We know that in physics the laws of physics take different forms when we change the scale, from the infinitely small to the infinitely large, or when we move very quickly. Quantum entanglement is well observed at ordinary energies with photons, but does it also occur at energies that only the LHC, the Large Hadron Collider, on Earth can study, and at exotic particles like quarks? Researchers have just answered this question by using the giant Atlas detector to study the fundamentals of quantum mechanics.

By the end of the 19th century, no one doubted that with enough energy we could make a material body exceed the speed of light and reach any speed. Newton’s laws of gravity seemed so well confirmed by their predictions about the motions of the planets that there was no real reason to doubt them.

However, we know that relativity theory would soon contradict all of these beliefs and nonlinear equations corresponding to general relativity would replace the linear gravitational equations of Laplace and Poisson.

Today it seems that the equations of quantum mechanics are equally reliable, but this is not the case. There are attempts to find alternatives to these equations, which in turn involve nonlinear equations, in this case a nonlinear version of the famous Schrödinger equation (be careful, under this name we combine two different equations, one of which is not one). Modification of the laws of quantum mechanics).

Just as simple wavelets on the surface of water are described by a linear wave equation that is replaced by a nonlinear equation to describe violent collisions between unwanted waves, some wonder whether this is the case at very high energies, such as those seen in proton collisions LHC (Large Hadron Collider), deviations from the predictions of quantum mechanics could not occur, signs of a theory that goes beyond the theory discovered by Heisenberg, Born and Schrödinger.

Quantum entanglement is a phenomenon that closely links the properties of two particles, regardless of the distance between them. This leads to such strange effects that Albert Einstein himself doubted it! The debate was settled in 1982 when Alain Aspect at the Institute of Optics conducted an experiment that demonstrated the physical reality of quantum entanglement on light particles – photons. This experience earned him the Nobel Prize in Physics in 2022. Entanglement has now become an indispensable tool for the development of ultra-high-performance cryptography devices and the design of quantum computers. Produced to mark the 80th anniversary of the CNRS, this video traces the history of this strange phenomenon, from the conceptual debate of the 1930s to contemporary laboratory experiments. © Institute of Optics

In recent years there has been a lot of talk about the phenomenon of quantum entanglement and, more generally, about the new field of physics, quantum information. We believe we can use it to make efficient quantum computers. However, we know that quantum mechanics, and in particular this phenomenon of quantum entanglement, poses serious conceptual problems, especially when we try to apply it to black holes and quantum cosmology.

What if experiments in particle physics at very high energies gave us the key to these puzzles by showing that we actually need to modify the equations of quantum mechanics? Most quantum entanglement experiments have been performed at very low energies using photons and electrons. What would they look like with hadrons, particles made of quarks?

We have actually carried out such experiments with particle accelerators and neutral K mesons from quark-antiquark pairs, but at energies much lower than the 13 TeV that we can achieve by collisions of protons from three quarks and antiquarks.

However, now we learn that particle physicists were able to verify the phenomenon of quantum entanglement at these energies by studying the products of collisions of protons containing pairs of top quarks and antiquarks. These quarks are particularly heavy and also very unstable.

## Decay products of a pair of top quark and antiquark

This did not prevent the researchers from highlighting the phenomenon of entanglement with top quarks and antiquarks when analyzing the secondary products of top pair decay using the LHC’s giant Atlas detector. The secondary products decay in certain spatial directions and by measuring the particle flows in these directions we could return to the original quantum state of the quark pairs. The data used in the new atlas measurements comes from 13 TeV collisions collected between 2015 and 2018. This means researchers could explore areas with energy scales of more than 12 orders of magnitude, a trillion times better than classic laboratory experiments such as those developed by Nobel Prize winner Alain Aspect and his colleagues in the early 1980s.

The measured entanglement signal exceeded 5 sigma, which is another way of saying that the probability of it being a fictitious signal created by statistical fluctuations in the giant detector is almost one in a million.

The question of whether particle physics can study subtle quantum mechanical effects is relatively old, and researchers such as John Ellis (in numerous papers) have been interested in possible quantum decoherence effects, such as those that enable solving Schrödinger’s riddle caused by the foam of the cat Spacetime.

Did you know ?

In quantum mechanics, the Einstein-Podolski-Rosen paradox or EPR paradox is famous. It is so spectacular that it is now known to the general public and its various avatars can often be found in the media. It all started in 1935, when Albert Einstein and his two young colleagues published a paper in which they tried to prove that quantum mechanics cannot be the ultimate description of the quanta of light or matter. If this were the case, in their opinion it would lead to phenomena that would contradict the spirit of special relativity.

In its modern form, the paradox is often studied and represented using pairs of photons produced by the decay of another particle, such as a pi meson, or using a nonlinear optical device. We can also use electrons and nuclei. To describe the respective state of these pairs of particles in quantum mechanics, one speaks of entangled pairs of particles. A mathematical theory makes it possible to define what kind of physical systems are meant by “entanglement” and what the degree of entanglement is.

After quantum entanglement, two photons then appear as an inseparable whole. Therefore, any measurement of certain properties of one of these particles (resulting in a change in its state) immediately produces results (according to known equations, but all we really know is that when a signal is emitted between the entangled particles, this at least must be the case). much faster than light) a change in the state of the other particle, even if they are several million light years apart. We understand that this conclusion seemed completely incompatible with Einstein’s theory of relativity, which states that no signal can travel faster than light in the universe.

A careful analysis of the phenomenon shows, as physicist Niels Bohr did, that it is still possible to preserve both Einstein’s theory and the laws of quantum mechanics if we admit that there is a kind of “non-locality”. Objects in the universe would fundamentally not be in space and time. Through some kind of perspective effect, we would divide a reality consisting of a single block into a series of particles or waves in a spacetime that we can understand. In fact, this reality would fundamentally lie outside this spatiotemporal framework.