Hello, and welcome to the Physics World Weekly Podcast. I'm Hamish Johnston. This episode is part of our ongoing celebration of the International Year of Quantum Science and Technology, and I'm in conversation with the quantum cryptography pioneer and theoretical physicist, Arthur Eckert. Arthur has academic appointments at the University of Oxford and the National University of Singapore, where he founded the Centre for Quantum Technologies.
I caught up with him at Oxford's Merton College, where he is professorial fellow in quantum physics and cryptography. Hi, Arthur. Welcome to the podcast. Yeah. Thank you for having me. So in Arthur, in 1991, you created a quantum cryptography protocol called e 91. And that has its roots in a paradox of quantum physics that was first identified back in 1935 by Albert Einstein and his colleagues Boris Podolsky and Nathan Rosen.
So I thought we we could start off by maybe talking a bit about this EPR paradox and how it important it was for the development of quantum mechanics. So what was Einstein what what what upset Einstein about quantum mechanics, and how did he express it in this paradox? Yeah. Yes. I guess it it is a good starting point, for this, fascinating narrative.
Well it's, you know, it's really difficult today to say what Einstein really had in mind, and volumes are written about it really, what Einstein could possibly have in mind. But but one thing we know is that he was, unhappy with, quantum theory simply because he thought that it was not complete. And by that, we mean that, it you like, in quantum theory, you had to use probabilities. It looked like, randomness was inherent part of it. And to Einstein, that was not a good
theory to describe the real world. Right? So so it should be predictable. Once you know, initial conditions, you should be able to predict what's going to happen and not just only with certain probability. And if you do it with certain probabilities, that that can only reflect the lack of knowledge on your side. But the laws of physics should be such that, you should have deterministic predictions.
I'm simplifying the whole story. There was a little bit of, about two systems, that when separated, they shouldn't be able to communicate instantaneously and so on and so forth. So Einstein was combining, his understanding that he developed working on, the special theory of relativity, with, his, with sort of belief how how the world should be, described.
And, we call this kind of description, local, and we refer to, his program as a local hidden variable model by which we mean that according to Einstein, the quantum physics didn't provide complete description. The reason why we see this, probabilistic behavior is due to the fact that what people in quantum physics would say, oh, this is a the description of quantum state. To Einstein, it was just a description of quantum
state. It was not actually complete that if you add some extra parameters, maybe then you should be able to make those precise predictions. So so it was about, predictability and about, what some people case say, spooky action at the distance. So so so to understand, everything should be described locally. You should be able by looking at the state of a given object here at a given location, you should be able to make predictions, and be able to say for sure what's
going to happen. And if you don't have this, so that means that your theory is not complete. It's not quite right. So Einstein didn't really argue against quantum physics. He just simply thought it was just the job is not done, that you have to work harder and and work out details. I see. And I suppose like a lot of things in the early, formulations or approaches to to quantum physics, this remained just an idea for for a long time and a sort of a quirky
way of thinking about things. But then, later on, John Bell, the Northern Irish physicist, I think it was in 1964, built a mathematical way of expressing this EPR, paradox. Yeah. And and that turned out to be a very profound thing, didn't it? That's right. Even though, you know, when we talk about it today, right, so, with the benefit of hindsight and and looking what happened so many years ago, we could see that those were very important things, but at the time, they were hardly noticed really.
So, Einstein just for, you know, the EPR paper, the Einstein Podolsky and Rosen paper that you cited from 1935, the starting point for our story today. Right? That that one, when we look at it today, we can see that this was actually a very, very important paper. But at the time, I don't think people really were too much bothered about it. Most physicists have this, very instrumental approach. Take a theory, make predictions. If it works,
fine. That's good enough. Einstein went one step further, and he wanted he was seeking good explanations, not only good predictions, but also good explanation, how it really, really works. And, so after that, there was silence, really. Nobody bothered too much. It was the number of beautiful results that people, worked out in quantum theory, mostly looking at atomic spectrum, atomic physics.
So John Bell was one of those, again, an outlier, I would say, who, apart from having a real job, right, working at CERN designing accelerators, after hours, he was interested in the foundations.
