- Hi, my name is Olivia
Lanes from IBM Quantum and I'm here today to celebrate
and explain the relevance and the excitement around
the 2022 Physics Nobel Prize. So, just a few days ago, the Physics Nobel Prize for this year was awarded to three gentlemen: John Clauser, Alaine Aspect, and Anton Zeilinger. These gentlemen won the award
for groundbreaking research and experiments in quantum mechanics. And all of this research was based upon some original
experiments that were done by a gentleman named John Bell, who tragically passed away before he was able to see a Nobel Prize or something of that magnitude. But we are going to discuss it here today and show why the
experiments that were built off of John Bell's original
theorem are so important. So we have to go back a little bit in history in order to set the stage and the context of why
this is so important. So, in the sixties, John Stewart Bell was
reading the EPR paper which was a paper written by Einstein and some of his colleagues
explaining how they believed that quantum mechanics
was an incomplete theory. Now, in quantum mechanics, we say there exists this thing
called the "wave function." Which describes all of the properties of your quantum system. And upon measurement, it
collapses into a single quantity. That was only described probabilistically before you measured it. Anyway, in the paper, Einstein said that quantum mechanics must be an incomplete theory because this could not be possible. This would necessitate
things that he could not believe to be true. And so we were either missing
something in the theory, somehow these particles were interacting in a way that we didn't
actually understand or didn't have a description for yet. But basically it was not finished. And sometime later, John Stewart Bell was reading this paper, and he sat down and he wrote
what is come to be known as "Bell's Theorem." In just a few lines of algebra, he showed that if you
only assume two things. Those things being local realism. Locality, meaning you can't travel faster than the speed of light. And realism, which means that things
have definite values whether or not you measure them. So for instance, we always say, "if a tree falls in the forest and no one is around to hear
it, does it make a noise?" If the answer is yes, this would be an example
of definite realism. So he assumed only those two things in his mathematical derivation. And he was able to show that
there are certain qualities, quantities that you can
measure that are bounded by classical physics if you
assume only local realism. And essentially he was able to show, on paper at least, that quantum mechanics was incompatible with any classical theory. So anything that was basically
including local realism could not possibly work
with quantum mechanics. So now a few years later, John Clauser came along and
he was reading Bell's Theorem and he thought that
this should be something that was able to be done experimentally. So he was able to actually
perform an example in the laboratory for the first time, a violation of this equality and show that indeed, nature behaves
as weirdly as predicted. And so in essence, Einstein was wrong in his EPR paper. And this led to
groundbreaking work later on from Alaine Aspect who was able to perform even more tests of Bell's inequality and close some of the
loopholes, so to speak. And then this led to Anton Zeilinger who was able to show the demonstration for the first time of
quantum teleportation. Which is not what you might think. it's not at all like Star Trek. People are not teleporting
across the room, but instead, quantum teleportation is a way to entangle or correlate quantum particles in such a way that you can
transfer quantum information from one to another. So in order to really understand
why this is so important and why this is so groundbreaking, I want to go back and look at a very specific demonstration
of Bell's tests for you. There are in fact many
different Bell type theorems that can be shown but we're gonna look at really one specific one today which is called the CHSH test or the CHSH inequality. And so what you need to do for this thought
experiment is first imagine a person named Alice and a gentleman named Bob and they are standing
some distance far away from one another. And then there's another
guy in the middle, let's just call him Victor. And he's going to send a particle to Alice and to Bob at the same time. And Alice and Bob are going to perform, every single time this happens,
one of two measurements. They're going to either
measure the X projection or the Y projection of this particle. And so you can only measure one at a time. So in order to have a good understanding of the measurements of both, they have to perform this
experiment multiple times. And it's important to
note that every time Alice receives a particle, Bob
also receives a particle. And again, the only thing
really assuming here, is local realism. So these particles are not moving faster than the speed of light. Bob and Alice can't call each other faster than the speed of light and
communicate information, nothing like that. And these particles also
have these definite values that are either going
to be one or minus one because, let's just say they
are, they're normalized. So the values for X and Y for Alice and X and Y for Bob, can only be one or minus one. They are binary. So now at this point, we need to write down what
is known as the CHSH value. So, this is like so, Alice's measurement of X times Bob's measurement of x plus Alice's measurement of X, Bob's measurement of Y, Alice's measurement of Y, Bob's measurement of X. And then you subtract from that sum Alice's measurement Of Y and Bob's measurement of Y. And it really doesn't matter
