --> 00:44:42,125
is that a function of the the sort

1161
00:44:42,125 --> 00:44:44,684
of no go or go no go nature

1162
00:44:44,684 --> 00:44:47,164
of the Bell test? Yeah. It's it's a

1163
00:44:47,164 --> 00:44:48,545
function of something called,

1164
00:44:51,920 --> 00:44:53,840
we we say that the Bell result is

1165
00:44:53,840 --> 00:44:55,619
rigid. So that means that

1166
00:44:56,400 --> 00:44:58,320
to simplify things a little bit, that means

1167
00:44:58,320 --> 00:45:00,000
if you see the full violation of the

1168
00:45:00,000 --> 00:45:00,739
Bell inequality

1169
00:45:01,200 --> 00:45:03,059
so suppose you run the test

1170
00:45:03,840 --> 00:45:04,239
and,

1171
00:45:04,640 --> 00:45:07,644
you reach the maximum violation of the Bell

1172
00:45:07,644 --> 00:45:08,144
inequality

1173
00:45:08,605 --> 00:45:09,344
that is

1174
00:45:09,804 --> 00:45:10,704
that is possible.

1175
00:45:13,164 --> 00:45:14,304
Then you know

1176
00:45:14,844 --> 00:45:17,264
from this one number, you know that

1177
00:45:18,010 --> 00:45:20,329
there's no any other way for this to

1178
00:45:20,329 --> 00:45:22,970
happen but just to set up something that

1179
00:45:22,970 --> 00:45:24,829
is equivalent to the Bell test.

1180
00:45:26,090 --> 00:45:27,769
So you can argue the other way around.

1181
00:45:27,769 --> 00:45:29,610
So the the the violation of the Bell

1182
00:45:29,610 --> 00:45:30,110
inequality

1183
00:45:30,410 --> 00:45:31,230
tells you

1184
00:45:33,204 --> 00:45:35,224
that only by by looking at the data,

1185
00:45:35,284 --> 00:45:37,284
even just, you know, looking at one number

1186
00:45:37,284 --> 00:45:39,684
tells you that there's no way that could

1187
00:45:39,684 --> 00:45:41,925
have happened without setting up the Bell test.

1188
00:45:41,925 --> 00:45:43,144
So that was

1189
00:45:44,969 --> 00:45:46,650
then then, of course, you know, the we

1190
00:45:46,650 --> 00:45:47,309
know that,

1191
00:45:48,889 --> 00:45:50,889
it's very unlikely that you will get,

1192
00:45:51,530 --> 00:45:54,489
the maximum violation. There's always noise in the

1193
00:45:54,489 --> 00:45:56,010
system and so on and so forth. So

1194
00:45:56,010 --> 00:45:57,070
so you have to

1195
00:45:58,025 --> 00:46:00,825
look at some parameters, and from that parameters,

1196
00:46:00,825 --> 00:46:01,805
you can estimate,

1197
00:46:02,985 --> 00:46:06,105
the upper bound on information that any third

1198
00:46:06,105 --> 00:46:07,965
party eavesdropper included

1199
00:46:08,585 --> 00:46:10,765
and could have gained from from the system.

1200
00:46:11,900 --> 00:46:14,059
So, yeah, so that's that's that's that's a

1201
00:46:14,059 --> 00:46:15,039
beauty of,

1202
00:46:16,300 --> 00:46:17,360
of the Bell inequality

1203
00:46:17,660 --> 00:46:20,380
approach, which you cannot achieve with anything else.

1204
00:46:20,380 --> 00:46:21,180
So so this,

1205
00:46:22,140 --> 00:46:24,960
the system that Charlie and Gilles proposed

1206
00:46:26,554 --> 00:46:27,614
may work in

1207
00:46:28,154 --> 00:46:30,414
a in a scenario where you

1208
00:46:31,674 --> 00:46:34,014
you are confident that your devices are

1209
00:46:34,954 --> 00:46:37,454
exactly what they are and you are confident

1210
00:46:37,674 --> 00:46:38,174
about,

1211
00:46:38,630 --> 00:46:40,789
using more technical terms, the dimension of the

1212
00:46:40,789 --> 00:46:42,630
Hilbert space of your carrier and so on

1213
00:46:42,630 --> 00:46:43,369
and so forth,

1214
00:46:43,829 --> 00:46:46,150
but you cannot achieve the level of,

1215
00:46:48,070 --> 00:46:50,809
devising the penthouse that you can achieve with,

1216
00:46:51,190 --> 00:46:53,690
Bell test kind of crypto that I proposed.

