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

An episode of [Physics World Weekly Podcast](https://invisiblenetwork.ai/podcast/1400686733), 20 Mar 2025, 55 min.

Audio: https://media.blubrry.com/physicsworldweeklypodcast/content.blubrry.com/physicsworldweeklypodcast/PWWeekly20Mar2025ArturEkert.mp3

Our podcast guest is the theoretical crypto-physicist Artur Ekert

## Transcript

[0:09] Hello, and welcome to the Physics World Weekly
[0:12] Podcast.
[0:13] I'm Hamish Johnston.
[0:14] This episode is part of our ongoing
[0:18] celebration
[0:18] of the International Year of Quantum Science and
[0:22] Technology,
[0:23] and I'm in conversation
[0:25] with the quantum cryptography
[0:27] pioneer and theoretical physicist,
[0:30] Arthur Eckert.
[0:32] Arthur has academic appointments at the University of
[0:35] Oxford and the National University of Singapore,
[0:39] where he founded the Centre for Quantum Technologies.
[0:43] I caught up with him at Oxford's
[0:46] Merton College, where he is professorial
[0:49] fellow
[0:50] in quantum physics and cryptography.
[1:01] Hi, Arthur. Welcome to the podcast. Yeah. Thank
[1:04] you for having me.
[1:05] So in Arthur, in 1991,
[1:08] you created a quantum cryptography protocol called e
[1:13] 91.
[1:14] And that has its roots in a paradox
[1:17] of quantum physics
[1:19] that was first identified back in 1935
[1:23] by Albert Einstein
[1:25] and his colleagues Boris Podolsky
[1:27] and Nathan Rosen.
[1:29] So I thought we we could start off
[1:31] by maybe talking a bit about this EPR
[1:34] paradox and how it important it was for
[1:37] the development of quantum mechanics.
[1:40] So what was Einstein what what what upset
[1:43] Einstein about quantum mechanics, and how did he
[1:46] express it in this paradox?
[1:49] Yeah. Yes. I guess it it is a
[1:50] good starting point, for this, fascinating narrative.
[1:55] Well it's, you know, it's really difficult
[1:57] today to say what Einstein really had in
[2:00] mind,
[2:01] and volumes are written about it really, what
[2:03] Einstein
[2:04] could possibly
[2:05] have in mind. But but one thing we
[2:07] know is that he was, unhappy with, quantum
[2:11] theory simply because
[2:12] he thought that it was not complete.
[2:15] And by that,
[2:16] we mean that,
[2:19] it
[2:19] you
[2:20] like, in quantum theory, you had to use
[2:23] probabilities.
[2:24] It looked like,
[2:26] randomness was inherent part of it.
[2:28] And to Einstein, that was not a
[2:31] good
[2:33] theory to describe the real world. Right? So
[2:35] so it should be predictable. Once you know,
[2:38] initial conditions, you should be able
[2:41] to predict what's going to happen
[2:43] and not just only with certain probability. And
[2:46] if you do it with certain probabilities,
[2:48] that that can only reflect the lack of
[2:50] knowledge on your side. But the laws of
[2:53] physics should be such that, you should have
[2:55] deterministic
[2:56] predictions.
[2:57] I'm simplifying the whole story. There was a
[2:59] little bit of,
[3:01] about
[3:04] two systems,
[3:05] that when separated, they shouldn't be able to
[3:08] communicate instantaneously
[3:09] and so on and so forth. So Einstein
[3:11] was combining,
[3:14] his
[3:15] understanding
[3:15] that he developed working on,
[3:18] the special theory of relativity,
[3:21] with,
[3:22] his,
[3:24] with sort of belief how how the world
[3:28] should be,
[3:29] described.
[3:30] And, we call this kind of description,
[3:34] local,
[3:36] and
[3:38] we refer to,
[3:41] his program as a local hidden
[3:43] variable model by which we mean that
[3:47] according to Einstein, the quantum physics didn't provide
[3:50] complete description. The reason why we see
[3:53] this,
[3:54] probabilistic
[3:55] behavior
[3:56] is due to the fact that
[3:58] what
[4:00] people in quantum physics would say, oh, this
[4:02] is a the description of quantum state. To
[4:05] Einstein, it was just a description of quantum
[4:07] state. It was not actually
[4:11] complete that if you add some extra parameters,
[4:13] maybe then you should be able to make
[4:15] those precise predictions.
[4:17] So so it was about,
[4:19] predictability
[4:21] and about,
[4:22] what some people case say, spooky action at
[4:25] the distance. So
[4:26] so so to understand, everything should be
[4:30] described
[4:31] locally. You should be able
[4:34] by looking at the state of a given
[4:36] object here at a given location, you should
[4:38] be able to make predictions,
[4:41] and be able to say for sure what's
[4:43] going to happen. And if you don't have
[4:45] this, so that means that your theory is
[4:46] not complete. It's not quite right. So Einstein
[4:49] didn't really argue against quantum physics. He just
[4:51] simply thought it was just the job is
[4:54] not done, that you have to work harder
[4:55] and and work out details.
[4:57] I see. And I suppose like a lot
[4:59] of things in the early,
[5:03] formulations
[5:04] or approaches to to quantum physics, this
[5:08] remained just an idea
[5:10] for for a long time and a sort
[5:12] of a quirky
[5:14] way of thinking about things. But then,
[5:17] later on, John Bell, the Northern Irish physicist,
[5:21] I think it was in 1964,
[5:24] built a mathematical
[5:26] way of expressing
[5:28] this EPR,
[5:30] paradox.
[5:31] Yeah. And and that turned out to be
[5:33] a very profound thing, didn't it?
[5:35] That's right. Even though, you know, when we
[5:37] talk about it today, right, so, with the
[5:40] benefit of hindsight and and looking
[5:43] what happened so many years ago, we could
[5:45] see that those were very important things, but
[5:48] at the time, they were hardly noticed really.
[5:50] So,
[5:51] Einstein
[5:52] just for, you know, the EPR paper,
[5:55] the Einstein Podolsky and Rosen paper that you
[5:57] cited from 1935,
[6:00] the starting point for our story today. Right?
[6:03] That that one, when we look at it
[6:05] today, we can see that this was actually
[6:07] a very, very important paper.
[6:10] But at the time, I don't think people
[6:12] really
[6:13] were too much bothered about it. Most physicists
[6:15] have this,
[6:16] very instrumental approach.
[6:19] Take a theory, make predictions. If it works,
[6:21] fine. That's good enough. Einstein went one step
[6:25] further, and he wanted he was seeking
[6:27] good explanations, not only good predictions, but also
[6:30] good explanation, how it really, really works.
[6:33] And,
[6:35] so after that,
[6:38] there was silence, really. Nobody
[6:40] bothered too much. It was
[6:43] the number of beautiful results that people,
[6:46] worked out in quantum theory, mostly looking at
[6:49] atomic spectrum,
[6:50] atomic physics.