And and almost thirty years later, what he did, looking at this philosophical paper and trying to understand whether there are additional parameters that that we may need if we want to have a very precise description, whether he can translate the whole idea into something that is testable, something that's at least falsifiable so that you can set up an experiment. And you run this experiment, and it can refute the local hidden variable model, of Einstein. So that was a a great achievement of,
John Bell. So he was able to take those what looked like a philosophical idea, Einstein paper, and translate it into something that you can show to experimentalists and say, go to your lab, do this experiment, and and then you can, refute one possible worldview that, Einstein was subscribing to. So Bell's theorem, which you've referred to, has been described by some as the most profound discovery in science. I mean that's that's a pretty strong statement. Yeah. Yeah. But so can you
I mean why would somebody say that? Obviously it's very important. It's a very important concept or a set of ideas. What why is it so important in the sort of history of quantum mechanics and and how we understand quantum theory today? Well, it's it is it is certainly very important, result. The whole quantum theory is a very important result. Right? It's a new way of looking at nature. It's not even
something that tells us about interactions. It's unlike gravity, electrodynamics, strong, or weak interaction. Quantum physics is a or quantum theory is a is a a meta statement. Right? It says no matter what the interaction are, then you have to describe them in a certain way. So so it gives you a, a framework in which you have to formulate any physical
theory. So so changing this framework from from classical to quantum, even though people maybe didn't quite realize it at the very beginning, but it it is like a changing of world view. And then there's Einstein there, right, who is saying, look, not so fast. Right? So maybe maybe it's not exactly that you have to live with inherent randomness. Perhaps it's not complete. So Bell, in a way, is coming with a with a mathematical
proposition which says, okay. Go to the lab, test it, and if you see a violation of this the so called Bell inequalities, then that's it. That you have to refute, that you have to drop the the the whole idea of, hidden variables. So there's no hope. There's no, like, you know, a a screenplay of some sort that tells exactly what's going to happen. The nature is inherently unpredictable. And and so that that is you know, when you think about it, that that is
quite a strong statement. That's almost like changing a world view, and and I think, it's it's perfectly justified to refer to it as, as one of the greatest thing that that happened in in in our understanding of of the of the nature of reality. I should add perhaps that, that there there there's more to this story.
I mean, just we don't have to go in because it may be part of the of of of of what we want to talk about, but, but nonetheless, it's connected that we make certain assumptions when when we look at, Bell theorem. And, so there are other ways to think about, quantum physics. And and, for example, it may well be that, our perception of the world is simplistic and maybe that there is a room for multiverse, which is the Everett approach.
And every approach is different in a sense that in in Bell's theorem, for example, we assume that, each each experiment has a outcome that we see and perceive, that in the Everett interpretation of all possible outcomes can happen. So so what I'm saying that there's more to this story. Right? So I may just say, okay. Bell, great. Wonderful idea. But then we also have Everett, great insight into the nature of reality and
many others. So so I'm I'm I'm I agree with the statement that it was a a profound discovery or statement that was made about our ability to distinguish between different world views, but but there's more to it. So Bell's theorem, I suppose, when he formulated it was still a theorem. And as you say, he he offered guidance to experimentalists, but it was still, I mean, it's still a very difficult experiment to do, in 1964.
Yeah. So, in 2022, Alain Aspe, John Klauser, and Anton Zeilinger, won the Nobel Prize for physics for their pioneering work in actually doing Bell tests in the lab. And this was work, I think, that was done is it eighties, nineties, roughly then? Sadly, of course, John Bell died in 1990, so he couldn't share the Nobel Prize, for that work. Can you give us a flavor of what Aspe, Clauser, and Zollinger did in the lab? How did they translate John Bell's ideas into actual experiments?