1217
00:46:54,704 --> 00:46:56,945
So that that I see as, as an

1218
00:46:56,945 --> 00:46:59,204
interesting way to go. Now your question is,

1219
00:47:00,144 --> 00:47:02,385
will it ever happen that we will,

1220
00:47:03,744 --> 00:47:05,905
go fully quantum? I don't think so. So

1221
00:47:05,905 --> 00:47:08,405
I think that there's always a classical element

1222
00:47:08,465 --> 00:47:09,285
around it

1223
00:47:09,800 --> 00:47:12,440
if we were to divide things into classical

1224
00:47:12,440 --> 00:47:13,420
and quantum, but,

1225
00:47:13,880 --> 00:47:15,800
but that's fine. So I I think, there's

1226
00:47:15,800 --> 00:47:17,260
still a work you know,

1227
00:47:17,800 --> 00:47:19,159
lots of work to be done, I I

1228
00:47:19,159 --> 00:47:20,380
would say, to make it

1229
00:47:20,760 --> 00:47:22,940
practical. First of all, the quantum technologies

1230
00:47:26,025 --> 00:47:26,605
is still

1231
00:47:27,545 --> 00:47:29,964
noisy, so you have to improve it.

1232
00:47:30,585 --> 00:47:31,885
But it's possible.

1233
00:47:32,585 --> 00:47:33,625
The same year that,

1234
00:47:35,224 --> 00:47:36,045
John Clauser,

1235
00:47:36,505 --> 00:47:39,244
Alan Asper, Anton Zeilinger got the Nobel Prize,

1236
00:47:39,690 --> 00:47:42,030
There were three experiments showing the

1237
00:47:42,889 --> 00:47:45,530
proof of principle of the device independent crypto.

1238
00:47:45,530 --> 00:47:47,150
One of them here in Oxford,

1239
00:47:48,089 --> 00:47:49,929
one of them in Munich, and one of

1240
00:47:49,929 --> 00:47:50,750
them in China.

1241
00:47:51,369 --> 00:47:53,389
So all the three groups show that

1242
00:47:54,215 --> 00:47:56,074
technology is good enough to implement

1243
00:47:56,855 --> 00:47:59,835
device independent cryptography. From there to,

1244
00:48:01,335 --> 00:48:04,054
make make it commercial is still a way

1245
00:48:04,054 --> 00:48:04,715
to go.

1246
00:48:05,175 --> 00:48:07,335
But I think that that eventually, we will

1247
00:48:07,335 --> 00:48:10,880
use crypto in combination with, quantum crypto in

1248
00:48:10,880 --> 00:48:13,300
combination with various classical methods.

1249
00:48:13,760 --> 00:48:16,000
Quantum crypto is good for point to point

1250
00:48:16,000 --> 00:48:16,500
communication,

1251
00:48:18,639 --> 00:48:19,539
not so great

1252
00:48:20,704 --> 00:48:23,824
for things like digital signature, password protections, and

1253
00:48:23,824 --> 00:48:25,824
many other crypto tasks that you would like

1254
00:48:25,824 --> 00:48:26,565
to implement.

1255
00:48:27,025 --> 00:48:27,525
But

1256
00:48:28,144 --> 00:48:29,585
but, obviously, with,

1257
00:48:30,065 --> 00:48:32,085
the progress in in quantum

1258
00:48:32,545 --> 00:48:33,045
computing,

1259
00:48:36,230 --> 00:48:38,550
the whole notion of security in the new

1260
00:48:38,550 --> 00:48:39,449
quantum world

1261
00:48:40,710 --> 00:48:43,769
partially relies on the development in in quantum

1262
00:48:43,829 --> 00:48:46,545
cryptography. So there's probably no way around it.

1263
00:48:47,025 --> 00:48:49,184
And why not? You know, once we are

1264
00:48:49,184 --> 00:48:51,744
more familiar with quantum technology, why not taking

1265
00:48:51,744 --> 00:48:53,585
advantage of it and and using it? So

1266
00:48:53,585 --> 00:48:55,605
I'm actually quite optimistic. Even today,

1267
00:48:56,065 --> 00:48:58,704
there are some companies that, can provide you

1268
00:48:58,704 --> 00:48:59,204
with

1269
00:49:01,230 --> 00:49:02,130
very decent

1270
00:49:02,829 --> 00:49:03,809
quantum cryptosystems,

1271
00:49:04,829 --> 00:49:06,530
just to mention ID Quantique,

1272
00:49:08,429 --> 00:49:08,929
or

1273
00:49:09,230 --> 00:49:11,010
Spectral in in Singapore.