[6:53] So John Bell was one of those,
[6:56] again,
[6:57] an outlier, I would say, who,
[7:00] apart from having a real job, right, working
[7:03] at CERN designing accelerators,
[7:06] after hours, he was interested in the foundations.
[7:08] And
[7:10] and almost thirty years later,
[7:13] what he did,
[7:15] looking at this philosophical
[7:17] paper and trying to understand
[7:20] whether there are additional parameters that
[7:25] that we may need if we want to
[7:27] have a very precise description,
[7:30] whether he can translate the whole idea into
[7:33] something that is testable, something that's at least
[7:37] falsifiable
[7:37] so that you can set up an experiment.
[7:40] And you run this experiment,
[7:42] and it can
[7:43] refute the local hidden variable model,
[7:47] of Einstein.
[7:48] So that was a a great achievement of,
[7:51] John Bell. So he was able to take
[7:53] those what looked like a philosophical
[7:55] idea,
[7:56] Einstein paper, and translate it into something that
[7:59] you can show to experimentalists
[8:01] and say, go to your lab,
[8:02] do this experiment, and
[8:05] and then you can,
[8:07] refute one
[8:09] possible worldview that, Einstein was
[8:12] subscribing to.
[8:14] So Bell's theorem, which you've referred to, has
[8:17] been described by some as the most profound
[8:21] discovery in science. I mean that's that's a
[8:24] pretty strong statement. Yeah. Yeah. But so
[8:27] can you
[8:28] I mean why would somebody say that? Obviously
[8:31] it's very important.
[8:33] It's a very important concept or a set
[8:35] of ideas. What why is it so
[8:38] important in the sort of history of quantum
[8:40] mechanics and and how we understand quantum theory
[8:44] today?
[8:45] Well, it's
[8:47] it is it is certainly very important,
[8:50] result.
[8:51] The whole quantum theory is a very important
[8:54] result. Right? It's a new way of looking
[8:56] at nature. It's not even
[8:58] something that tells us about interactions. It's unlike
[9:01] gravity, electrodynamics,
[9:03] strong, or weak interaction.
[9:05] Quantum physics is a or quantum theory is
[9:07] a is a a meta statement. Right? It
[9:09] says no matter what the interaction are,
[9:12] then you have to describe them in a
[9:14] certain way.
[9:15] So so it gives you a,
[9:19] a framework
[9:20] in which you have to formulate any physical
[9:22] theory. So so changing this framework from from
[9:25] classical to quantum,
[9:28] even though people maybe didn't quite realize it
[9:30] at the very beginning, but it it is
[9:32] like a changing of world view.
[9:34] And then there's Einstein there, right, who is
[9:37] saying, look, not so fast. Right? So maybe
[9:40] maybe it's not
[9:42] exactly that you have to live with inherent
[9:45] randomness.
[9:47] Perhaps it's not complete.
[9:49] So Bell,
[9:51] in a way,
[9:53] is coming with a with a mathematical
[9:55] proposition which says, okay. Go to the lab,
[9:58] test it, and if you see a violation
[10:00] of this the so called Bell inequalities,
[10:03] then that's it. That you have to refute,
[10:05] that you have to drop the the the
[10:08] whole idea of, hidden variables. So there's no
[10:11] hope.
[10:12] There's no, like,
[10:15] you know,
[10:17] a a screenplay of some sort that tells
[10:20] exactly what's going to happen. The nature is
[10:22] inherently unpredictable.
[10:24] And and so that that is you know,
[10:27] when you think about it, that that is
[10:28] quite a strong statement. That's almost like changing
[10:31] a world view, and and I think,
[10:33] it's it's perfectly justified to refer to it
[10:36] as, as one of the
[10:38] greatest thing that that happened in in in
[10:41] our understanding
[10:42] of of the of the nature of reality.
[10:44] I should add perhaps that,
[10:48] that there there there's more to this story.
[10:50] I mean, just we don't have to go
[10:51] in because it may be part of the
[10:53] of of of of what we want to
[10:55] talk about, but, but nonetheless, it's connected that
[10:59] we make certain assumptions when when we look
[11:02] at,
[11:02] Bell theorem. And,
[11:05] so there are other ways to think about,
[11:09] quantum physics. And and, for example,
[11:12] it may well be that, our perception of
[11:15] the world is simplistic and maybe that there
[11:18] is a room for
[11:20] multiverse,
[11:21] which is the Everett approach.
[11:24] And every approach is different in a sense
[11:26] that in in Bell's theorem, for example, we
[11:28] assume that, each each experiment has a outcome
[11:32] that we see and perceive,
[11:33] that in the Everett interpretation of all possible
[11:36] outcomes can happen. So so what I'm saying
[11:38] that there's more to this story. Right? So
[11:40] I may just say, okay. Bell, great. Wonderful
[11:43] idea. But then we also have Everett,
[11:45] great insight into the nature of reality and
[11:48] many others. So so I'm
[11:51] I'm I'm I agree with the statement that
[11:54] it was a a profound
[11:57] discovery or statement that was made about our
[12:00] ability to distinguish between different world views,
[12:04] but but there's more to it.
[12:07] So
[12:07] Bell's theorem, I suppose,
[12:10] when he formulated it was still a theorem.
[12:13] And as you say, he he offered guidance
[12:16] to experimentalists,
[12:19] but it was still, I mean, it's still
[12:21] a very difficult experiment to do,
[12:24] in 1964.
[12:26] Yeah.
[12:27] So,
[12:28] in 2022,
[12:30] Alain Aspe,
[12:32] John Klauser,
[12:33] and Anton Zeilinger,
[12:35] won the Nobel Prize for physics
[12:38] for their pioneering work in actually doing
[12:42] Bell tests
[12:43] in the lab.
[12:45] And this was work, I think, that was
[12:47] done is it eighties, nineties,
[12:50] roughly then?
[12:53] Sadly, of course, John Bell died in 1990,
[12:56] so he couldn't
[12:57] share the Nobel Prize,
[13:00] for that work.
[13:02] Can you give us a flavor of what
[13:04] Aspe, Clauser,
[13:06] and Zollinger did in the lab? How did
[13:09] they translate
[13:10] John Bell's ideas into
[13:12] actual experiments?
[13:15] Right. So,
[13:18] so John Bell, as you pointed out, John
[13:20] Bell came up with a a mathematical statement.
[13:23] What was good about this mathematical statement was
[13:25] that, it
[13:27] it could be taken to,
[13:31] could be translated into an experiment.
