La quantique change-t-elle notre réalité ? | 42 - La réponse à presque tout | ARTE

Imagine that this cat is not only here, but also there. And this simultaneously. Does it seem crazy to you? Of course it is. Because even if felines are known for their agility, they are not capable of such a prodigy. But in the microscopic world of the Kanta, this kind of phenomenon occurs all the time.

No, it's not magic, it's quantum physics. If I could suddenly reduce myself to the scale of an atom, the world would seem very strange to me. All the laws, all the behaviors that I would see around me would defile my understanding in some way. Could reality be very different from what we think? And how can this discovery be useful to us? We realized that quantum physics allowed us to invent new materials, new absolutely incredible concepts.

So how to pull out the tiny quantum particles in our vast world? Even if it may seem curious, our world is actually composed of two worlds. The visible one with the naked eye, governed by the laws of classical physics, and the infinitely small one, the world of the quanta. But this one obeys its own laws. Explanation:

So this is an apple. Everybody knows what an apple is. And it's something that can drop and try to predict where it will fall. If we would know exactly what its position was, its speed, what was the velocity of this apple, then it would be relatively easy to predict exactly what would be the point at which it would fall. In the everyday world, nothing complicated. But in the world of the Cantas, it's another pair of sleeves.

The apple is a good example of a macroscopic object of everyday life.

And now let's imagine that we zoom inside this apple, which is made up of a multitude of tiny particles. Each of these particles, whether they are atoms, electrons or other quantum objects, behave in a surprising way. So if this apple was an atom, if it was a quantum object, I wouldn't see it very clearly here. It would occupy a kind of cloud of probability. Imagine it floating a little everywhere in space.

Okay.

It's a bit like an apple being on a branch and on the ground. Or a whale evolving at the same time in the sea and in the Grand Canyon. In quantum physics, we talk about superposition. Welcome to the microcosm of quantum particles. A world that puts our common sense to the test. Let's try to understand.

These microscopic objects have a particularity. Their properties, for example their position, are indeterminate. These objects are in a superposition. They have several positions and speeds simultaneously. As long as we don't observe them, they exist in all these states at the same time. It's only when we observe them that they are in a given state. This theory is specific to quantum physics.

In other words, before we can observe it, a quantum particle is like a blank page, without a specific property. And it's only by looking at this blank page that it becomes written. Words appear as we look at it. It's the same thing in the microscopic world. Positions and colors are only determined when we look at them.

What is the point of putting an object in two places at once? First, intellectual pleasure. To discover that nature behaves like that on a small scale, it makes me happy. In addition, it has applications. For example, it is used to make the best sensors in the world to measure things very precisely. Or we try to use it to make the most powerful computers in the world.

The only problem is that quantum particles are not easily approachable. The peculiar properties of quantum particles that we want to exploit are very unstable.

Everything that interacts with the particle can make the superposition disappear. In quantum physics, we call these interactions with the environment noise. That is to say that there are too many particles nearby. Between my hands, for example, there are 10 to the power of 20, 10 to the power of 30 particles, or 1 followed by 30 zeros, which makes millions and millions of particles.

A macroscopic object placed in the hollow of my hands would interact with all these particles and we would no longer see the slightest superposition. So quantum particles are extremely sensitive. Niels Bohr, Nobel Prize winner and one of the pioneers of quantum physics, knew it well. The quantum particle feels observed, he would have said. But what bothers a quantum particle so much in being measured and therefore observed? Because to get back to the apple,

If we make it fall, it will fall, whether we observe it or not. We understood that what is played in the measurement is the fact that I bring a measuring tool which itself is composed of billions and billions of atoms. And so when my atom sees all these atoms coming in front of it, it feels forced to choose. It tries to speak to all the atoms at the same time and by interacting like that, hop, it forces it to choose one of these states. Is it possible to bypass this problem by making the particle not feel observed?

To make the particle invisible, it must be perfectly isolated. In our case, it works as follows: we place the particle in an absolutely empty space, then we control it with an optical tweezers that works with laser beams.

And the whole idea is to make sure that these laser rays, these little tweezers, interact as little as possible with the quantum particle. This is the main difficulty. Once this obstacle is overcome, quantum particles will open up many perspectives. For example, in the prevention of volcanic risks.