1274
00:49:11,710 --> 00:49:12,190
So,

1275
00:49:13,389 --> 00:49:15,409
so, yeah, so the quantum is there.

1276
00:49:16,324 --> 00:49:18,424
It's much more mature than

1277
00:49:18,885 --> 00:49:20,885
it was when I was working on it.

1278
00:49:20,885 --> 00:49:21,284
And,

1279
00:49:22,005 --> 00:49:24,085
I I think that in years to come,

1280
00:49:24,085 --> 00:49:25,464
we will see more of it.

1281
00:49:25,924 --> 00:49:28,569
So, Arthur, you're probably most famous for the

1282
00:49:28,569 --> 00:49:30,889
development of e 91, but, of course, you

1283
00:49:30,889 --> 00:49:32,969
do lots of other research. What what are

1284
00:49:32,969 --> 00:49:34,429
you working on at the moment?

1285
00:49:35,609 --> 00:49:36,489
I think that

1286
00:49:37,049 --> 00:49:38,730
thank you. I don't know. It's just, you

1287
00:49:38,730 --> 00:49:39,230
know,

1288
00:49:40,554 --> 00:49:43,195
being famous for it's it's kind of dangerous

1289
00:49:43,195 --> 00:49:45,434
to be famous for one thing. Right? It's

1290
00:49:45,434 --> 00:49:47,835
like an actor. You play one role, and

1291
00:49:47,835 --> 00:49:49,135
then you always associate

1292
00:49:49,434 --> 00:49:51,295
with that, buddy. Being the typecast. Yeah.

1293
00:49:51,914 --> 00:49:53,835
No. No. No. No. But you're absolutely right.

1294
00:49:53,835 --> 00:49:56,250
So most people recognize this early work.

1295
00:49:58,090 --> 00:50:00,670
I I I did some work later on

1296
00:50:01,210 --> 00:50:03,389
on various aspects of quantum computation,

1297
00:50:03,690 --> 00:50:05,389
in particular, on the,

1298
00:50:06,809 --> 00:50:07,309
universality

1299
00:50:07,690 --> 00:50:09,949
of quantum components in in computation

1300
00:50:10,409 --> 00:50:10,889
and,

1301
00:50:11,369 --> 00:50:13,905
some early quantum error correction and and a

1302
00:50:13,905 --> 00:50:15,525
way of stabilizing computation.

1303
00:50:16,385 --> 00:50:18,325
But, I think that the randomness

1304
00:50:18,704 --> 00:50:19,765
has been always,

1305
00:50:20,704 --> 00:50:21,505
something that,

1306
00:50:22,224 --> 00:50:23,045
I'm fascinated

1307
00:50:23,825 --> 00:50:26,005
about. So so I think it's still randomness.

1308
00:50:26,065 --> 00:50:28,489
It's still trying to understand the the nature

1309
00:50:28,489 --> 00:50:29,070
of randomness.

1310
00:50:30,409 --> 00:50:30,909
And,

1311
00:50:31,769 --> 00:50:33,210
and then and, of course, you know, I

1312
00:50:33,210 --> 00:50:34,030
I also

1313
00:50:34,409 --> 00:50:36,650
would like to see randomness not only in

1314
00:50:36,650 --> 00:50:38,889
the context of quantum, but see whether this

1315
00:50:38,889 --> 00:50:40,730
can emerge in in any other way. So

1316
00:50:40,730 --> 00:50:42,349
I've been recently working on

1317
00:50:42,664 --> 00:50:44,824
few crazy ideas with a colleague of mine

1318
00:50:44,824 --> 00:50:46,684
from Warsaw, Andre Dragon,

1319
00:50:47,224 --> 00:50:48,525
trying to see how,

1320
00:50:49,505 --> 00:50:50,005
the

1321
00:50:50,505 --> 00:50:51,644
including superluminal

1322
00:50:56,130 --> 00:50:57,670
part of Lorentz transformation

1323
00:50:58,130 --> 00:50:58,630
can

1324
00:50:59,250 --> 00:51:00,069
lead into,

1325
00:51:00,929 --> 00:51:02,150
quantum like appearance.