[13:33] And,
[13:34] so
[13:36] so John Bell proposed what we refer today
[13:39] as Bell inequalities. In fact, there is just
[13:41] not, like, one
[13:43] Bell inequality, but there are many versions of
[13:45] it. So the most common one is known
[13:47] as a CHSH inequality, but,
[13:50] but let let's refer to them as Bell
[13:52] inequalities. So so Bell inequality is a statement
[13:55] which to an experimentalist, it means go to
[13:58] the lab, set up experiment,
[14:00] and observe certain correlations. So,
[14:03] you have to have two particles, like two
[14:05] photons,
[14:08] and you send them into two different locations,
[14:10] and you want to, say, measure polarization of
[14:12] those photons. And when you do that, you
[14:15] realize that, the outcomes are not completely independent
[14:18] from each other. Those photons, which we call
[14:20] entangled photons,
[14:22] they re they respond to the measurements in
[14:25] a in a very correlated way.
[14:29] And then you you study those correlations.
[14:32] You measure several different types, like the four
[14:34] different types of correlations, and you set up
[14:36] inequality.
[14:38] And if this inequality is satisfied, then you
[14:41] say fine.
[14:42] That is consistent with what Einstein wanted to
[14:45] see. But if it is violated,
[14:48] then you know that Einstein
[14:50] view of local hidden variable model is is
[14:54] not correct, is refuted.
[14:57] And, again, you know, John Bell, as you
[14:59] said, in the early sixties, John Bell comes
[15:02] with this,
[15:03] Bell inequality
[15:05] statement.
[15:06] One would say
[15:08] from today's perspective, great thing. You should rush
[15:11] to the lab immediately and do all those
[15:13] kind of experiments. That didn't happen for for
[15:15] maybe for two reasons. First of all,
[15:17] most people didn't actually
[15:21] look at it as or didn't consider this
[15:24] as an interesting result.
[15:26] It was still on the fringe. It was
[15:28] still mostly philosophical thing.
[15:31] The other reason was the technology to,
[15:33] implement this kind of experiment was not, so
[15:36] mature at the time.
[15:39] So to the, you know, another thirty years
[15:42] or so, right,
[15:43] from the sixties to,
[15:47] like, eighties and nineties
[15:49] for people to start playing with it. And,
[15:51] again,
[15:52] not not mainstream physics, really. If you look
[15:55] at, those early experiments,
[15:58] again, a bunch of outliers, a postdoc who,
[16:01] like John Clauser, who didn't really want to
[16:04] do what his supervisor was asking him to
[16:06] do and started playing with that, Alain Aspair,
[16:10] a colorful,
[16:12] character himself who
[16:14] with a passion to really understand how it
[16:16] all works,
[16:18] and many others, and and later Anton Zeilinger
[16:20] as well.
[16:22] So what happened was that the Klaus who
[16:25] set up this experiment,
[16:28] he was the first to see the violation
[16:30] of the Bell inequalities. But I want to
[16:32] also add that at the time, there were
[16:34] experiments
[16:36] which were pointing in the other direction saying
[16:38] that, you know,
[16:39] maybe Einstein is right. So it was not
[16:42] entirely obvious. It was one of those interesting
[16:45] experiments where
[16:47] it's it's I wouldn't call it a cliffhanger,
[16:49] but some or something of that kind. Right?
[16:51] So so you are there as an experiment.
[16:54] Imagine Hamish, you go to this lab of
[16:57] Alain Aspair and look at this,
[16:59] set of correlations and thinking which way will
[17:01] it go. So there's the suspense. Right? Is
[17:04] it going to be local hidden variable or
[17:06] quantum rules the world and forget about,
[17:09] any determinism?
[17:11] So,
[17:13] so the the the sequence of those experiments
[17:16] starting from John Clauser, then Alain Aspair,
[17:19] and then Nicolas Gisen,
[17:22] and then,
[17:23] many other colleagues to mention, even, our British
[17:27] colleagues here, like John Rarity and and Paul
[17:29] Tapster
[17:30] at the Defense Research Agency
[17:33] that I had the pleasure to work with.
[17:36] So they
[17:37] they they implemented those experiments. And and then
[17:40] Anton Zeilinger who,
[17:44] really managed to close
[17:46] many loopholes in those experiments.
[17:51] So so the so the that that sequence
[17:53] of experiments over a period of time
[17:57] made it quite clear
[17:58] that,
[18:00] the local hidden variable model is not sustainable,
[18:03] so it doesn't really work. So it it
[18:05] you have to think about inherent randomness if
[18:07] you want to
[18:09] if you drop the multiverse,
[18:11] just then there's inherent randomness in nature.
[18:14] I see. And
[18:16] how
[18:18] how do you interpret
[18:21] or or or how should one interpret a
[18:24] well, let's say a positive
[18:26] bell result, positive in the sense that it
[18:28] points towards
[18:29] quantum physics. Do do you interpret that as,
[18:33] yes,
[18:34] entanglement as predicted by
[18:37] quantum physics is a real thing,
[18:40] and that the system that you are looking
[18:42] at must be quantum,
[18:44] it simply cannot be a classical system. Is
[18:47] that the the right way to interpret? Yes,
[18:50] of course. If you if you want to
[18:51] use this dichotomy,
[18:53] classical and quantum, so that's one way of
[18:55] looking at it. Usually, you just take the
[18:57] view that everything is quantum and classical. It's
[18:59] just an approximation to quantum.
[19:02] And,
[19:03] so having this, Bell test,
[19:06] it it shows that, there's just no way
[19:08] to explain what you see,
[19:11] using, kind of a classical tricks where things
[19:15] can be deterministic. You can you have to
[19:18] just drop your hope
[19:19] that you will be able in some experiments
[19:22] to say exactly that your photon will go
[19:24] into this photo detector and not into that
[19:27] one. So so that's that's not possible.
[19:29] And,
[19:30] so that's that's a big thing. Right. And
[19:32] and
[19:33] would it mean,
[19:35] you know, I know
[19:36] going back to 1935,
[19:37] Einstein
[19:39] and his colleagues had their concerns.
[19:42] If, you know, if Einstein were around when
[19:45] those experiments were finally done, and Podolsky and
[19:49] Rosen, Do you think that they would be
[19:51] satisfied that they would,
[19:53] Well, what are Their their their question would
[19:55] be answered by those experiments. Of course, a
[19:57] very interesting question. So I I would it
[19:59] would be fantastic to
[20:01] have Einstein at the time
[20:04] and,
[20:04] and see his reaction to to those experiments.
[20:09] I I think there are two ways around.
[20:11] So first of all, there's still still a
[20:13] possibility, but I don't think Einstein would subscribe
[20:16] to this.
[20:17] But there's still a possibility
[20:19] that everything is deterministic
[20:20] because when you run the Bell measurement,
[20:25] in the process of running this
[20:28] experiment,
[20:29] you have to make some random choices.
[20:32] It's very important that you randomly choose the
[20:34] setting,
[20:36] of,
[20:36] measuring devices
[20:38] which measure, say, polarization.