Right. So, so John Bell, as you pointed out, John Bell came up with a a mathematical statement. What was good about this mathematical statement was that, it it could be taken to, could be translated into an experiment. And, so so John Bell proposed what we refer today as Bell inequalities. In fact, there is just not, like, one Bell inequality, but there are many versions of it. So the most common one is known as a CHSH inequality, but, but let let's refer to them as Bell
inequalities. So so Bell inequality is a statement which to an experimentalist, it means go to the lab, set up experiment, and observe certain correlations. So, you have to have two particles, like two photons, and you send them into two different locations, and you want to, say, measure polarization of those photons. And when you do that, you realize that, the outcomes are not completely independent
from each other. Those photons, which we call entangled photons, they re they respond to the measurements in a in a very correlated way. And then you you study those correlations. You measure several different types, like the four different types of correlations, and you set up inequality. And if this inequality is satisfied, then you say fine. That is consistent with what Einstein wanted to
see. But if it is violated, then you know that Einstein view of local hidden variable model is is not correct, is refuted. And, again, you know, John Bell, as you said, in the early sixties, John Bell comes with this, Bell inequality statement. One would say from today's perspective, great thing. You should rush to the lab immediately and do all those kind of experiments. That didn't happen for for
maybe for two reasons. First of all, most people didn't actually look at it as or didn't consider this as an interesting result. It was still on the fringe. It was still mostly philosophical thing. The other reason was the technology to, implement this kind of experiment was not, so mature at the time. So to the, you know, another thirty years or so, right, from the sixties to, like, eighties and nineties for people to start playing with it. And, again,
not not mainstream physics, really. If you look at, those early experiments, again, a bunch of outliers, a postdoc who, like John Clauser, who didn't really want to do what his supervisor was asking him to do and started playing with that, Alain Aspair, a colorful, character himself who with a passion to really understand how it all works, and many others, and and later Anton Zeilinger as well. So what happened was that the Klaus who set up this experiment, he was the first to see the violation
of the Bell inequalities. But I want to also add that at the time, there were experiments which were pointing in the other direction saying that, you know, maybe Einstein is right. So it was not entirely obvious. It was one of those interesting experiments where it's it's I wouldn't call it a cliffhanger, but some or something of that kind. Right? So so you are there as an experiment.
Imagine Hamish, you go to this lab of Alain Aspair and look at this, set of correlations and thinking which way will it go. So there's the suspense. Right? Is it going to be local hidden variable or quantum rules the world and forget about, any determinism?
So, so the the the sequence of those experiments starting from John Clauser, then Alain Aspair, and then Nicolas Gisen, and then, many other colleagues to mention, even, our British colleagues here, like John Rarity and and Paul Tapster at the Defense Research Agency that I had the pleasure to work with. So they they they implemented those experiments. And and then Anton Zeilinger who, really managed to close many loopholes in those experiments.
So so the so the that that sequence of experiments over a period of time made it quite clear that, the local hidden variable model is not sustainable, so it doesn't really work. So it it you have to think about inherent randomness if you want to if you drop the multiverse, just then there's inherent randomness in nature. I see. And how how do you interpret or or or how should one interpret a well, let's say a positive bell result, positive in the sense that it points towards
quantum physics. Do do you interpret that as, yes, entanglement as predicted by quantum physics is a real thing, and that the system that you are looking at must be quantum, it simply cannot be a classical system. Is that the the right way to interpret? Yes, of course. If you if you want to use this dichotomy, classical and quantum, so that's one way of looking at it. Usually, you just take the view that everything is quantum and classical. It's just an approximation to quantum.
And, so having this, Bell test, it it shows that, there's just no way to explain what you see, using, kind of a classical tricks where things can be deterministic. You can you have to just drop your hope that you will be able in some experiments to say exactly that your photon will go into this photo detector and not into that one. So so that's that's not possible. And,
so that's that's a big thing. Right. And and would it mean, you know, I know going back to 1935, Einstein and his colleagues had their concerns. If, you know, if Einstein were around when those experiments were finally done, and Podolsky and Rosen, Do you think that they would be satisfied that they would, Well, what are Their their their question would be answered by those experiments. Of course, a
very interesting question. So I I would it would be fantastic to have Einstein at the time and, and see his reaction to to those experiments. I I think there are two ways around. So first of all, there's still still a possibility, but I don't think Einstein would subscribe to this. But there's still a possibility that everything is deterministic because when you run the Bell measurement, in the process of running this experiment, you have to make some random choices.
