The ATLAS and CMS experiments at the Large Hadron Collider (LHC) at the European Laboratory for Particle Physics (CERN) in Geneva (Switzerland) have independently observed, for the first time, quantum entanglement between two top quarks, the heaviest known elementary particle. This phenomenon allows two particles to be bound together regardless of their distance. Moreover, this observation has been made at the highest energy achieved to date in an experiment, opening a new perspective in the complex world of quantum physics. The Institute of Corpuscular Physics (IFIC), located at the University of Valencia Science Park, is participating in the discovery.
Quantum entanglement is possibly the most genuine and fascinating feature of quantum physics, the theory of the very small. If two particles are quantum entangled, the state of one particle is linked to that of the other, no matter how far apart they are. This amazing phenomenon, which has no analog in classical physics, has been observed in a wide variety of systems and has found several important applications, such as quantum cryptography and quantum computing. In 2022, the Nobel Prize in Physics was awarded to Alain Aspect, John F. Clauser and Anton Zeilinger for their pioneering experiments with entangled photons. These experiments confirmed the predictions about the manifestation of entanglement made by theoretical physicist John Bell, and gave rise to quantum information science.
Entanglement has remained largely unexplored at high energies, accessible only at large particle colliders like the LHC. In a paper published in Nature, the ATLAS collaboration reports how it succeeded in observing quantum entanglement at the LHC for the first time between elementary particles called top quarks and at the highest energy yet, 13 teraelectronvolts (TeV). First observed by ATLAS in September 2023 and subsequently confirmed in independent observations by the CMS collaboration, this result has opened up a new perspective on the complex world of quantum physics.
“While particle physics is deeply rooted in quantum mechanics, the observation of quantum entanglement in a new particle system and at the highest energy ever achieved is a remarkable milestone,” says Andreas Hoecker, ATLAS spokesperson. “It opens the way for new research into this fascinating phenomenon, offering a rich menu of exploration as our data volume continues to grow.”
IFIC has been involved in the LHC ATLAS experiment since its inception, a cathedral-sized detector involving more than 3,000 physicists and engineers from all over the world. Specifically, two IFIC researchers, Carlos Escobar and Marcel Vos, have participated in the review process of all aspects of the analysis now published in Nature
The ATLAS and CMS teams observed quantum entanglement between a top quark and a top antiquark, its antiparticle. The observations are based on a recently proposed method for using top quark pairs produced at the LHC as a new system to study entanglement. The top quark is the heaviest known elementary particle. Normally, it decays into other particles before it has time to combine with other quarks, transferring its properties to the particles into which it decays. It is precisely one of these properties, spin, that allows the study of entanglement between two particles.
To observe entanglement between top quarks, the ATLAS and CMS collaborations selected top quark pairs from proton-proton collision data that took place during the second LHC data-taking period, called Run2, between 2015 and 2018. The ATLAS and CMS teams observed spin entanglement between top quarks with statistical significance greater than five standard deviations.
Participation of the Institute of Corpuscular Physics of Valencia
The Institute of Corpuscular Physics (IFIC), a Severo Ochoa center of excellence, is a pioneer in Spain in research in particle physics, nuclear physics and astroparticle physics, as well as in its applications in medical physics and other fields of science and technology. IFIC has been involved in the LHC ATLAS experiment since its inception, a cathedral-sized detector involving more than 3,000 physicists and engineers from all over the world. Specifically, two IFIC researchers, Carlos Escobar and Marcel Vos, have participated in the review process of all aspects of the analysis now published in Nature.
“As is normal with a groundbreaking result, it has been an intense review process, both within the collaboration and with the journal. After three years of work, we are very happy to see this result published in Nature,” says Carlos Escobar, CSIC Ramón y Cajal researcher at IFIC. “This ATLAS measurement and the CMS confirmation mark the beginning of a new way of studying the foundations of quantum mechanics. There are many new ideas exploring the potential of the LHC to clarify the interpretation of quantum physics,” says Marcel Vos, CSIC researcher at IFIC.