Scientists have managed to push back the time of a split second

In The Curious Case of Benjamin Button, a story of F. Scott Fitzgerald that will later become a film starring Brad Pitt, a man is developing backwards: born as an old man and getting younger over the years He dies like a baby with arms.

This is not something we see in real life. The question is why not.

In what amounts to a technological triumph to become the Benjamin Button of the virtual world, a team of quantum physicists announced earlier this year that they have managed to create a computer algorithm that serves as a "fountain of youth". .

With the help of an IBM quantum computer, they were able to reverse the aging of a millionth of a second from a single simulated elementary particle. However, it was at best a Pyrrhic victory, because it required manipulations so unlikely that it occurred naturally, which only reinforced the idea that we were desperately stuck in time.

Most of us believe that the atoms of a scrambled egg can not be restored to the inside of a blank shell. It seems that, under general conditions, even a single particle can probably not go up without help or meticulous modification.

"We show that returning a single quantum particle in time is an insurmountable task for nature itself," said Valerii M. Vinokur, of the Argonne National Laboratory, one of five candidates for the application of "gentlemen", in an email. of Time ", led by Gordey B. Lesovik of the Moscow Institute of Physics and Technology.

"A system that includes two particles is even more irreversible, not to mention the eggs (made up of billions of particles) that we break to make an omelette."

On paper, the fundamental laws of physics are reversible; they work mathematically, as time advances or decreases, but, if time is only another dimension of space-time, as Einstein said, it's about time. 39, a strange dimension that only goes in one direction. In the real world, we can get out of the metro and turn right or left, but we do not have the opportunity to move forward or backward in time. We are always moving towards the future.

The principle of uncertainty, which is at the center of quantum mechanics, indicates that at one point you can specify the location or velocity of a subatomic particle, but not both. Therefore, a particle as an electron, or a particle system, is represented by a mathematical entity called a wave function, whose amplitude is a measure of the probability of finding a particle on a site or in a specific condition.

The wave function extends through space and time. The law that describes its evolution, known as the Schrödinger equation, in the honor of the Austrian physicist Erwin Schrödinger, is equally valid, whether it is to advance or retreat, but the to reverse a wave function in return is not a trivial matter.

Here, the quantum computer appears.

Unlike ordinary computers, which process a series of zeros and ones, or bits, quantum computers are made up of what are called cubits or quantum bits, each of which can be zero and one at a time. A quantum computer can perform thousands, even millions of calculations simultaneously, provided that no one sees the answer until the end.

Lesovik and his colleagues are trying to reduce the wave function with the help of an IBM quantum computer, accessible online to the public.

"It remains to be seen," writes the team in an article published online in February, "if the irreversibility of time is a fundamental law of nature or if, on the contrary, it can be avoided".

The IBM computer they used represents a tiny step in the direction of what theorists call "quantum supremacy". It had only 5 cubits (there are also IBM devices at 16 and 20 cubits), compared to the computer "Bristlecone" of 72 cubic inches of Google, which is much more advanced. To simplify things, the group used only one or sometimes three cubits.

The experiment to reverse time was a four-step process. First, the cubits were prepared in a simple, initial state that imitated an "artificial atom," Vinokur explained. In addition, the cubits were closely related to what Einstein called "a ghostly action at a distance": everything that happened to a cube affected the measures of the other (or the other two, depending on the number of cubits deployed) .

Then, the team played the cubits with a series of microwave radio pulses, which allowed him to move from a simple state to a more complex state. After a millionth of a second, scientists stopped this phase – "the evolution program" – and submitted the cubes to another microwave pulse, to reverse their phase and prepare them to bring them back to their youth .

"In graphic language, we are turning rings from the expansion of a pond into rings ready to return to their source," Vinokur explained. It took another millionth of a second.

Finally, the team relaunched the "evolution" program. And the cubits have returned to their original alignment, to their own past. In fact, they have become a millionth of a second younger.

The algorithm has almost always worked. He managed to bring the cubits back to their juvenile state 85% of the time when the calculation included 2 cubits, but only half the time when 3 cubits were used. The authors attributed the reduced reliability to the imperfections of the quantum computer and the tendency of the cubits to desynchronize as their number increases.

Eventually, machines of several hundred cubits will be needed to realize the ambitions of quantum mathematicians. When such computers are available, the team's time inversion algorithm could be used to test them, said Andrey V. Lebedev, a physicist at ETH Zurich in Switzerland and author of the article, in a press release from the Institute of Physics and Technology. of Moscow.

* Copyright: 2019 The New York Times Press Office