It's very important that you randomly choose the setting, of, measuring devices which measure, say, polarization. Now if those choices are predetermined, not random, then, you can see the violation of the Bell inequalities even though you can still explain everything using
this local hidden variable model. So what I'm saying is that if you don't have free will as experimenter to have this free freedom of choosing the setting so that everything can be then scripted, your action so if you if you involve absolutely everything, the whole universe, so call it superdeterminism, then, of course, we cannot exclude this possibility. So this particular experiment assumes that, the the settings of those devices are independent from devices itself.
Not so many people would subscribe to this worldview that everything is absolutely super deterministic. We would like to believe that we have free will, that the whole point of science is to make choices and and set up devices and and see what happens, but that cannot be excluded. So it has to be mentioned that there is still this
possibility. Another possibility, of course, I don't know, again, whether Einstein will subscribe to it, would be the Everett interpretation, which has all kind of interesting features, which somehow tells you that that there are just that we live in this multiple of realities, and, and it's not as science fiction as it sounds. Right? I know people just, like Everett interpretation because it has all these strange connotations to many parallel unitises and so on and so
forth. But, honestly, if you take quantum physics seriously and, say that there's no border between classical and quantum, that there's not that much you are left with. If you if you're serious about this description, then you have to somehow, lean towards the Everett interpretation. So I I guess my hope my hope is, you know, is that Einstein would endorse the Everett interpretation. That that's my guess.
So, Archer, that brings us very nicely to, quantum cryptography and in particularly and in particular, the, e 91 protocol that, that you came up with. Can you explain how it is or how it relies on Bell's theorem or is inspired by Bell's theorem? Yes. So that's, what happened was that when I was a PhD student in Oxford, I was supposed to be working on improving quantum communication with using what quantum opticians at that time called, the squeezed states of light.
It was interesting topic even though, I have to say improve improving signal to noise ratio and the hopes we had with squeezed states were, well, never really materialized the way we wanted, them. That doesn't mean that we don't use those states of light. I mean, we use them in very precise interferometers. But but for me, it was a starting point to look into the foundations of, quantum physics.
And, with great interest, I was reading some historical papers and in particular, this 1935 paper by Einstein, Podolsky, and Rosen. And Einstein and I have to say that I really like, the way he writes. So I'm assuming here that the paper was, mostly or influenced in style by Einstein. So Einstein's style is is is very clear.
And, so EPR paper is carefully structured so that the Einstein wants to define what does it mean for something to exist, for example, and, and by exist meaning prior to the measurement. Because, you know, once you measure something, it's usually you say, okay. There was something there. It had a certain property. I measured it, and I reveal this property. If there's a a box, I look at it, switch on the light, and it's red, I measure a property,
redness of the box. Right? So and but but but I assume, of course, that me looking at it didn't make it red. It used to be red. Right? So it had to be red before I look at it. Einstein wants to define what it means for something to exist. And, he has this statement. There was just, like, one statement in that paper that fired my imagination, really, and that was my starting
point. And that statement roughly says that the element of reality that's Ishan just called something that that, you know, exists. The element of reality according to Einstein is defined in such a way that if you can find out about the value of a certain physical property without disturbing this property, then there exists an element of reality associated with this property.
So that means, like, if we can measure something and use without messing up this property So that that that means that there's a notion of passive measurement. Right? So that that really reveals the value of this property without disturbing it. And then to me and and I had some from my previous interest in in secure communication. I had some, interest in the notion of eavesdropping in in a secure communication system. And then an eavesdropper wants to do exactly
this, to measure something without disturbing this. So if there's a carrier of information that has certain properties and those properties carry data that you want to read. An eavesdropper wants to measure it without disturbing it, find out the value of this physical property without disturbing it. So then I made this mental connection. I think, gosh. You know? So Einstein is talking about eavesdropping even though it's not called eavesdropping, and maybe I can put it in in a different context.
And then I knew about the Bell work, and I was, at the time, happily reading Alan Aspert, thesis with with description of his experiments. Then I thought, okay. We know that there are physical systems or experiments where you can show that there is no element of reality, that the whole thing, if the Bell inequality is violated, you cannot say that you have this physical the element of physical reality.
It simply can be then translated into the statement that there's just no way there was an eavesdropping.
And so that was the connection. So from Einstein definition of the element of reality, knowing about the Bell theorem that that shows that Einstein approach, can be refuted, knowing that it can be implemented, by people, you know, people who implemented it to to mention Clauser and, Aspe and Nicolas Giza and Anton Zeilinger and and, Jan Wei Pan in China and, late and that was later, by the way, and and our colleagues here in in Malvern.
