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