News

Employees of the Department of Subnuclear Physics of the Institute of Experimental Physics SAS (ÚEF SAV) in Košice participated as members of the team that first measured the so-called quantum entanglement at high energies. We´ve already informed about the unique research of ÚEF SAV scientists HERE. These results open new insights into the complex world of quantum physics and were published in Nature on September 18, 2024. CERN issued a press release on the occasion of the publication of this article.

We´ve already informed about the unique research of ÚEF SAV scientists HERE. Quantum entanglement is a fascinating feature of quantum physics – the theory of very small things. If two particles are quantum entangled, the state of the first particle is bound to the state of the second, regardless of how far apart the particles are. This mind-blowing fact, which has no analogue in classical physics, has been observed in a wide variety of systems and has found several important applications, such as quantum cryptography and quantum computers. In 2022, Alain Aspect, John F. Clauser and Anton Zeilinger were awarded the Nobel Prize in Physics for their groundbreaking experiments with entangled photons. These experiments confirmed the manifestations of quantum entanglement predicted by the late CERN theorist John Bell and contributed to the beginnings of quantum computing.

Quantum entanglement has remained more or less unexplored at the high energies available at particle accelerators such as the Large Hadron Collider (LHC). In a paper published in Nature on Thursday, September 19, 2024, the ATLAS collaboration described how it successfully observed quantum entanglement at the LHC for the first time and at the highest energies to date, between fundamental particles called top quarks. This result, first announced by ATLAS in September 2023, and subsequently confirmed by two observations made by the CMS collaboration, opened a new perspective on the complex world of quantum physics.

"Even though particle physics has deep roots in quantum mechanics, the observation of quantum entanglement in a new particle system and at much higher energy than previously possible is remarkable," said ATLAS spokesman Andreas Hoecker. “The continued accretion of data paves the way for new investigations of this fascinating phenomenon and opens up a rich offer for further research."

The ATLAS and CMS collaborations observed the quantum entanglement between the top quark and its antiparticle. These observations are based on a recently proposed method that uses top quarks produced at the LHC as a new system for studying quantum entanglement.

The top quark is the heaviest known fundamental particle. Under normal circumstances, it decays into other particles before it has time to combine with other quarks. Therefore, its spin and other quantum traits are transferred to its decay products. Physicists observe the properties of the decay products to infer the spin orientation of the original top quark.

To observe entanglement between top quarks, the ATLAS and CMS collaborations selected pairs of top quarks from data from proton–proton collisions that took place at an energy of 13 teraelectronvolts during the second run of the LHC, between 2015 and 2018 They focused on pairs in which the top quarks have a small mutual momentum. This is where the spins of the two quarks are expected to be strongly entangled.

The existence and degree of spin entanglement can be inferred from the angle between the directions in which the electrically charged decay products of the two quarks are emitted.  By measuring these angular separations and correcting for experimental effects that could alter the measured values, the ATLAS and CMS teams each observed spin entanglement between top quarks with a statistical significance larger than five standard deviations.

In its second study, the CMS collaboration also looked for pairs of top quarks in which the two quarks are simultaneously produced with high momentum relative to each other. In this domain, for a large fraction of top quark pairs, the relative positions and times of the two top quark decays are predicted to be such that classical exchange of information by particles traveling at no more than the speed of light is excluded, CMS experiment observed spin entanglement between top quarks also in this case as well.

Measurements of quantum entanglement and other quantum concepts in a new particle system and at an energy range beyond what was previously accessible offer new possibilities for testing the Standard Model of particle physics, as well as new ways of looking for signs of new physics that may lie beyond it.

Source: CERN Press Release

Edited by: Pavol Stríženec, ÚEF SAV, v. v. i.

Photo: ÚEF SAV, v. v. i.