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Quantum entanglement is a fascinating feature of quantum physics—the theory of the infinitely small. When two particles are in a state of quantum entanglement, the state of one determines the state of the other, no matter how far apart they are. This puzzling phenomenon, which has no counterpart in classical physics, has been observed in a wide variety of systems and has led to several important applications, such as quantum cryptography and quantum computing.
Artist's impression of a pair of entangled top quarks.
Image: CERN
In 2022, the Nobel Prize in Physics was awarded to Alain Aspect, John F. Clauser, and Anton Zeilinger for their pioneering experiments on entangled photons. These experiments confirmed the predictions about the manifestation of entanglement made by John Bell, a late theorist from CERN, and paved the way for the science of quantum information.
Entanglement had remained little studied at the high energies reached in particle colliders like the Large Hadron Collider (LHC). In an article published in Nature, the ATLAS collaboration describes how it managed to observe quantum entanglement for the first time between fundamental particles called top quarks and at unprecedented energies.
First reported by ATLAS in September 2023 and subsequently confirmed by a first and a second observation from the CMS collaboration, this result opened a new perspective on the complex world of quantum physics.
"While particle physics is deeply rooted in quantum physics, observing quantum entanglement in a new particle system and at a much higher energy than was previously possible is remarkable," explains Andreas Hoecker, ATLAS spokesperson. "This paves the way for further research into this fascinating phenomenon, and a multitude of studies as our data samples continue to grow."
The ATLAS and CMS teams observed quantum entanglement between a top quark and its antiparticle. These observations are based on a recently proposed method of using top quark pairs produced at the LHC as a new system to study quantum entanglement.
The top quark is the heaviest known fundamental particle. It normally decays into other particles before it has time to combine with other quarks, transferring its spin and other quantum characteristics to the particles resulting from its decay. By observing the products of these decays, physicists can infer the top quark's spin orientation.
To observe the entanglement between top quarks, the ATLAS and CMS collaborations selected top quark pairs from data generated in proton-proton collisions at an energy of 13 teraelectronvolts during the LHC's second operational period, from 2015 to 2018. They specifically looked for pairs in which both quarks are produced simultaneously with low momentum relative to each other. In this case, the spins of both quarks should be strongly entangled.
The existence of the entanglement phenomenon and the degree of entanglement of the spins can be inferred from the angle between the directions in which the electrically charged products from the two quarks' decays are emitted. By measuring these angular separations and correcting for experimental effects that might distort the measured values, the ATLAS and CMS teams each observed spin entanglement between top quarks, with a statistical significance greater than five standard deviations.
In its second study, the CMS collaboration also looked for top quark pairs produced simultaneously with high momentum relative to each other. In this configuration, for a large fraction of the top quark pairs, the relative positions and times of the two top quark decays should, according to theory, rule out the classical exchange of information by particles traveling at most at the speed of light; CMS also observed spin entanglement between top quarks in this case.
"With measurements of entanglement and other quantum concepts in a new particle system and within an energy range exceeding what was previously accessible, we can test the Standard Model of particle physics in new ways and search for signs of potential new physics beyond this model," explains Patricia McBride, CMS spokesperson.
To learn more:
- ATLAS Nature article
- First CMS study
- Second CMS study