An insight on the physics behind it and its importance for quantum computers.
Today — on the 4th of October 2022 — once again a nobel price was given for great achievements in the field of physics. And this year it has been given to the 3 scientists Alain Aspect, John Clauser and Anton Zeilinger for their experiments with
“[…]entangled photons, establishing the violation of the Bell inequalities and pioneering quantum information science”.
There are a couple of things in that sentence that might seem confusing at first. What are entangled photons? What are the Bell inequalities? And how is all of that related to quantum information science?
When we are talking about quantum information science, we are talking about the foundation for quantum computing. So apparently what these 3 scientists showed plays a crucial role in the future of computing.
I think we should have a look at the physics behind all of that.
It all starts with entanglement, a pretty strange state that quantum particles can be in. And when we talk about quantum particles we mean microscopic particles like molecules or atoms or electrons. Those are particles that no longer obey the classical laws of physics. Which is a bad way of saying it, because the classical laws of physics were just not sufficient enough, to also describe the behavior of particles in the microscopic world. Classical physics might work brilliantly in the macroscopic world to describe the movement of cars or rockets or planets, but one cannot use the same laws to describe the movement of an electron.
One needs quantum physics to do that. And quantum physics comes with a lot of strange properties. Strange, because they completely contradict our expectations from classical physics. Quantum particles suddenly can be at two places in space at the same time. Or they can be entangled.
But what is this entanglement anyway?
Let’s take 2 particles A and B and “entangle” them. Since they are entangled, they are somehow connected to each other. To be more precise: what happens to one particle directly influences what happens to the other particle.
Okay, that does not sound too crazy at first. But now we take those particles and put them far apart from each other. Really far apart from each other.
We place them at opposite sites of the observable universe. Then we do something to particle A and in the exact same moment — instantaneously — this triggers an action in particle B. This would mean that the information of something happening has been transported in between those particles faster than the speed of light.
Because both particles are on the opposite ends of the observable universe. Which means that not even light — which is the fastest thing in the universe- could have travelled from one particle to the other. Because the amount of time it would have needed would be more than the age of the universe.
But quantum physics predicts exactly that. An instantaneous reaction on the other side, no matter the distance.
In 1935 the 3 physicists Albert Einstein, Boris Podolsky and Nathan Rosen did not like that kind of “spooky action at a distance”. Especially Einstein had a problem with it, because it was contradicting his own special theory of relativity.
In a scientific publication they stated that quantum physics must be an “incomplete” theory. Especially because the principle of locality is not fulfilled. Let’s have a look at what that means.
Here we are again with another complicated word. But let me calm you down, the principle of locality is actually quite simple. It simply states that a particle can only be influenced by its direct surroundings. To be more clear about this: for an action at one point X to have an influence on another point Y, something in space in between these points must mediate that action. Practically one can see this mediation with electric or magnetic fields for example. The magnetic attraction between an electron and a proton is transported by the magnetic field. And Einstein showed with his special theory of relativity, that this action cannot be mediated faster than the speed of light.
A physical theory that fulfills that principle is called a “local theory”. Obviously physicists wanted to keep all their theories local, because otherwise one would suggest that things can happen without knowing why they are happening. Because something from extremely far away has influenced it without any information being transported — which makes a measurement almost impossible.
This is why Einstein, Podolsky and Rosen thought that quantum physics must be incomplete. They thought, that quantum physics does not give a good understanding of the world. They just could not accept that a non-local theory would describe the very reality that we are living in. So they saw only 2 possibilities for entanglement to work:
- either the information between the 2 particles is exchanged at a speed greater than the speed of light or
- the entangled particles have some unmeasured property which predetermined their final quantum state before they have been separated
Since the first probability is contradicting with Einstein’s special theory of relativity, the 3 scientists thought, that there must be some hidden variables.
Hidden variables, that were not yet discovered by quantum physicists.
Therefore quantum physics — in their eyes- was not the correct way of seeing how the world works.
Decades later John Steward Bell picked up the topic of entanglement in his own research and worked on the so called EPR-Problem. He was especially looking at the case of the hidden variables.
How could one investigate, whether entangled particles contain hidden variables or not?
This was the question John Bell tried to answer. And his way of answering it was the so called Bell inequality. He claimed that in order for a quantum particle to contain hidden variables, it has to satisfy the Bell inequality. But he also found out that quantum physics predicts processes that violate this inequality.
And a violation of this inequality would mean, that we indeed live in a non-local reality.
Since then a big discussion both in physics and philosophy came up lasting until now. But now that those 3 nobel laureates have experimentally verified a violation of the Bell inequalities, we have entered a new era of understanding how our world works. But how is that related to quantum computing?
Quantum computers are known to be able to execute many tasks significantly faster in the future than a classical computer would be able to. But in order for quantum computers to work properly, one need quantum algorithms. Those are algorithms especially designed to work on quantum computers and on quantum computers only. One example for a quantum algorithm would be the famous Shor- algorithm, proposed by Peter Shore in the 1990’s. With this algorithm it would be possible to factorize large numbers faster than a classical computer would do. And this could be used to crack any security system on the planet, since almost all of them are based on the mathematical problem of factorizing large numbers.
Most quantum algorithms are still theoretical and not yet properly tested on a quantum computer. But more importantly: most of them are based on entanglement between particles. So by proving the violation of the Bell inequalities one also has a proof that the mechanisms of entanglement predicted through quantum physics indeed work and no hidden properties are neglected in the calculations. Which is quite some good reassurance that the quantum algorithms will work in the near future.
Quantum physics is a weird part of physics and almost every physicist will answer “I have no idea what is happening” when being asked about the processes in the quantum world. We physicists come up with mathematical models to get an explanation for our observations, but especially in quantum physics it is extremely hard to understand, why those calculations actually work. They just do, although they completely contradict what we are used to from our every day life, but we accept them as a physicist. Just because they work.