[20:40] Now
[20:43] if those choices
[20:45] are predetermined,
[20:46] not random,
[20:47] then,
[20:48] you can see the violation of the Bell
[20:51] inequalities
[20:52] even though you can still explain everything using
[20:54] this local hidden variable model. So what I'm
[20:57] saying is that
[20:58] if you don't have free will as experimenter
[21:01] to have this free freedom of choosing the
[21:04] setting
[21:05] so that everything can be then scripted, your
[21:08] action
[21:09] so if you if you involve absolutely everything,
[21:12] the whole universe, so call it superdeterminism,
[21:16] then, of course, we cannot exclude this possibility.
[21:18] So this particular experiment assumes that,
[21:22] the the settings of those devices are independent
[21:25] from devices itself.
[21:28] Not so many people would subscribe to this
[21:30] worldview that everything is absolutely super deterministic.
[21:34] We would like to believe that we have
[21:36] free will, that the whole point of science
[21:39] is to make choices and and set up
[21:41] devices and and see what happens,
[21:44] but that cannot be excluded. So it has
[21:45] to be mentioned that there is still this
[21:47] possibility. Another possibility, of course, I don't know,
[21:49] again, whether Einstein will subscribe to it, would
[21:52] be the Everett interpretation, which has all kind
[21:54] of interesting features, which somehow
[21:57] tells you that
[21:59] that there are just
[22:01] that we live in this multiple of realities,
[22:04] and,
[22:05] and it's not as science fiction as it
[22:07] sounds. Right? I know people just, like Everett
[22:09] interpretation
[22:10] because it has all these strange connotations to
[22:13] many parallel unitises and so on and so
[22:15] forth. But, honestly, if you take quantum physics
[22:18] seriously
[22:19] and,
[22:20] say that there's
[22:22] no
[22:23] border between classical and quantum, that there's not
[22:26] that much you are left with. If you
[22:28] if you're serious about this description, then you
[22:30] have to somehow,
[22:31] lean towards the Everett interpretation. So I I
[22:34] guess
[22:35] my hope my hope is, you know,
[22:38] is that Einstein would endorse the Everett interpretation.
[22:41] That that's my guess.
[22:43] So, Archer, that brings us very nicely to,
[22:47] quantum cryptography and in particularly
[22:51] and in particular,
[22:52] the, e 91
[22:54] protocol that,
[22:56] that you came up with.
[22:58] Can you explain how it is or how
[23:01] it relies on Bell's theorem or is inspired
[23:04] by Bell's theorem?
[23:06] Yes. So that's,
[23:08] what happened was that
[23:11] when I
[23:12] was a PhD student in Oxford,
[23:15] I was supposed to be working on
[23:18] improving
[23:20] quantum communication with
[23:23] using what quantum opticians at that time called,
[23:26] the squeezed states of light.
[23:28] It was interesting topic even though,
[23:31] I have to say improve
[23:33] improving signal to noise ratio and the hopes
[23:35] we had with
[23:37] squeezed states were,
[23:40] well, never really materialized the way we wanted,
[23:44] them. That doesn't mean that we don't use
[23:46] those states of light. I mean, we use
[23:47] them in very precise interferometers.
[23:49] But but for me, it was a starting
[23:51] point to look into the foundations of,
[23:54] quantum physics.
[23:56] And, with great interest, I was reading some
[23:59] historical papers and in particular, this 1935
[24:03] paper by Einstein, Podolsky, and Rosen.
[24:05] And Einstein and I have to say that
[24:08] I really like,
[24:09] the way he writes.
[24:12] So I'm assuming here that the paper was,
[24:15] mostly or influenced
[24:17] in style by Einstein. So Einstein's style
[24:20] is is is very clear.
[24:23] And,
[24:25] so EPR
[24:27] paper
[24:29] is carefully structured so that the Einstein wants
[24:32] to define
[24:33] what does it mean for
[24:35] something to exist, for example, and,
[24:38] and by exist meaning prior to the measurement.
[24:42] Because, you know, once you measure something, it's
[24:44] usually you say, okay. There was something there.
[24:47] It had a certain property.
[24:49] I measured it, and I reveal this property.
[24:53] If there's
[24:54] a a box,
[24:56] I look at it,
[24:57] switch on the light, and it's red,
[24:59] I measure a property,
[25:01] redness of the box. Right? So and but
[25:04] but but I assume, of course, that me
[25:06] looking at it didn't make it red.
[25:08] It used to be red. Right? So it
[25:10] had to be red before
[25:11] I look at it. Einstein wants to
[25:15] define what it means for something to exist.
[25:18] And, he has this statement. There was just,
[25:21] like, one statement in that paper that fired
[25:23] my imagination, really, and that was my starting
[25:26] point. And that statement roughly says that the
[25:29] element of reality
[25:30] that's Ishan
[25:31] just
[25:32] called something that that,
[25:34] you know, exists.
[25:36] The element of reality according to Einstein is
[25:39] defined in such a way that
[25:41] if you can
[25:42] find out about the value of a certain
[25:45] physical property without
[25:47] disturbing this property, then there exists an element
[25:50] of reality associated
[25:51] with this property.
[25:53] So that means, like, if we can
[25:57] measure something and use
[25:59] without messing up this property
[26:01] So that that that means that there's a
[26:03] notion of passive measurement. Right? So that that
[26:05] really reveals the value of this property without
[26:08] disturbing it.
[26:10] And then to me and and I had
[26:12] some
[26:14] from my
[26:15] previous interest in in secure communication.
[26:17] I had some,
[26:20] interest in the notion of eavesdropping
[26:22] in in a secure communication system. And then
[26:26] an eavesdropper
[26:29] wants to do exactly
[26:31] this, to measure something without disturbing this. So
[26:35] if there's a carrier of information that has
[26:37] certain properties and those properties carry
[26:40] data that you want to read.
[26:42] An eavesdropper wants to
[26:45] measure it without disturbing it, find out the
[26:47] value of this physical property without disturbing it.
[26:50] So then I made this mental connection. I
[26:52] think, gosh. You know?
[26:53] So Einstein is talking about eavesdropping
[26:56] even though it's not called eavesdropping,
[27:00] and maybe I can put it in in
[27:01] a different context.
[27:03] And then I knew about the Bell
[27:06] work,
[27:07] and I was, at the time, happily reading
[27:10] Alan Aspert,
[27:12] thesis
[27:14] with with description of his experiments.
[27:17] Then I thought, okay. We know that there
[27:20] are physical
[27:21] systems or experiments
[27:23] where you can show that there is no
[27:25] element of reality, that the whole thing, if
[27:28] the Bell inequality is violated,
[27:30] you cannot say that you have this physical
[27:33] the element of physical reality.
[27:35] It simply
[27:37] can be then translated into the statement that
[27:39] there's just no way there was an eavesdropping.