So that's knowing all this, you know, I just went on and and and and said, well, I can construct a system where the violation of Belling Equality certifies the lack of eavesdropping. And so that that that is sort of like a vagues a very very vague explanation.
This is where the idea comes. And, of course, you know, you have the idea, and there's a long way to go because you have to do all kind of mathematical gymnastics to make sure that, you look at it as a cryptographer, that you look at the different property, you look at the secret key rate, and you introduce new things. So that's, that's I had to, work a little bit on that. And and that was, yeah, that was it, actually.
So so so the idea, if I understand it correctly, is that information is sent, and it is, I suppose in in in a quantum form. Is that the right the right way to explain it? If an eavesdropper makes a measurement on it, it loses some or all of its quantumness. Correct. And then when you make the Bell test, if if if if you don't get a thumbs up on quantum from the Bell test, you know that somebody has fiddled around with that information. Could have in
principle fiddled with that. Yeah. Yeah. That's right. So you're absolutely right. So the the the quantumness, the that is indicated by the violation of the Bell inequality is lost when there is an eavesdropper because that eavesdropper would make would introduce this element of reality. And and so, so, yeah, so that you can translate it into something that, is called key distribution cryptography.
So that means that two individuals using exactly the experiment that was, set up by, those pioneers who started playing with the Bell inequalities, then you can you can simply just, use this experiment with some modification and minor variation of those experiments, to allow two individuals in different locations to share private random sequences that later on can be used for cryptographic purposes.
So you you've come up with this idea, and, I suppose the next step is to to show that it's possible, And, which I'm guessing was probably not not a very easy thing. No. No. Yeah. You're right. So you joined forces with, John Rarity and Paul Tapster. And they were I'm right in thinking they were at The UK's Defense Research Agency in Malvern? That's correct. And, which is in, I suppose in, Southwest England, or is it the Midlands, or on the border between, the West Country and the Midlands.
And this was in 1991. Can you give us an idea of how you did that experiment or how you Yeah. And John and Paul did that ex So it's interesting, you know, because I I had this, I wrote this paper and and the the two things happened. First of all, you know, as a PhD student, you you work on something that that is a bit bizarre. It was another mainstream. And I had very low confidence in myself at the time, so I didn't think that, well, at least I was not so sure
that that that that really makes sense. And then two things happened. The one thing was that I learned from, a colleague of mine that, Charlie Bennett and Gilles Brassard, my colleagues from, one from US, One from Canada, actually worked on something very similar, namely using quantum phenomena to set up a key distribution in cryptography. So that was to me even though I
didn't know this work. There was no Internet at the time, and, and they published the work in a very obscure conference proceedings from Bangalore, which, you know, didn't exist in even in in Oxford, you couldn't find such a thing. And and, you know, to find it, you would have to know what to look for, but but I didn't. I didn't think that anyone was working
on this. And so, but with the help of, David Deutsch, my, mentor here in Oxford, and, and Peter Knight, who was my supervisor at the time, Imperial College. I I had a chance to meet and talk to Charlie Bennett and and Gilles Brassard. And, I was both happy and unhappy, I have to say, at the same time. Happy because, that gave me confidence that what I was working on,
was relevant, was maybe important. And if those guys rather established an eminent, physicist and computer scientist, I think at the time Gilles would refer to himself as a computer scientist. I think, so that was great, you know, that that that that means that I was working on something important. So that that gave me sort of a boost of confidence.
I'm happy because, you know, would be nice to know that nobody thought about this before even though what Gilles and Charlie did was completely different. So they were looking at, how disturbance can be detected using different phenomena. So it was not really that much of a fusion of fundamental, physics with cryptography. It was Heisenberg answers the principle based on previous ideas introduced by Steven Wiesner, by late
Steven Wiesner. So so I learned about that work, after I already did mine, and and that was interesting. So so I was confident that at least it's it's worth trying. It's worth doing something about it, but not so many people were really interested in. And and then another interesting thing happened. I was, giving, I was showing a poster, in fact, in one of those skiing conferences in Cortina D'Ampezzo, in fact. And on the slopes, I met generality.
