2023-10-03 19:32:56
We know that in physics, when we change scale, from the infinitely small to the infinitely large, or when we go very quickly, the physical laws take different forms. 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, can probe on Earth and with exotic particles like quarks? Researchers have just answered this question by probing the foundations of quantum mechanics using the giant Atlas detector.
Towards the end of the 19th century, no one doubted that if we had enough energy, we could make a material body exceed the speed of light to reach an arbitrarily high speed. Newton’s laws of gravitation seemed so well verified by the predictions they made regarding the movements of the planets that there was no real reason to doubt them.
However, we know that the theory of relativity would soon contradict all these beliefs and non-linear equations, those of general relativity, would replace the linear gravitation equations of Laplace and Poisson.
Today, it would appear that equal reliance is placed on the equations of quantum mechanics but it’s nothing. There are attempts concerning alternatives to these equations again involving non-linear equations, in this case a non-linear version of the famous Schrödinger equation (be careful, under this name we group together two different equations, one of which does not is not a modification of the laws of quantum mechanics).
Just as simple wavelets on the water surface are described by a linear wave equation which is replaced by a nonlinear equation to describe violent collisions between rogue waves, some wonder whether at very high energies, like those that can be achieved in proton collisions at the LHC (Large Hadron Colliderin English), deviations from the predictions of quantum mechanics could not occur, signs of a theory beyond that discovered by Heisenberg, Born and Schrödinger.
Quantum entanglement is a phenomenon that intimately links the properties of two particles, regardless of the distance separating them. This leads to such strange effects that Albert Einstein himself doubted it! The debate was settled in 1982, when Alain Aspect carried out an experiment at the Institute of Optics demonstrating the physical reality of quantum entanglement on particles of light – photons. This experience earned him the Nobel Prize in Physics in 2022. Entanglement has today become an essential tool for developing ultra-high-performance cryptography devices and designing quantum computers. This video, produced to mark the 80th anniversary of the CNRS, traces the history of this strange phenomenon, from the conceptual debate of the 1930s to contemporary experiments carried out in laboratories. © 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 that is quantum information. We think we can exploit it to make efficient quantum computers. But 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 enigmas by showing that we must indeed modify the equations of quantum mechanics? Most experiments on quantum entanglement have been done at very low energies with photons and electrons. What would they look like with hadrons, particles made of quarks?
We have actually carried out experiments of this kind with particle accelerators and neutral K mesons made of quark-antiquark pairs but at energies much lower than the 13 TeV that we can reach with collisions of protons made of three quarks and antiquarks.
However, we now learn that particle physicists have been 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 they are also very unstable.
Decay products of a pair of top quark and antiquark
This did not prevent the researchers, analyzing with the LHC’s giant Atlas detector the secondary products of the decay of the top pairs, to highlight the phenomenon of entanglement with top quarks and antiquarks. The secondary products decay in particular directions of space and by measuring the particle flows in these directions, we could go back to the initial quantum state of the quark pairs. The data used in the new Atlas measurements were obtained from 13 TeV collisions collected between 2015 and 2018. This means that researchers could explore territory with energy scales of more than 12 orders of magnitude, so a trillion times superior to classic laboratory experiments like those carried out by Nobel Prize winner Alain Aspect and his colleagues in the early 1980s.
The measured entanglement signal exceeded 5 sigmas, which is another way of saying that there is almost a one in a million chance that it is a fictitious signal produced by statistical fluctuations in the giant detector.
The question of whether particle physics can probe subtle quantum mechanical effects is a relatively old one and researchers, such as John Ellis (in many articles), were interested in possible quantum decoherence effects, such as those making it possible to solve the enigma of Schrödinger’s cat, caused by the foam of space-time.
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 are often found in the media. It all began in 1935, when Albert Einstein and his two young colleagues published a paper attempting to prove that quantum mechanics could not be the ultimate description of the quanta of light or matter. If this were the case, according to them, it would lead to phenomena in contradiction with the spirit of special relativity.
In its modern form, the paradox is often studied and presented 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 particular state of these particle pairs in quantum mechanics, we speak of entangled particle pairs. A mathematical theory makes it possible to define what is meant by “entanglement” for physical systems and the degree of entanglement.
According to quantum entanglement, two photons then appear as an inseparable whole. Thus, any measurement of certain characteristics of one of these particles (producing a modification of its state), results instantly (according to known equations but all that we really know is that if a signal is emitted between the entangled particles it must at least be much faster than light) a modification of the state of the other particle, even if they are separated by a distance of several million light years. We understand that this conclusion seemed to Einstein to be very incompatible with his theory of relativity, which implies that no signal can move faster than light in the Universe.
A careful analysis of the phenomenon shows, as did the physicist Niels Bohr, that it is nevertheless possible to preserve both Einstein’s theory and the laws of quantum mechanics, if we admit that there exists a sort of “non-locality”. Objects in the Universe would not be fundamentally in space and time. By a sort of perspective effect, we would split a reality made up of a single block into a series of particles or waves in a space-time that we can apprehend. However, this reality would in fact be fundamentally beyond this spatiotemporal framework.
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