[27:44] And so that was the connection. So from
[27:46] Einstein definition of the element of reality,
[27:49] knowing about the Bell
[27:51] theorem that that shows that Einstein approach,
[27:54] can be refuted,
[27:56] knowing that it can be implemented,
[27:59] by people,
[28:00] you know, people who implemented it to to
[28:03] mention Clauser and,
[28:05] Aspe and Nicolas Giza and Anton Zeilinger and
[28:09] and,
[28:10] Jan Wei Pan in China and, late and
[28:13] that was later, by the way, and
[28:16] and our colleagues here
[28:17] in in Malvern.
[28:19] So that's
[28:20] knowing all this,
[28:22] you know, I just went on and and
[28:25] and and said, well, I can construct a
[28:27] system
[28:28] where
[28:30] the violation of Belling Equality
[28:32] certifies the lack of eavesdropping.
[28:35] And so that that that is sort of
[28:37] like a vagues a very very vague explanation.
[28:39] This is where the idea comes. And, of
[28:41] course, you know, you have the idea,
[28:43] and there's a long way to go because
[28:45] you have to do all kind of mathematical
[28:47] gymnastics to make sure that,
[28:51] you look at it as a cryptographer,
[28:53] that you look at the different property, you
[28:55] look at the secret key rate, and you
[28:57] introduce new things.
[29:00] So that's, that's I had to,
[29:02] work a little bit on that.
[29:05] And and that was,
[29:07] yeah, that was it, actually.
[29:09] So so so the idea,
[29:10] if I understand it correctly, is that
[29:14] information is sent, and it is,
[29:18] I suppose in in in a quantum form.
[29:21] Is that the right the right way to
[29:22] explain it? If an eavesdropper
[29:24] makes a measurement on it, it loses
[29:27] some or all of its quantumness. Correct. And
[29:30] then when you make the Bell test,
[29:33] if if if if you don't get a
[29:35] thumbs up on quantum from the Bell test,
[29:37] you know
[29:38] that somebody has
[29:40] fiddled around with that information. Could have in
[29:42] principle fiddled with that. Yeah. Yeah. That's right.
[29:44] So
[29:45] you're absolutely right. So the the the quantumness,
[29:48] the that is indicated by the violation of
[29:51] the Bell inequality
[29:53] is lost when there is an eavesdropper
[29:56] because that eavesdropper would make
[29:59] would introduce this
[30:00] element of reality.
[30:02] And and so,
[30:05] so, yeah, so that you can translate it
[30:06] into something that, is called key distribution cryptography.
[30:10] So that means
[30:11] that two individuals
[30:13] using exactly the experiment that was, set up
[30:16] by,
[30:17] those pioneers who started playing with the Bell
[30:20] inequalities,
[30:22] then you can you can simply just,
[30:24] use this experiment with some
[30:27] modification and minor variation
[30:29] of those experiments,
[30:31] to allow two individuals in different locations
[30:34] to share private random sequences that later on
[30:38] can be used for cryptographic purposes.
[30:40] So you you've come up with this idea,
[30:43] and, I suppose the next step is to
[30:45] to show that it's possible,
[30:47] And, which I'm guessing was probably not not
[30:49] a very easy thing. No. No. Yeah. You're
[30:52] right.
[30:53] So you joined forces with,
[30:55] John Rarity
[30:56] and Paul Tapster.
[30:58] And they were I'm right in thinking they
[31:00] were at The UK's
[31:01] Defense
[31:02] Research Agency
[31:04] in Malvern? That's correct.
[31:06] And, which is in, I suppose in, Southwest
[31:09] England, or is it the Midlands, or
[31:12] on the border between, the West Country and
[31:15] the Midlands.
[31:16] And this was in 1991.
[31:20] Can you give us an idea of how
[31:22] you did that experiment or how you Yeah.
[31:24] And John and Paul did that ex So
[31:26] it's interesting, you know, because I I had
[31:27] this,
[31:29] I wrote this paper and
[31:32] and the the two things happened.
[31:35] First of all, you know, as a PhD
[31:36] student,
[31:38] you you work on something that that is
[31:40] a bit bizarre. It was another mainstream.
[31:45] And I had very low confidence in myself
[31:48] at the time, so I didn't think
[31:50] that,
[31:53] well, at least I was not so sure
[31:54] that that that that really makes sense. And
[31:57] then two things happened. The one thing was
[31:59] that I learned from, a colleague of mine
[32:02] that,
[32:04] Charlie Bennett and Gilles Brassard,
[32:08] my colleagues from, one from US, One from
[32:10] Canada,
[32:12] actually worked on something very similar,
[32:16] namely using quantum phenomena to
[32:20] set up a key distribution in cryptography.
[32:22] So that was to me even though I
[32:24] didn't know this work. There was no Internet
[32:26] at the time, and, and they published
[32:29] the work in a very obscure
[32:31] conference proceedings from Bangalore,
[32:34] which, you know,
[32:36] didn't exist in even in in Oxford, you
[32:38] couldn't find such a thing. And and, you
[32:39] know, to find it, you would have to
[32:41] know what to look for, but but I
[32:44] didn't. I didn't think that anyone was working
[32:46] on this. And
[32:47] so,
[32:48] but with the help of,
[32:50] David Deutsch, my,
[32:52] mentor here in Oxford,
[32:55] and, and Peter Knight, who was my supervisor
[32:59] at the time, Imperial College.
[33:02] I I had a chance to meet and
[33:04] talk to Charlie Bennett and and Gilles Brassard.
[33:07] And, I was both happy and unhappy, I
[33:10] have to say, at the same time.
[33:12] Happy because,
[33:14] that gave me confidence that what I was
[33:16] working on,
[33:18] was relevant, was maybe important. And if those
[33:21] guys
[33:22] rather established an eminent,
[33:26] physicist
[33:27] and computer scientist, I think at the time
[33:29] Gilles would refer to himself as a computer
[33:31] scientist.
[33:34] I think,
[33:35] so that was great, you know, that that
[33:36] that that means that I was working on
[33:39] something important. So that that gave me sort
[33:41] of a boost of confidence.
[33:43] I'm happy because, you know,
[33:45] would be nice to
[33:47] know that nobody thought about this before even
[33:49] though
[33:50] what Gilles and Charlie did was completely different.
[33:53] So they were looking at,
[33:56] how disturbance can be detected
[33:58] using different phenomena. So it was not really
[34:01] that much of a fusion of fundamental,
[34:03] physics with
[34:05] cryptography. It was Heisenberg answers the principle
[34:09] based on
[34:11] previous ideas introduced by Steven Wiesner, by late
[34:14] Steven Wiesner. So
[34:16] so I learned about that work,
[34:18] after I already did mine, and and that
[34:20] was interesting. So so I was confident that
[34:23] at least it's it's worth trying. It's worth
[34:25] doing something about it, but not so many
[34:27] people were really interested in.
