Albert Einstein in 1952
SCIENCE – At the beginning of the 20th century, quantum physics revolutionized the way we saw the world and at times pitted thinkers in the discipline against each other. Alain Aspect’s work on entanglement and the connections between particles in the early 1980s earned him the 2022 Nobel Prize in Physics today. They are also the decisive blow to a theory of Albert Einstein, whose work continues to inspire researchers to make discoveries that could change everything in our daily lives.
To understand the magnitude of what the French researcher has demonstrated, we must first return to quantum physics itself and its field of action. : “These are the laws of the world on a very small scale”, summarizes Julien Bobroff, physicist and professor at the University of Paris-Saclay and author of Welcome to the new quantum revolution (Expenditure Flammarion).
Shaft and body at the same time
Because these laws, which determine the behavior of atoms, electrons or photons, have nothing in common with our macroscopic universe. When we think of an atom, we think of a kind of tiny marble: at this scale, the particles don’t really behave like marbles, but more like waves.
In other words, atoms passing through two separate holes should, in theory, hit two different points on a wall with precision, like marbles. However, this is not the case: if not measured, light behaves like a wave and propagates along a spectrum as shown in the figure below.
Illustration of the “Young’s Slit” experiment conducted in 1801
This discovery of the nature of the infinitesimal, which we owe to Max Planck and Albert Einstein in 1910-1920, was the beginning of quantum physics and the basis of Alain Aspect’s work on entanglement. . A term that in turn has something to change the entire understanding of the world around us.
Atoms united for life…
“Imagine mixing two grains of light”enthuses Julien Bobroff. “They therefore ensure that they share the same condition, a real common destiny”. And this shared destiny has no distance limit, unbelievable as it may seem. They do not merge, but share an inseparable bond.
A photon, i.e. a grain of light, has two states of polarization, like a sphere, which would have two colors depending on whether it rotates to the left or to the right. But as long as it has not been measured, it can be in these two polarizations at the same time, as if the sphere had two colors at the same time! This is the principle of quantum superposition, well known to fans of the famous “Schrödinger cat” paradox, found below.
Schrödinger’s cat experiment
From the moment we measure the polarization of a photon, its nature changes, going from a small wave to a particle, as we have seen. At the same time, it chooses its polarization: either it rotates to the left or to the right, as if the sphere had suddenly chosen its color… But when it shares the fate of another photon, a strange phenomenon occurs.
“Imagine I have two entangled photons and then pull them apart”explains the physicist. “I measure the first who chooses his condition completely at random”, i.e. right or left. Immediately, the other photon placed at a distance will systematically choose to turn the other way. “The incredible thing is that you can take as much from them as you want: they continue to share their destiny”explains Julien Broboff.
…but a long-distance relationship
This is exactly what Alain Aspect demonstrated. And vice versa, which Albert Einstein rejected. The legendary physicist of German origin believed that the two photons must inevitably exchange information in order for one to systematically go in the opposite direction to the other, or share a secret, a hidden variable in advance. So there must be a tiny, barely measurable moment between the moment the first photon was observed and decided on a direction and the moment the second did the same. But how to demonstrate this absolute simultaneity or its absence?
“Alain Aspect solved the problem”says Julien Broboff, “Be sure they don’t have time to communicate”. In 1980, Nobel 2022 set up a system that left no doubt, demonstrating that even at a distance of 12 meters, entangled photons can continue to share an instantaneous fate without the need to exchange information. Entanglement becomes a demonstrated and incurable reality. The great Albert Einstein was wrong, and the promise of this experiment is staggering.
Cryptography, imaging, but above all quantum computing: Entanglement is already enabling spectacular achievements today. The computer language is based on ones and zeros: by being able to mix superposition (the fact that a photon rotates both left and right) and entanglement, you get unparalleled computing power.
“Imagine, we manage to entangle not two, but seventy photons”, and whose condition we would stabilize, “We would store all information produced by mankind”. The challenges remain immense, but progress continues. China is not lagging behind on this issue: in 2017 it announced that it had tested the entanglement of two photons… at a distance of more than 1,000 kilometers. The legacy of Alain Aspect’s experience.
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