And while we were skiing, we were talking science, and, and John, who work at the Defense Research Agency in Malvern at the time, got interested. He said, well, look. We essentially have all the setup that that you are talking about so we can implement that thing. And so I was invited to, see their labs later. I traveled to Malvern, and it was it was really interesting to work with a duo of John Rarity and Paul Tapster.
I I, you know, I like them because they were so different and so complimentary in a way. So John was, rather an extrovert, and and very much willing to talk about everything that they're working on and and traveling places, going to conferences, giving good talks. Paul, in contrast, was an introvert. So he worked in the lab, and, I think, for Paul, most of the world was confined to a triangle that was defined by his house, his lap in DRA, and his bridge clap. So he played bridge.
So that was Paul. But Paul was one of the most amazing experimental physicists that I came across, so he could just make magic happen. So working with them was a was a great pleasure. I have to say as a theorist, you know, I just I was I was quite happy if I could just go to the lab and identify all the components on the Octopi table. So so I wouldn't claim that I really contributed that much to this
experiment, but we worked together on this. And, we managed to demonstrate that it really works in this different setting. Violation of Bell inequalities can be, understood, or can be used for for practical purposes. So so at this point, the interesting fusion happened. On one hand, you have cryptographers working on, secure communication, and then there's this big problem that they call the key distribution problem. And this is like a holy grail of
cryptography. Find the solution to the key distribution problem. On the other hand, you have a bunch of, outliers in physics working on the foundations, doing experiments, and nobody cares about those experiments, honestly, because most working physicists say, okay. Fine. You guys are telling us that quantum is the way to go. But we we had known this for a long time. Right? So so so why why bother so much? It's purely philosophical.
But here comes a moment, in in 1991 and then in 1992 when when we do these experiments that shows to the world that it's not completely irrelevant what those weirdos doing, quantum foundations, think. And their tools can be useful for something very practical for secure communication. So this fusion happens. Quantum foundations offers a solution to the key distribution problem, and the cryptographers at the same time, getting a new set of tools to play with.
And from this moment onwards, another interesting thing happens, namely the experiments that show the violation of Bell inequalities were not perfect. They were not perfect because there were various loopholes, and those loopholes assumed something that most physicists would just dismiss. For example, they assume that nature may cheat you by implementing such and such local hidden variable model in one experiment and then experimenting, maybe implementing maybe another one in another experiment.
Or that if you don't have a perfect detection that nature will conspire against you. So you, in the data that you see, you see the violation of the Bell inequalities. But if you were to include all possible data, including those that you didn't register so if you combine them together, you wouldn't see the violation of the Bell inequalities. And then, you know, physicists would say, come on. Honestly, nature conspiring against you? No way. But if you put the whole thing into a crypto setting,
it's not nature. It's your adversary there, and that adversary can cheat you in any way possible. Right? So you have to close those loopholes. So that gave the final push to the foundations. Right? So so now that the closing those loopholes was not just question of being sort of, like, pedanting from a philosophical point of view. It was just more to say, if you want to show the security of the system, you have to close the loopholes
full stop. And here comes, I I think, people like Rasmus Hansen and Anton Zeilinger and and other who had technology at the time to close the loopholes, and that happened. So so you could see at this point a beautiful synergy of the two. Right? Cryptographers telling people in the foundations, come on, guys. You have to close the loopholes. And people on the foundation side say, oh, great. You know? So far, nobody cared about
this, and now someone does. So let's close the loopholes, and let's have those experiments. So so the Nobel Prize that's in 2022 went to Closer and Asper and Zeilinger, that really shows evolution of, people starting from doing really, blue sky, curiosity driven research to beautiful implementation, full implementation of the Bell inequalities that already comes with understanding that it's relevant. Clauser and Asper were mostly driven by curiosity.
I'm Anton, I I think, was driven both by curiosity. So working with him was always a pleasure, and we talk a lot about the philosophy and foundations, but he also had a benefit of understanding that this is important from the practical point of view. So, Archer, today, they're they're actually commercially available quantum cryptography systems. Although I'm guessing that, there's probably many improvements that can be made.