where this quantity comes from. The fact is, if we look
at it a little bit closer, we can factor out the Alice quantities from the Bob quantities. And you see, we would
get something like this. And I drew this for you here, a little equal sign there at the end, so you can see that either this value every time the experiment is performed or this value is going
to be equal to zero. Because either you are
measuring one minus one or you're gonna have minus one plus one. The values again, can
only be one or minus one. So the maximum value this
quantity could take on is equal to two, at most. But remember, we're running this
experiment many, many times. So say we ran this experiment, you know, hundreds of times and we're measuring a few different values every
time Alice is measuring X and Y and a few different values every time Bob is measuring X and Y. That means that this is
actually bounded by two again but it can be equal to two or less than two. Once we take the averages, and that's what these
little brackets mean here. We're gonna take the average
of all of these measurements. All right. So this value has to be less than two. If the only assumptions
we are making are locality and realism. However, let's think about
this experiment again. Slightly differently. Let's now assume that
instead of a particle, just a random object Alice
and Bob are receiving, they are going to be
receiving entangled qubits. These are qubits. It doesn't matter how they
were entangled originally at the beginning, let's
just say that they are. What you're actually going to measure, once you perform this
experiment hundreds of times, is that this quantity is
going to be approximately 2.8. Not two, not less than two. We know for sure that
2.8 is greater than two. Which means that if these
qubits obey the laws of quantum mechanics,
which we know they do, quantum mechanics violates
Bell's inequality. It is incompatible with local realism. So either something is moving faster than the speed of light or, these particles do not
have definite values before they are measured. And now a lot of people, I think, misconstrue what this actually means. Some people think that
this opens the gateway for faster than light communication. This is not the case. Let me be very clear about that, because this is a really
easy mistake to make. Bob and Alice are not communicating with each other faster
than the speed of light. There's no possible way to do that. The particles become entangled, and when you measure them,
say Alice measures one, we know that instantaneously
the other particle is going to choose the opposite correlated value, negative one. But that does not mean that Alice and Bob are able to communicate with each other faster than the speed of light. No information is being
transferred at that speed. So that's really important to know. And since there's no evidence of anything in the universe traveling
faster than the speed of light, the way that most scientists
have interpreted this is that we have to give
up the idea of realism. These quantum particles
actually do not have values that are specific to
them before you measure them. They are instead
described by what Einstein (chuckles) called and
resented the "wave function." And so it has some probability of being in either state one or minus
one before you measure it. And you might be wondering, where does this 2.8 number come from? Certainly it's greater than two, and we can understand that it does indeed
violate this inequality. But where does this 2.8 come from? And so, if you're interested, I challenge you to go online
to the Qiskit textbook, and the link is linked below in the bio, and to try your hand at this experiment for yourself right now. There is a tutorial actually
already published online on the Qiskit textbook,
on the CHSH inequality. And you can run the
experiment from your couch or wherever you are right now, and see indeed that you
are going to get a number that is approximately 2.8. And while you might not win a Nobel Prize for doing this because lots of experiments in this vein have been done at this point, I still think it's miraculous that we have gone in a few decades from an experiment that
is Nobel Prize winning to an experiment that you can do, that I can do, from anywhere in the world. So what this showed is
that quantum mechanics is even weirder and even more mysterious than we initially thought it was. It's completely incompatible
with any classical theory that we can come up with. If you assume locality and realism. And reading this you might
think that makes no sense. That's impossible. I hate it. But what these gentlemen
who just won the Nobel were able to do is read this, this proof, this thought experiment, and then go to the lab
and actually demonstrate it in real life. So it's not just weird on paper, it's weird in the real world. Quantum mechanics brings us to the brink of the impossible and it
brings us to the point at which we shouldn't be able to fully comprehend what's going
on, but no further than that. And so that's why it's just a really, really exciting time for
us in the field today. Because these scientists showed that not only is quantum
mechanics weird and mysterious and counterintuitive, it is practical. And everything that we are working on today at IBM Quantum, everything that other
quantum computing companies are working on, quantum
sensing, quantum cryptography, are all built upon these experiments which showed for the first
time that entanglement, quantum entanglement, really is something that is
so fundamentally different than classical physics can describe. So I want to wish congratulations to the Nobel Laureates, again, from myself and everybody here at IBM and thank them for their work and for our jobs as well. (gentle music)
