Reviewed by Lexie CornerSep 19 2024
Quantum entanglement between a top quark and its antimatter counterpart was observed by the ATLAS and CMS teams at CERN. The observations utilize a recently proposed method for studying entanglement using pairs of top quarks produced at the Large Hadron Collider (LHC) as a novel system. The study was published in Nature.
Artist’s impression of a quantum-entangled pair of top quarks. Image Credit: CERN
One fascinating aspect of quantum physics is quantum entanglement, where two particles remain interconnected regardless of the distance between them. This phenomenon, which has no parallel in classical physics, has been observed in various systems and has significant applications in fields such as quantum computing and cryptography.
Alain Aspect, John F. Clauser, and Anton Zeilinger were awarded the 2022 Nobel Prize in Physics for their pioneering work with entangled photons. Their experiments confirmed the predictions of the late CERN theorist John Bell, solidifying the foundations of quantum information science.
Until recently, entanglement at the high energies found in particle colliders like the Large Hadron Collider (LHC) remained largely unexplored. The ATLAS collaboration has now reported the first observation of quantum entanglement at the LHC, specifically between top quarks—fundamental particles—at the highest energies. This groundbreaking result, initially announced by ATLAS in September 2023 and later confirmed by the CMS collaboration through a first and second observation, offers new insights into the complex world of quantum physics.
Quantum entanglement between a top quark and its antimatter counterpart has been observed by the ATLAS and Compact Muon Solenoid (CMS) teams. The observations are based on a newly proposed approach to studying entanglement using pairs of top quarks produced at the LHC as a novel system.
The top quark is the heaviest known fundamental particle. It decays into other particles before it can combine with other quarks, transferring its spin and other quantum properties to these decay products. Physicists then observe these decay products to deduce the spin orientation of the top quark.
To observe entanglement between top quarks, the ATLAS and CMS collaborations analyzed data from proton-proton collisions collected at an energy of 13 teraelectron volts during the second run of the LHC, which took place between 2015 and 2018. They focused on pairs of top quarks produced with low relative momentum, where the spins of the two quarks are expected to be highly entangled.
The angle between the directions of the electrically charged decay products of the two quarks can be used to infer the presence and extent of spin entanglement. The ATLAS and CMS teams measured these angular separations and found evidence of spin entanglement between top quarks with a statistical significance exceeding five standard deviations, after accounting for experimental effects that could influence the measurements.
In a separate study, the CMS collaboration also investigated top quark pairs produced with high momentum relative to each other. In this regime, for a significant fraction of top quark pairs, the relative positions and decay times are such that the classical exchange of information through particles traveling at the speed of light is ruled out. Even under these conditions, CMS observed spin entanglement between top quarks.
With measurements of entanglement and other quantum concepts in a new particle system and at an energy range beyond what was previously accessible, we can test the Standard Model of particle physics in new ways and look for signs of new physics that may lie beyond it.
Patricia McBride, Spokesperson, Compact Muon Solenoid Team, CERN
Journal Reference:
The ATLAS Collaboration (2024). Observation of quantum entanglement with top quarks at the ATLAS detector. Nature. doi.org/10.1038/s41586-024-07824-z