LHC experiments observe quantum entanglement at the highest energy yet

24 September 2024

Photo: CERN

Artist’s impression of a quantum-entangled pair of top quarks

For the first time, quantum entanglement has been observed at energies as high as 13 TeV. This remarkable result stems from top quark measurements by the ATLAS and CMS experiments at CERN. Alexander Grohsjean, a key researcher in the Excellence Cluster Quantum Universe, currently leads the CMS TOP Physics Analysis Group, which conducted two of the three key measurements related to entanglement at LHC. This breakthrough opens new avenues for exploring quantum phenomena at unprecedented energy scales. Quantum entanglement, once famously referred to as "spooky action at a distance" by Albert Einstein, describes a phenomenon where the quantum states of two particles remain correlated, even when separated by vast distances.

Quantum entanglement is a fascinating feature of quantum physics – the theory of the very small. If two particles are quantum-entangled, the state of one particle is tied to that of the other, no matter how far apart the particles are. This mind-bending phenomenon, 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 computing. In 2022, the Nobel Prize in Physics was awarded to Alain Aspect, John F. Clauser and Anton Zeilinger for groundbreaking experiments with entangled photons. These experiments confirmed the predictions for the manifestation of entanglement made by the late CERN theorist John Bell and pioneered quantum information science.

Entanglement has remained largely unexplored at the high energies accessible at particle colliders such as the Large Hadron Collider (LHC). In an article published today in Nature, the ATLAS collaboration reports how it succeeded in observing quantum entanglement at the LHC for the first time, between fundamental particles called top quarks and at the highest energies yet. First reported by ATLAS in September 2023 and since confirmed by a first and a second observation made 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 much higher energy than previously possible is remarkable,” says ATLAS spokesperson Andreas Hoecker. “It paves the way for new investigations into this fascinating phenomenon, opening up a rich menu of exploration as our data samples continue to grow."

The ATLAS and CMS teams observed quantum entanglement between a top quark and its antimatter counterpart. The observations are based on a recently proposed method to use pairs of top quarks produced at the LHC as a new system to study 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 traits to its decay particles. Physicists observe and use these decay products to infer the top quark’s spin orientation.

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, and CMS observed spin entanglement between top quarks also in this case.

“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.” says CMS spokesperson Patricia McBride.

(Adapted from the press release by CERN)