1326
00:51:03,809 --> 00:51:05,969
I I think I I'm enjoying this work

1327
00:51:05,969 --> 00:51:08,769
because it's crazy enough that, it's it's certainly

1328
00:51:08,769 --> 00:51:10,789
not mainstream, not at the moment.

1329
00:51:11,775 --> 00:51:14,574
It's quite possible that it's wrong, but if

1330
00:51:14,574 --> 00:51:16,255
it is wrong, it may be wrong for

1331
00:51:16,255 --> 00:51:19,074
a good reason. So by by pointing to

1332
00:51:19,694 --> 00:51:21,454
where it goes wrong, we can learn a

1333
00:51:21,454 --> 00:51:23,234
lot, I think. So

1334
00:51:23,650 --> 00:51:25,969
until, until we are proven wrong, I think

1335
00:51:25,969 --> 00:51:28,269
Andrei and I and our colleagues are

1336
00:51:28,610 --> 00:51:30,449
who are working on this field in in

1337
00:51:30,449 --> 00:51:31,989
this field are are prepared

1338
00:51:32,449 --> 00:51:34,929
to continue for a while. I see. Okay.

1339
00:51:34,929 --> 00:51:35,670
So so

1340
00:51:37,090 --> 00:51:38,550
here are you talking about

1341
00:51:39,025 --> 00:51:41,284
classical physics but allowing for

1342
00:51:41,585 --> 00:51:43,664
faster than the speed of light? That's right.

1343
00:51:43,664 --> 00:51:45,824
If you can reproduce some sort of quantum

1344
00:51:45,985 --> 00:51:49,025
That's right. Like phenomenon. Exactly. Exactly. I see.

1345
00:51:49,025 --> 00:51:50,485
Okay. Oh, that's fascinating.

1346
00:51:51,480 --> 00:51:53,960
Well, that was really fascinating, Arthur. Thanks for

1347
00:51:53,960 --> 00:51:55,019
coming on the podcast.

1348
00:51:55,320 --> 00:51:56,859
Yeah. Thank you for having me.

1349
00:51:57,400 --> 00:51:59,559
So the the next question I'm I'm sort

1350
00:51:59,559 --> 00:52:01,340
of asking everybody quantum.

1351
00:52:01,719 --> 00:52:03,019
This question. And

1352
00:52:03,480 --> 00:52:04,699
the plan is to gather

1353
00:52:05,480 --> 00:52:06,920
all the time we have for this week's

1354
00:52:06,920 --> 00:52:10,195
quantum cast. But the celebration of the international

1355
00:52:10,414 --> 00:52:12,594
year of quantum science and technology

1356
00:52:13,375 --> 00:52:15,635
continues here at Physics World.

1357
00:52:16,015 --> 00:52:17,715
So to hear you part of my

1358
00:52:18,015 --> 00:52:18,675
live feed.

1359
00:52:19,055 --> 00:52:20,195
Go to our website. Intellectual

1360
00:52:21,055 --> 00:52:21,954
events. Quantum

1361
00:52:22,359 --> 00:52:23,900
under the topics tab.

1362
00:52:24,280 --> 00:52:25,719
I'd like to take the parts of the

1363
00:52:25,719 --> 00:52:26,219
instrumental

1364
00:52:26,519 --> 00:52:27,260
level. Fascinating conversation.

1365
00:52:28,599 --> 00:52:31,260
And also Poses lots of challenges to understand

1366
00:52:31,559 --> 00:52:32,699
this problem. Podcast.

1367
00:52:33,079 --> 00:52:34,934
We'll be back again next week.

1368
00:52:35,894 --> 00:52:38,635
It means multiverse and Everett interpretation.

1369
00:52:39,015 --> 00:52:39,994
I'm trying to

1370
00:52:40,934 --> 00:52:43,675
understand and reconcile this worldview

1371
00:52:44,055 --> 00:52:44,454
with,

1372
00:52:45,655 --> 00:52:46,795
my everyday experience.

1373
00:52:48,239 --> 00:52:50,260
I think my life just wouldn't be,

1374
00:52:51,360 --> 00:52:52,659
at the intellectual level,

1375
00:52:54,320 --> 00:52:56,019
interesting enough without quantum.