Researchers have developed a quantum sensor that they tested on the highest active volcano in Europe, Etna. But how can quantum particles predict eruptions? So you imagine that you have your column there, and at the top of the column, here, you drop atoms, you drop atoms. So there you see the atom falling because of gravity, that's all simple.

But during its fall, you will superimpose it in two states. So you throw a laser beam at it. There you see, it is doubling. And then there is one that falls a little faster than the other. There they are falling. Then you throw a laser again so that the one who was late catches the other. So it's a single atom, all that, which doubles. So there it is catching up, catching up and hop! Here it combines with itself.

the way it recombines will depend on the gravity it suffered during the fall. You will see that sometimes it changes a little bit this gravity just because under your feet there is magma that has moved for example. So just before a volcanic eruption, you will see gravity very slightly vary. Thanks to the hypersensitivity of quantum particles, it is therefore possible to detect an eruption in time. In the future, quantum gravimeters could serve to the permanent surveillance of volcanoes.

The quanta can therefore allow us to explore our world with unprecedented precision. The mystery of observation or measurement remains, which fixes the particle in a defined state. In quantum mechanics, the world seems to become real in its very essence only when we come into contact with it. So is it the act of observing that determines the shape that the world takes? Is it the observer? Highly philosophical questions.

What is important to realize is that we don't choose how objects are defined. This is a completely random process. By observing, we define their state, but we cannot say "oh, I want to pick this or that property as such". So we define the reality but we have no influence on this property that is appearing.

In summary, we do not decide what is happening, but we do determine reality. The founding fathers of quantum physics have looked at this enigma with divergent concepts. For Werner Heisenberg and Niels Bohr, it is impossible to say anything about reality outside of observation. Albert Einstein sees things differently.

For him, reality exists objectively, independently of our observations, and does not owe anything to chance. According to legend, he and Niels Bohr would have had the following exchange during a walk: "You are not going to tell us that the moon does not exist when no one looks at it? But finally, how could I know, Mr. Einstein?

In quantum physics, we start from the principle that even if we knew everything that can be known, it would be impossible to calculate or to know what will happen. And that's what bothered Einstein. The problem does not lie in our lack of knowledge, but in the fact that it is impossible, by essence, to predict a result. Superposition, inexplicable coincidence. Quantum physics puts our understanding of the behavior of matter to harsh tests.

It forces us to rethink our vision of the world, in particular our conception of the infinitely small.

This kind of schizophrenia completely facilitates the fact that the world around us, we know gravity, action, reaction. Well, at the same time, there are laws on a very small scale that have nothing to do with these laws and that defy our understanding. What we don't understand again is why nature is like that. But how it behaves, we understand it beautifully. By allowing the development of new technologies, quantum physics deeply influences our daily lives.

This is the case with laser, made up of particles with the same energy and propagating in the same direction. Another application: transistors. These electric current amplifiers are present in the micro-pulses of all our computers and smartphones. Without our knowledge of the quanta, the current world would be very different.

And in the future, it could be possible to realize many other wonders considered as impossible. Because there is another phenomenon that no one can really explain and which is even more surprising: quantum intricacy.

"Entanglement is a property of quantum physics when we have more than one particle in one position. What is remarkable about entanglement is that we observe one of the particles, then properties are defined in this particle but also in the other particle. And so this was something that was recognized by many scientists, but the one who realized the power of the particle was Einstein."

In 1935, Albert Einstein and two colleagues carry out the following thought experiment. Two electrons collide, then go in different directions. According to quantum physics laws, they should be bound by a kind of telepathic connection. Unthinkable for Einstein, who talks about a "ghostly action at a distance".

His colleague, Erwin Schrödinger, gives a name to the phenomenon: the intrigue. Only at the time, it cannot be proved. Does the intrigue really exist? Let's start by clarifying what we mean exactly by that.

I'm going to give you an indirect trick between Paris and Munich. Let's take a quantum object, for example a photon. I put it in two states at once. Look, it's both yellow and green. I'll do the same with a second light grain. I have two light grains in two states at once. I'm going to rub them together, I'm going to trick them. That means that now their destinies are shared.

So if I send this one thousands of miles away to Munich, look what will happen. I leave this one alone, I don't measure it, I send this one, and now I measure mine. I see it yellow, and at the same time in Munich, it sees it green. I can do that a billion times, every time, I will see anything here, but my colleague in Munich will see the opposite. This means that the state of intricate quantum particles is closely linked.