[34:29] And and then another interesting thing happened. I
[34:31] was,
[34:33] giving,
[34:34] I was showing a poster, in fact, in
[34:36] one of those
[34:37] skiing conferences in Cortina D'Ampezzo, in fact.
[34:41] And
[34:43] on the slopes, I met generality.
[34:46] And while we were skiing, we were talking
[34:48] science, and,
[34:50] and John,
[34:51] who work at the Defense Research Agency in
[34:54] Malvern at the time,
[34:55] got interested. He said, well, look. We essentially
[34:58] have all the setup that
[35:01] that you are talking about so we can
[35:03] implement that thing.
[35:05] And so I was invited to,
[35:07] see their labs later. I traveled to Malvern,
[35:11] and it was it was really interesting to
[35:13] work with a duo of John Rarity and
[35:14] Paul Tapster.
[35:17] I I, you know, I like them because
[35:18] they were so different and so complimentary in
[35:20] a way.
[35:21] So John was,
[35:23] rather
[35:24] an extrovert,
[35:26] and and very
[35:28] much
[35:29] willing to talk about everything that they're working
[35:31] on and
[35:33] and traveling places, going to conferences, giving good
[35:36] talks.
[35:37] Paul, in contrast, was an introvert. So he
[35:39] worked in the lab, and, I think,
[35:43] for Paul,
[35:44] most of the world
[35:45] was confined to a triangle that was defined
[35:48] by his house,
[35:49] his lap in DRA,
[35:51] and his bridge clap. So he played bridge.
[35:54] So that was Paul. But Paul was one
[35:56] of the most amazing
[35:59] experimental physicists that I came across,
[36:01] so he could just make magic happen. So
[36:04] working with them was a was a great
[36:06] pleasure. I have to say as a theorist,
[36:07] you know, I just I was I was
[36:09] quite happy if I could just go to
[36:10] the lab and identify all the components on
[36:13] the Octopi table. So so I wouldn't claim
[36:16] that I really contributed that much to this
[36:18] experiment, but we worked together on this. And,
[36:21] we managed to demonstrate that it really works
[36:23] in this different setting.
[36:27] Violation of Bell inequalities
[36:30] can
[36:30] be, understood,
[36:32] or can be used for for practical purposes.
[36:35] So so at this point, the interesting fusion
[36:38] happened.
[36:39] On one hand, you have cryptographers
[36:42] working on,
[36:43] secure communication, and then there's this big problem
[36:46] that they call the key distribution problem.
[36:49] And this is like a holy grail of
[36:50] cryptography. Find the solution to the key distribution
[36:53] problem.
[36:54] On the other hand, you have a bunch
[36:56] of,
[36:57] outliers in physics working on the foundations, doing
[37:00] experiments, and nobody cares about those experiments, honestly,
[37:03] because most working physicists say, okay. Fine. You
[37:06] guys are telling us that
[37:08] quantum is the way to go. But we
[37:10] we had known this for a long time.
[37:12] Right? So so so why why bother so
[37:15] much? It's purely philosophical.
[37:17] But here comes a moment,
[37:21] in in 1991
[37:23] and then in 1992
[37:24] when when we do these experiments
[37:27] that shows to the world that it's not
[37:30] completely irrelevant what those
[37:33] weirdos doing,
[37:35] quantum foundations,
[37:37] think. And their tools can be useful for
[37:40] something very practical for secure communication.
[37:43] So this fusion happens.
[37:45] Quantum foundations
[37:47] offers a solution to the key distribution problem,
[37:50] and the cryptographers at the same time,
[37:53] getting a new set of tools to play
[37:55] with.
[37:56] And from this moment onwards,
[37:59] another interesting thing happens, namely
[38:02] the experiments
[38:03] that show the violation of Bell inequalities were
[38:06] not perfect.
[38:08] They were not perfect because there were various
[38:10] loopholes,
[38:11] and those loopholes assumed something that
[38:15] most physicists would just dismiss.
[38:17] For example,
[38:18] they assume that nature may cheat you
[38:23] by implementing
[38:25] such and such local hidden variable model in
[38:27] one experiment
[38:29] and then
[38:31] experimenting, maybe implementing maybe another one in another
[38:34] experiment.
[38:35] Or that
[38:36] if you don't have a perfect detection
[38:39] that
[38:40] nature will conspire
[38:41] against you. So you,
[38:43] in the data that you see, you see
[38:45] the violation of the Bell inequalities.
[38:47] But if you were to include all possible
[38:50] data, including those that you didn't register
[38:53] so if you combine them together, you wouldn't
[38:55] see the violation of the Bell inequalities.
[38:58] And then, you know, physicists would say, come
[39:00] on. Honestly,
[39:02] nature conspiring against you? No way.
[39:05] But if you put the whole thing into
[39:07] a crypto setting,
[39:09] it's not nature. It's your adversary
[39:11] there, and that adversary can cheat you in
[39:14] any way possible. Right? So you have to
[39:16] close those loopholes.
[39:18] So that gave the final push to the
[39:20] foundations. Right? So so now that the closing
[39:23] those loopholes was not just question of being
[39:27] sort of, like, pedanting from a philosophical point
[39:30] of view. It was just more to say,
[39:32] if you want to show the security of
[39:33] the system, you have to close the loopholes
[39:36] full stop. And here comes, I I think,
[39:39] people like Rasmus Hansen and Anton Zeilinger and
[39:42] and other
[39:44] who had technology at the time
[39:46] to close the loopholes, and that happened.
[39:49] So so you could see at this point
[39:51] a beautiful synergy of the two. Right? Cryptographers
[39:54] telling
[39:55] people in the foundations, come on, guys. You
[39:57] have to close the loopholes.
[39:58] And people on the foundation side say, oh,
[40:00] great. You know? So far, nobody cared about
[40:03] this, and now someone does. So let's close
[40:05] the loopholes, and let's have those experiments.
[40:08] So
[40:09] so the Nobel Prize that's in 2022
[40:12] went to Closer
[40:14] and Asper and Zeilinger,
[40:18] that really shows
[40:19] evolution
[40:20] of,
[40:21] people starting from
[40:23] doing
[40:25] really,
[40:26] blue sky,
[40:29] curiosity driven research
[40:31] to
[40:32] beautiful implementation,
[40:34] full implementation of the Bell inequalities
[40:38] that
[40:39] already comes with understanding that it's relevant.
[40:42] Clauser and Asper were mostly driven by curiosity.
[40:46] I'm Anton, I I think, was driven both
[40:48] by curiosity.
[40:50] So working with him was always a pleasure,
[40:52] and we talk a lot about the philosophy
[40:54] and foundations, but he also had a benefit
[40:56] of understanding that this is important from the
[40:58] practical point of view.
[41:01] So, Archer, today, they're they're actually commercially available
[41:05] quantum cryptography
[41:06] systems.
[41:08] Although I'm guessing that,
[41:10] there's probably many improvements that can be made.