Do do you think that at some point in time, quantum cryptography is going to replace the the sort of classical or standard cryptography systems that we use today. Well, I don't think that there will be a complete replacement. But I think that quantum crypto will certainly play a significant or increasingly significant role in in our secure, communication environment.
To start with, one thing that happened after my work was, I wouldn't well, improvement in understanding of the system that I proposed, and, that is known as a device independent cryptography. And that that is a from the fundamental point of view, it's it's simply absolutely unbelievable. It essentially says that you can purchase devices from your adversary, from your enemy, run the test on those devices. And if they pass CHSH or Bell test, essentially, then those devices are good. I mean, you
can use them. You don't have to inspect devices. You don't have to open them. You don't have to tinker with unknown technology. You know, usually, you don't have expertise to do that. So unlike today where we have to certify those devices, so you produce a a device that say generates random numbers for crypto purposes. So how do you know that this is really generating random numbers? And, the only way is to trust some sort of authority. So there's a certifying agency that will say,
okay. You know, a bunch of wise guys looked into this, and we can get you trust us, and we guarantee that this is this is fine. But we don't want this necessarily in the future. Right? So having the self testifying devices, and it's not so much about necessarily buying them from the enemy. Nobody would do it. But but sometimes when you desire a device, you unwillingly, you may introduce some bugs into the system and,
and then you want to avoid this. So so this self testing is a wonderful thing because you don't have to do anything inside the device. You don't touch the device. You just provide those devices with some inputs that generate some outputs. You run a statistical test, and you say, okay. Those devices are fine. No matter where they come from, they pass the test. They pass the test. They're good.
And, so this device independent crypto is, is fantastic because you can then put your devices into location, so, like, on satellite, for example. And you don't have to worry too much about someone tinkering with your devices there because if that happens, you can see it in the test. Right? So so they self testify they self, self test themselves. So that that is from the fundamental point of view. That's, actually
a very interesting new development. I think it's pushing the the the limits of privacy to the limits. So that that self self testing, is that is that a function of the the sort of no go or go no go nature of the Bell test? Yeah. It's it's a function of something called, we we say that the Bell result is
rigid. So that means that to simplify things a little bit, that means if you see the full violation of the Bell inequality so suppose you run the test and, you reach the maximum violation of the Bell inequality that is that is possible. Then you know from this one number, you know that there's no any other way for this to happen but just to set up something that is equivalent to the Bell test. So you can argue the other way around.
So the the the violation of the Bell inequality tells you that only by by looking at the data, even just, you know, looking at one number tells you that there's no way that could have happened without setting up the Bell test. So that was then then, of course, you know, the we know that, it's very unlikely that you will get, the maximum violation. There's always noise in the
system and so on and so forth. So so you have to look at some parameters, and from that parameters, you can estimate, the upper bound on information that any third party eavesdropper included and could have gained from from the system. So, yeah, so that's that's that's that's a beauty of, of the Bell inequality approach, which you cannot achieve with anything else.
So so this, the system that Charlie and Gilles proposed may work in a in a scenario where you you are confident that your devices are exactly what they are and you are confident about, using more technical terms, the dimension of the Hilbert space of your carrier and so on and so forth, but you cannot achieve the level of, devising the penthouse that you can achieve with, Bell test kind of crypto that I proposed. So that that I see as, as an
interesting way to go. Now your question is, will it ever happen that we will, go fully quantum? I don't think so. So I think that there's always a classical element around it if we were to divide things into classical and quantum, but, but that's fine. So I I think, there's still a work you know, lots of work to be done, I I would say, to make it practical. First of all, the quantum technologies is still noisy, so you have to improve it. But it's possible.
The same year that, John Clauser, Alan Asper, Anton Zeilinger got the Nobel Prize, There were three experiments showing the proof of principle of the device independent crypto. One of them here in Oxford, one of them in Munich, and one of them in China. So all the three groups show that technology is good enough to implement device independent cryptography. From there to, make make it commercial is still a way to go.