1376
00:52:57,599 --> 00:52:59,679
It would make sense. I don't know. It's

1377
00:52:59,679 --> 00:53:01,440
very subjective, you know. It's just like

1378
00:53:02,465 --> 00:53:05,125
it's interesting. Right? So you you start learning

1379
00:53:05,184 --> 00:53:07,505
about the whole thing. You learn about you

1380
00:53:07,505 --> 00:53:09,905
learn about it in a very instrumental way.

1381
00:53:09,905 --> 00:53:12,164
You're happy that you can solve some equations.

1382
00:53:12,224 --> 00:53:13,985
You are happy that you can make provision.

1383
00:53:13,985 --> 00:53:15,764
You start asking deeper questions.

1384
00:53:16,385 --> 00:53:18,780
How does it really, really work? And then

1385
00:53:18,780 --> 00:53:20,940
you get some sort of answers, and you

1386
00:53:20,940 --> 00:53:22,320
come across this Copenhagen

1387
00:53:22,699 --> 00:53:23,199
interpretation.

1388
00:53:23,579 --> 00:53:24,079
And

1389
00:53:24,380 --> 00:53:26,300
and then you you look at it and

1390
00:53:26,300 --> 00:53:28,380
and try to understand it and think, well,

1391
00:53:28,380 --> 00:53:30,400
it doesn't quite make sense. You know?

1392
00:53:31,105 --> 00:53:33,824
Where is this difference between classical and quantum?

1393
00:53:33,824 --> 00:53:34,885
Where is this collapse?

1394
00:53:35,505 --> 00:53:38,405
And then you you are sort of standing

1395
00:53:38,545 --> 00:53:38,704
in

1396
00:53:39,824 --> 00:53:40,644
and and

1397
00:53:41,025 --> 00:53:43,585
and the roads by four k's at this

1398
00:53:43,585 --> 00:53:44,144
point to the

1399
00:53:44,980 --> 00:53:47,880
one side, you have to explain this collapse,

1400
00:53:47,940 --> 00:53:48,440
and,

1401
00:53:50,019 --> 00:53:52,739
and that's intellectually honest approach if you're a

1402
00:53:52,739 --> 00:53:55,219
realist. And then you can have people say,

1403
00:53:55,219 --> 00:53:57,640
like Roger Penrose who would say, yes.

1404
00:53:58,574 --> 00:54:01,054
There is another phenomenon if you look into

1405
00:54:01,054 --> 00:54:03,635
gravity, and then you can have this collapse.

1406
00:54:04,414 --> 00:54:06,255
But then if you are not prepared to

1407
00:54:06,255 --> 00:54:08,574
take that road, there's another road that which

1408
00:54:08,574 --> 00:54:10,574
I took, and that was the road towards

1409
00:54:10,574 --> 00:54:12,750
the Everly interpretation. I have to say that

1410
00:54:12,829 --> 00:54:14,530
suddenly I was influenced by

1411
00:54:14,909 --> 00:54:16,289
David Deutsch, my

1412
00:54:16,670 --> 00:54:18,210
supervisor here in Oxford,

1413
00:54:19,309 --> 00:54:19,809
who,

1414
00:54:21,309 --> 00:54:24,289
who is a proponent for of this view.

1415
00:54:24,844 --> 00:54:26,605
But I'm I'm happy to be in this

1416
00:54:26,605 --> 00:54:27,904
camp, I have to say.

1417
00:54:28,844 --> 00:54:30,924
Oh, that's great. Thanks. And if I can

1418
00:54:31,085 --> 00:54:32,864
if you if you can indulge me,

1419
00:54:33,324 --> 00:54:34,944
I've been asked by a colleague

1420
00:54:35,404 --> 00:54:36,464
to ask you

1421
00:54:37,910 --> 00:54:40,550
three more questions. Absolutely. Yes. But,

1422
00:54:41,349 --> 00:54:41,849
if,

1423
00:54:43,190 --> 00:54:45,110
feel free to don't you don't have to

1424
00:54:45,110 --> 00:54:47,269
give long answers if you don't want. Okay.

1425
00:54:47,269 --> 00:54:49,269
Okay. So so the the way the these

1426
00:54:49,269 --> 00:54:50,410
questions are presented

1427
00:54:51,146 --> 00:54:51,646
is,

1428
00:54:53,146 --> 00:54:55,406
actually what I'm gonna do is I'm gonna,

1429
00:54:55,626 --> 00:54:57,406
to make this easier for me,

1430
00:54:58,266 --> 00:54:59,086
I'm going to