[41:13] Do do you think that at some point
[41:15] in time, quantum cryptography
[41:17] is going to replace
[41:18] the the sort of classical or standard cryptography
[41:21] systems that we use today.
[41:25] Well, I don't think that there will be
[41:27] a complete replacement.
[41:30] But I think that
[41:33] quantum crypto will certainly play a significant
[41:36] or increasingly significant role
[41:38] in in our secure,
[41:41] communication environment.
[41:43] To start with,
[41:45] one thing that happened
[41:47] after my work
[41:49] was,
[41:52] I wouldn't well, improvement in understanding
[41:55] of the system that I proposed,
[41:58] and,
[41:58] that is known as a device independent
[42:02] cryptography.
[42:03] And that that is a
[42:06] from the fundamental point of view, it's it's
[42:08] simply
[42:09] absolutely unbelievable.
[42:11] It essentially
[42:13] says that
[42:15] you can purchase devices
[42:18] from your adversary,
[42:20] from your enemy,
[42:22] run the test on those devices.
[42:24] And if they pass CHSH or Bell test,
[42:27] essentially,
[42:28] then those devices are good. I mean, you
[42:30] can use them. You don't have to
[42:33] inspect devices. You don't have to open them.
[42:35] You don't have to tinker with unknown technology.
[42:37] You know, usually, you don't have expertise to
[42:39] do that.
[42:40] So unlike today where we have to certify
[42:43] those devices, so you produce a a device
[42:46] that say
[42:47] generates random numbers for crypto purposes.
[42:50] So how do you know that this is
[42:52] really generating random numbers?
[42:55] And,
[42:55] the only way is to trust
[42:59] some sort of authority. So there's a certifying
[43:02] agency that will
[43:04] say,
[43:05] okay. You know, a bunch of wise guys
[43:06] looked into this, and we can get you
[43:09] trust us, and we guarantee that this is
[43:11] this is fine. But we don't want this
[43:13] necessarily in the future. Right? So having the
[43:16] self testifying devices,
[43:19] and it's not so much about necessarily
[43:22] buying them from the enemy. Nobody would do
[43:24] it. But but sometimes when you desire a
[43:26] device,
[43:29] you unwillingly, you may introduce some
[43:32] bugs into the system and,
[43:34] and then you want to avoid this. So
[43:36] so this self testing is a wonderful thing
[43:38] because you don't have to do anything
[43:41] inside the device. You don't touch the device.
[43:43] You just provide those devices with some inputs
[43:46] that generate some outputs. You run a statistical
[43:49] test, and you say, okay. Those devices are
[43:50] fine. No matter where they come from,
[43:53] they pass the test. They pass the test.
[43:55] They're good.
[43:56] And, so this device independent crypto
[44:00] is,
[44:01] is fantastic because
[44:03] you can then put your devices
[44:05] into location, so, like, on satellite, for example.
[44:08] And you don't have to worry too much
[44:10] about
[44:11] someone
[44:14] tinkering with your devices there because if that
[44:16] happens, you can see it in the test.
[44:18] Right? So so they self testify
[44:20] they self,
[44:21] self test themselves.
[44:23] So that that is from the fundamental point
[44:26] of view. That's, actually
[44:28] a very interesting new development. I think it's
[44:31] pushing the the the limits of privacy to
[44:33] the limits.
[44:37] So that that self self testing, is that
[44:40] is that a function of the the sort
[44:42] of no go or go no go nature
[44:44] of the Bell test? Yeah. It's it's a
[44:47] function of something called,
[44:51] we we say that the Bell result is
[44:53] rigid. So that means that
[44:56] to simplify things a little bit, that means
[44:58] if you see the full violation of the
[45:00] Bell inequality
[45:01] so suppose you run the test
[45:03] and,
[45:04] you reach the maximum violation of the Bell
[45:07] inequality
[45:08] that is
[45:09] that is possible.
[45:13] Then you know
[45:14] from this one number, you know that
[45:18] there's no any other way for this to
[45:20] happen but just to set up something that
[45:22] is equivalent to the Bell test.
[45:26] So you can argue the other way around.
[45:27] So the the the violation of the Bell
[45:29] inequality
[45:30] tells you
[45:33] that only by by looking at the data,
[45:35] even just, you know, looking at one number
[45:37] tells you that there's no way that could
[45:39] have happened without setting up the Bell test.
[45:41] So that was
[45:44] then then, of course, you know, the we
[45:46] know that,
[45:48] it's very unlikely that you will get,
[45:51] the maximum violation. There's always noise in the
[45:54] system and so on and so forth. So
[45:56] so you have to
[45:58] look at some parameters, and from that parameters,
[46:00] you can estimate,
[46:02] the upper bound on information that any third
[46:06] party eavesdropper included
[46:08] and could have gained from from the system.
[46:11] So, yeah, so that's that's that's that's a
[46:14] beauty of,
[46:16] of the Bell inequality
[46:17] approach, which you cannot achieve with anything else.
[46:20] So so this,
[46:22] the system that Charlie and Gilles proposed
[46:26] may work in
[46:28] a in a scenario where you
[46:31] you are confident that your devices are
[46:34] exactly what they are and you are confident
[46:37] about,
[46:38] using more technical terms, the dimension of the
[46:40] Hilbert space of your carrier and so on
[46:42] and so forth,
[46:43] but you cannot achieve the level of,
[46:48] devising the penthouse that you can achieve with,
[46:51] Bell test kind of crypto that I proposed.
[46:54] So that that I see as, as an
[46:56] interesting way to go. Now your question is,
[47:00] will it ever happen that we will,
[47:03] go fully quantum? I don't think so. So
[47:05] I think that there's always a classical element
[47:08] around it
[47:09] if we were to divide things into classical
[47:12] and quantum, but,
[47:13] but that's fine. So I I think, there's
[47:15] still a work you know,
[47:17] lots of work to be done, I I
[47:19] would say, to make it
[47:20] practical. First of all, the quantum technologies
[47:26] is still
[47:27] noisy, so you have to improve it.
[47:30] But it's possible.
[47:32] The same year that,
[47:35] John Clauser,
[47:36] Alan Asper, Anton Zeilinger got the Nobel Prize,
[47:39] There were three experiments showing the
[47:42] proof of principle of the device independent crypto.
[47:45] One of them here in Oxford,
[47:48] one of them in Munich, and one of
[47:49] them in China.
[47:51] So all the three groups show that
[47:54] technology is good enough to implement
[47:56] device independent cryptography. From there to,
[48:01] make make it commercial is still a way
[48:04] to go.
[48:05] But I think that that eventually, we will
[48:07] use crypto in combination with, quantum crypto in
[48:10] combination with various classical methods.
[48:13] Quantum crypto is good for point to point
[48:16] communication,
[48:18] not so great
[48:20] for things like digital signature, password protections, and
[48:23] many other crypto tasks that you would like
[48:25] to implement.