These two share a common destiny. So how to explain the inexplicable? Will the light grains exchange secret messages? Forget this hypothesis, because these photons are so far away from each other that it is impossible for a message to pass from one to the other. This experiment is done almost instantly, so that there is no possibility of communication of any kind whatsoever.

OK, let's drop this hypothesis. Quantum particles share common properties without exchanging information. What if the result was known in advance?

"After that, we thought that if it happened, they would share the information at the beginning, with a kind of hidden variable, a little clue, that they would keep in their pockets, in a way, and then after a thousand kilometers they would go out and say 'ah, well, I'm going to put it like that, you put it like that'. But in fact, there are very clear experiences that have shown that this was not the case either." "There is no explanation in this sense. We must accept what Einstein called 'the ghost action at a distance'. " What Albert Einstein had theoretically thought would be proven 50 years later by experiments.

Intrication is not a chimera. It remains to be seen whether the great scientist would have been delighted. Because the question remains: how do two intricate quantum particles manage to form only one system, without prior consultation and without exchange of messages? Listen to the coincidence:

Why does the quantum particle adopt precisely this or that state when we observe it? The particle itself probably has no answer. "God doesn't play dice anyway" Einstein would have said on a walk. "But finally, stop telling God what he has to do!" Bohr would have retorted. The riddle seems insoluble. "In order to find some interpretation, in order to find some interpretation,

you really have to postulate something even more extraordinary and surprising than quantum physics itself. We will have to assume that there exists a world of hidden variables, a world that we cannot detect, that we will never be able to see, where the laws of physics of our world don't apply. So let's go back to the laws of quantum physics that we understand rather well. They stipulate in particular that intricate particles form a unit.

And what's crazy is that this unity lasts. Even if the two particles are in different places, they will always be described as a unit. I can send a bullet to another country or give it to someone who takes the plane to go somewhere. As long as this unit exists, I know that if this one is yellow, the other is green and vice versa.

The whole point is to maintain this unity and to prevent an observation from taking place between the two particles. Because as soon as it happens, the intrusion unfortunately disappears. And how far can we maintain the intrusion?

The world record is held by Chinese who sent a satellite into space, a kind of small quantum laboratory, and this satellite sent intricate light grains to both sides of China. When they arrived at their respective terrestrial station, the two photons were measured. As a result, they were indeed always intricate, at nearly 1200 km away from each other.

The only downside is that the satellite had to send no less than a million pairs of photons until one of them arrived intact on Earth. All the others were lost on the way. Intrication over long distances is not yet at its peak. Its potential, however, is obvious. Because it goes without saying that scientists did not send these quantum particles into space for the simple pleasure of making them travel.

The fact that a particle disappears and reappears without going through anything opens the way for applications in the field of communication, particularly cryptography. And that's exactly what quantum teleportation does. Teleportation, do you remember? The Star Trek science fiction series anchored it in our imaginations. Since a teleportation platform, crew members could teleport in a blink of an eye to another planet. Teleportation, Scotty.

Were the creators of Star Trek visionaries? It's fascinating, it's dreamy. We imagine Star Trek where we disappear here and appear elsewhere. So in reality, the name is a bit misleading. We don't teleport matter when we do quantum teleportation because that's forbidden by the laws of physics. In fact, what we do is we do rather copy, you see, copy-paste, a bit like computer science where we will have a quantum object in a certain state, and we will be able to copy this state to paste it on another quantum object.

Teleporting people or objects will therefore remain science fiction. Too bad. But then, what can this invention be used for? It can, for example, allow to make an Internet that would be encrypted, that would be encrypted, thanks to quantum physics. So the Internet itself would be completely normal, we would transmit information, but we can encrypt this information thanks to quantum physics.

In other words, information is transmitted from one quantum particle to another without a hacker being able to intercept it on the go. That's for theory, because the quantum internet is only a matter of swallowing. It will still take some time to see it work. But one thing is for sure: quantum particles and their strange behavior will radically transform our world.