But I think that that eventually, we will use crypto in combination with, quantum crypto in combination with various classical methods. Quantum crypto is good for point to point communication, not so great for things like digital signature, password protections, and many other crypto tasks that you would like to implement. But but, obviously, with, the progress in in quantum computing, the whole notion of security in the new quantum world partially relies on the development in in quantum
cryptography. So there's probably no way around it. And why not? You know, once we are more familiar with quantum technology, why not taking advantage of it and and using it? So I'm actually quite optimistic. Even today, there are some companies that, can provide you with very decent quantum cryptosystems, just to mention ID Quantique, or Spectral in in Singapore. So, so, yeah, so the quantum is there. It's much more mature than it was when I was working on it.
And, I I think that in years to come, we will see more of it. So, Arthur, you're probably most famous for the development of e 91, but, of course, you do lots of other research. What what are you working on at the moment? I think that thank you. I don't know. It's just, you know, being famous for it's it's kind of dangerous to be famous for one thing. Right? It's like an actor. You play one role, and then you always associate with that, buddy. Being the typecast. Yeah.
No. No. No. No. But you're absolutely right. So most people recognize this early work. I I I did some work later on on various aspects of quantum computation, in particular, on the, universality of quantum components in in computation and, some early quantum error correction and and a way of stabilizing computation. But, I think that the randomness has been always, something that, I'm fascinated about. So so I think it's still randomness.
It's still trying to understand the the nature of randomness. And, and then and, of course, you know, I I also would like to see randomness not only in the context of quantum, but see whether this can emerge in in any other way. So I've been recently working on few crazy ideas with a colleague of mine from Warsaw, Andre Dragon, trying to see how, the including superluminal part of Lorentz transformation can lead into, quantum like appearance.
I I think I I'm enjoying this work because it's crazy enough that, it's it's certainly not mainstream, not at the moment. It's quite possible that it's wrong, but if it is wrong, it may be wrong for a good reason. So by by pointing to where it goes wrong, we can learn a lot, I think. So until, until we are proven wrong, I think Andrei and I and our colleagues are who are working on this field in in this field are are prepared to continue for a while. I see. Okay.
So so here are you talking about classical physics but allowing for faster than the speed of light? That's right. If you can reproduce some sort of quantum That's right. Like phenomenon. Exactly. Exactly. I see. Okay. Oh, that's fascinating. Well, that was really fascinating, Arthur. Thanks for coming on the podcast. Yeah. Thank you for having me. So the the next question I'm I'm sort of asking everybody quantum. This question. And the plan is to gather all the time we have for this week's
quantum cast. But the celebration of the international year of quantum science and technology continues here at Physics World. So to hear you part of my live feed. Go to our website. Intellectual events. Quantum under the topics tab. I'd like to take the parts of the instrumental level. Fascinating conversation. And also Poses lots of challenges to understand this problem. Podcast. We'll be back again next week. It means multiverse and Everett interpretation.
I'm trying to understand and reconcile this worldview with, my everyday experience. I think my life just wouldn't be, at the intellectual level, interesting enough without quantum. It would make sense. I don't know. It's very subjective, you know. It's just like it's interesting. Right? So you you start learning about the whole thing. You learn about you learn about it in a very instrumental way. You're happy that you can solve some equations. You are happy that you can make provision.
You start asking deeper questions. How does it really, really work? And then you get some sort of answers, and you come across this Copenhagen interpretation. And and then you you look at it and and try to understand it and think, well, it doesn't quite make sense. You know? Where is this difference between classical and quantum? Where is this collapse?
And then you you are sort of standing in and and and the roads by four k's at this point to the one side, you have to explain this collapse, and, and that's intellectually honest approach if you're a realist. And then you can have people say, like Roger Penrose who would say, yes. There is another phenomenon if you look into gravity, and then you can have this collapse. But then if you are not prepared to take that road, there's another road that which I took, and that was the road towards
the Everly interpretation. I have to say that suddenly I was influenced by David Deutsch, my supervisor here in Oxford, who, who is a proponent for of this view. But I'm I'm happy to be in this camp, I have to say. Oh, that's great. Thanks. And if I can if you if you can indulge me, I've been asked by a colleague to ask you three more questions. Absolutely. Yes. But, if, feel free to don't you don't have to give long answers if you don't want. Okay.
Okay. So so the the way the these questions are presented is, actually what I'm gonna do is I'm gonna, to make this easier for me, I'm going to