[48:27] But
[48:28] but, obviously, with,
[48:30] the progress in in quantum
[48:32] computing,
[48:36] the whole notion of security in the new
[48:38] quantum world
[48:40] partially relies on the development in in quantum
[48:43] cryptography. So there's probably no way around it.
[48:47] And why not? You know, once we are
[48:49] more familiar with quantum technology, why not taking
[48:51] advantage of it and and using it? So
[48:53] I'm actually quite optimistic. Even today,
[48:56] there are some companies that, can provide you
[48:58] with
[49:01] very decent
[49:02] quantum cryptosystems,
[49:04] just to mention ID Quantique,
[49:08] or
[49:09] Spectral in in Singapore.
[49:11] So,
[49:13] so, yeah, so the quantum is there.
[49:16] It's much more mature than
[49:18] it was when I was working on it.
[49:20] And,
[49:22] I I think that in years to come,
[49:24] we will see more of it.
[49:25] So, Arthur, you're probably most famous for the
[49:28] development of e 91, but, of course, you
[49:30] do lots of other research. What what are
[49:32] you working on at the moment?
[49:35] I think that
[49:37] thank you. I don't know. It's just, you
[49:38] know,
[49:40] being famous for it's it's kind of dangerous
[49:43] to be famous for one thing. Right? It's
[49:45] like an actor. You play one role, and
[49:47] then you always associate
[49:49] with that, buddy. Being the typecast. Yeah.
[49:51] No. No. No. No. But you're absolutely right.
[49:53] So most people recognize this early work.
[49:58] I I I did some work later on
[50:01] on various aspects of quantum computation,
[50:03] in particular, on the,
[50:06] universality
[50:07] of quantum components in in computation
[50:10] and,
[50:11] some early quantum error correction and and a
[50:13] way of stabilizing computation.
[50:16] But, I think that the randomness
[50:18] has been always,
[50:20] something that,
[50:22] I'm fascinated
[50:23] about. So so I think it's still randomness.
[50:26] It's still trying to understand the the nature
[50:28] of randomness.
[50:30] And,
[50:31] and then and, of course, you know, I
[50:33] I also
[50:34] would like to see randomness not only in
[50:36] the context of quantum, but see whether this
[50:38] can emerge in in any other way. So
[50:40] I've been recently working on
[50:42] few crazy ideas with a colleague of mine
[50:44] from Warsaw, Andre Dragon,
[50:47] trying to see how,
[50:49] the
[50:50] including superluminal
[50:56] part of Lorentz transformation
[50:58] can
[50:59] lead into,
[51:00] quantum like appearance.
[51:03] I I think I I'm enjoying this work
[51:05] because it's crazy enough that, it's it's certainly
[51:08] not mainstream, not at the moment.
[51:11] It's quite possible that it's wrong, but if
[51:14] it is wrong, it may be wrong for
[51:16] a good reason. So by by pointing to
[51:19] where it goes wrong, we can learn a
[51:21] lot, I think. So
[51:23] until, until we are proven wrong, I think
[51:25] Andrei and I and our colleagues are
[51:28] who are working on this field in in
[51:30] this field are are prepared
[51:32] to continue for a while. I see. Okay.
[51:34] So so
[51:37] here are you talking about
[51:39] classical physics but allowing for
[51:41] faster than the speed of light? That's right.
[51:43] If you can reproduce some sort of quantum
[51:45] That's right. Like phenomenon. Exactly. Exactly. I see.
[51:49] Okay. Oh, that's fascinating.
[51:51] Well, that was really fascinating, Arthur. Thanks for
[51:53] coming on the podcast.
[51:55] Yeah. Thank you for having me.
[51:57] So the the next question I'm I'm sort
[51:59] of asking everybody quantum.
[52:01] This question. And
[52:03] the plan is to gather
[52:05] all the time we have for this week's
[52:06] quantum cast. But the celebration of the international
[52:10] year of quantum science and technology
[52:13] continues here at Physics World.
[52:16] So to hear you part of my
[52:18] live feed.
[52:19] Go to our website. Intellectual
[52:21] events. Quantum
[52:22] under the topics tab.
[52:24] I'd like to take the parts of the
[52:25] instrumental
[52:26] level. Fascinating conversation.
[52:28] And also Poses lots of challenges to understand
[52:31] this problem. Podcast.
[52:33] We'll be back again next week.
[52:35] It means multiverse and Everett interpretation.
[52:39] I'm trying to
[52:40] understand and reconcile this worldview
[52:44] with,
[52:45] my everyday experience.
[52:48] I think my life just wouldn't be,
[52:51] at the intellectual level,
[52:54] interesting enough without quantum.
[52:57] It would make sense. I don't know. It's
[52:59] very subjective, you know. It's just like
[53:02] it's interesting. Right? So you you start learning
[53:05] about the whole thing. You learn about you
[53:07] learn about it in a very instrumental way.
[53:09] You're happy that you can solve some equations.
[53:12] You are happy that you can make provision.
[53:13] You start asking deeper questions.
[53:16] How does it really, really work? And then
[53:18] you get some sort of answers, and you
[53:20] come across this Copenhagen
[53:22] interpretation.
[53:23] And
[53:24] and then you you look at it and
[53:26] and try to understand it and think, well,
[53:28] it doesn't quite make sense. You know?
[53:31] Where is this difference between classical and quantum?
[53:33] Where is this collapse?
[53:35] And then you you are sort of standing
[53:38] in
[53:39] and and
[53:41] and the roads by four k's at this
[53:43] point to the
[53:44] one side, you have to explain this collapse,
[53:47] and,
[53:50] and that's intellectually honest approach if you're a
[53:52] realist. And then you can have people say,
[53:55] like Roger Penrose who would say, yes.
[53:58] There is another phenomenon if you look into
[54:01] gravity, and then you can have this collapse.
[54:04] But then if you are not prepared to
[54:06] take that road, there's another road that which
[54:08] I took, and that was the road towards
[54:10] the Everly interpretation. I have to say that
[54:12] suddenly I was influenced by
[54:14] David Deutsch, my
[54:16] supervisor here in Oxford,
[54:19] who,
[54:21] who is a proponent for of this view.
[54:24] But I'm I'm happy to be in this
[54:26] camp, I have to say.
[54:28] Oh, that's great. Thanks. And if I can
[54:31] if you if you can indulge me,
[54:33] I've been asked by a colleague
[54:35] to ask you
[54:37] three more questions. Absolutely. Yes. But,
[54:41] if,
[54:43] feel free to don't you don't have to
[54:45] give long answers if you don't want. Okay.
[54:47] Okay. So so the the way the these
[54:49] questions are presented
[54:51] is,
[54:53] actually what I'm gonna do is I'm gonna,
[54:55] to make this easier for me,
[54:58] I'm going to

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