For a few years, we've been experiencing a new quantum revolution. We are able to manipulate atoms one by one, to manipulate electrons and make them do what we want. The same with light bulbs. And that will allow us to play with effects we didn't play with before. We will be able to play with intricacy, with state superposition, and to do it on a measure, in a way. A particular application could revolutionize our lives. The physicist at the origin of this idea was called Richard Feynman.

he threw away the basics of the quantum computer. It is very difficult to solve certain problems with classical computers. According to Feynman, computers based on quantum physics, which he had named quantum computers, should be able to solve these problems more quickly, or even solve them in a short time. But how exactly does a computer made up of quantum particles work?

How does a quantum computer work? The basic principle is to use a quantum object, for example an atom, which is in two states at the same time. For example here, not excited and excited at the same time. And we will imagine that when it is not excited it is a zero, when it is excited it is a one. So it gives this. You have an object that can be both zero and one. And that as such has no interest, it's just nonsense. It starts to get interesting if you put several of them. Let's put three of them for example.

So now I have three intricate atoms, each in two states at the same time. That means I have a lot of possible combinations. But my quantum computer doesn't choose. It has them all at the same time. Thanks to superposition, a quantum computer can explore several calculations at the same time.

A quantum computer is therefore as efficient as a squad of classic computers working simultaneously. This superposition shows you that I can manipulate a lot of information at once. And here I only have three, it's not incredible, it gives me eight possible combinations. But if I add a fourth, it gives me 16 possible combinations. If I add a fifth, it gives me 32. If I add 70, I have all the information of humanity today. The addition of a single particle doubles the capacity to calculate.

To be honest, we don't know. We don't know exactly what the quantum computer will look like.

Research is underway to select the most appropriate particles to build such a computer. But anyway, the objective is not to produce a larger public computer more powerful than current computers thanks to a quantum processor. Sorry to disappoint you. The quantum computer will be used for specific applications.

to solve complex problems more efficiently. We still have to find out what problems we are talking about. Arte's YouTube community has listed the major existential questions to be solved in priority by the quantum computer: the origin of life and the universe and the mystery of the disappearance of the second sock in the washing machine.

The answer to the ultimate question of life, the universe and everything is 42. Would a quantum computer really give this kind of answer?

"Quantum computers are extremely efficient in solving the problems of materials science and chemistry. Thanks to their power of calculation superior to that of classical computers, they could help us to design materials, but also drugs and chemical compounds more efficient." Currently, it takes months, even years, before the development of a drug.

the quantum computer could accelerate considerably the process of simulation of the candidate molecules. Quantum computing can also find applications in the treatment of huge amounts of data and in the optimization of logistics systems. For some, we are at the dawn of a second quantum revolution. But if the quantum computer is as powerful as that, why don't we exceed the level of the simple prototype?

The problem is that it doesn't work, or rather it works very, very badly. You have to realize that the experiment is very delicate to do. Our work consists of manipulating individual particles, for example atoms or ions, really tiny particles. And all the difficulty lies in observation and measurement. Indeed, the slightest interaction, the slightest external influence, makes the fundamental properties of quantum mechanics disappear. It destroys them.

It is extremely difficult to control a large number of these particles simultaneously. This is the puzzle of quantum phenomena. The development of a quantum computer requires to control a large number of particles. But for this to keep their superposition state, whose computer draws its unparalleled computing power, we must not disturb them. The best current computers make mistakes roughly every thousand operations.

And the drama is that we can't debug, we can't look for where the errors are, because if you try to stop in the middle of the road to see where the error is, you can't start over. Did we rejoice too soon? Is the quantum computer a mirage? We know how to correct the errors of quantum computers as long as they are not too important. We are currently working on it. The development of the quantum computer is in progress.

Some researchers are optimistic, others are more cautious. Still, it is still too early to say that quantum computers will actually revolutionize our lives. But maybe this is not the most important thing.

I feel that what counts is to try to push back the boundaries of our knowledge. To understand what intricacy is, what state superposition is, where they come from, how to calculate them, how to predict them. And that's what makes quantum physics interesting for me. It's really one of the last contemporary fields of exploration in physics.

I think that the generation, like the previous ones, still have difficulty accepting quantum physics and its exotic properties. But this will not be the case for future generations, who will be born with quantum technology. It's like today with mobile phones. We have completely integrated their communication mode, completely dematerialized.

By raising the veil on the behavior of the smallest particles and their funny properties, quantum physics is changing our view of the world and gives us the chance to see the real as never before. A dive into the heart of matter as exciting as dizzying.