Physicists are using the LHC in the world’s largest quantum experiment

Physicists have used the Large Hadron Collider, CERN’s famous particle accelerator, to investigate the strange phenomenon of quantum entanglement at much higher energies than ever before.

The Large Hadron Collider (LHC) is known for its high-speed particle collisions. But now physicists have used this massive machine in a more precise experiment: studying quantum entanglement.

the First results He demonstrated that entanglement occurs between pairs of quarks, a type of subatomic particle. These first measurements launch new attempts to test whether exotic quantum phenomena also occur at extremely high energy levels. Research can lead to new insights into the fundamental nature of reality.

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They are inseparable

Entanglement is a quantum effect in which particles are closely linked. As a result, measuring a certain property of one particle immediately reveals the same property of the other particle.

Researchers have extensively studied entanglement in some elementary particles, including electrons and photons. However, measuring some elementary particles is much more difficult. Quarks in particular, the indivisible building blocks of protons and neutrons, are almost always bound together and therefore difficult to study on their own.

Baby particles

Now we have physicists Jay Howarth And his colleagues at CERN, the particle laboratory in Switzerland where the LHC is located, measured the entanglement between pairs of quarks. “This directly tests whether quantum mechanics still behaves the same way when you have an energy 10 trillion times higher than any other experiment,” Howarth says. “That seems to be the case overall.”

The team studied top quarks, a type of quark that is about 175 times the mass of a proton. The researchers produced these top quarks by accelerating two beams of protons to high energies and directing them so that they come very close to each other without actually colliding. When the beams collide with each other, pairs of top quarks form from the excess energy. It then quickly decomposes into a soup of other molecules that contain a lot of energy.

Whether or not the top quarks are entangled affects where their “subparticles” go. The researchers were thus able to reconstruct the quantum nature of top quarks by observing the directions of the particle fragments. This confirms that there is an overlap. “The top quarks are entangled, and then we measure the decay particles that bear the signature of that entanglement,” Howarth says.


Never before has quantum entanglement been measured at such high energies. The team also discovered something unexpected: The results were just below the level of entanglement predicted by the Standard Model of particle physics, our best theory of how particles interact. This perhaps highlights the fact that when we get to these very extreme areas, the theory lacks a little precision. “Maybe this isn’t some kind of weird new physics,” Howarth says.

“This is a particularly interesting new measurement, which provides the LHC with the opportunity to perform a type of physics with which accelerators are not traditionally associated,” says the physicist. Alan Barr From Oxford University. He did not participate in the work.

Future experiments measuring entanglement between other fundamental heavy particles, such as the Higgs particle and W and Z bosons, could help determine where the Standard Model fails. We would be surprised if quantum theory did not succeed in this area. “But nature has surprised us before in the past,” Barr says. “Until you actually take the measurements, you don’t know what the results will be.”

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Winton Frazier

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