LHC experiments observe quantum entanglement at highest energy ever

Quantum entanglement is a fascinating feature of quantum physics, the theory of the infinitely small. If two particles are quantumly entangled, the state of one is linked to that of the other, regardless of the distance between them. This astonishing phenomenon, which has no equivalent 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 groundbreaking experiments on entangled photons. These experiments confirmed predictions about the manifestation of entanglement made by the late CERN theorist John Bell and paved the way for quantum information science.

Entanglement has remained largely unexplored at the high energies accessible to particle colliders such as the Large Hadron Collider (LHC). In a paper published in NatureThe ATLAS collaboration reports how it managed to observe quantum entanglement for the first time at the LHC, between fundamental particles called top quarks and at the highest energies ever recorded.

First reported by ATLAS in September 2023 and since confirmed by two observations by the CMS collaboration, this result has opened a new perspective on the complex world of quantum physics.

“Particle physics is deeply rooted in quantum mechanics, but observing quantum entanglement in a new particle system and at much higher energy than was possible before is remarkable,” says Andreas Hoecker, spokesperson for ATLAS. “This paves the way for new research into this fascinating phenomenon, opening up a rich menu of exploration as our data samples continue to grow.”

The ATLAS and CMS teams have observed quantum entanglement between a top quark and its antimatter counterpart. These observations build 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 characteristics to its decay particles. Physicists observe and use these decay products to infer the orientation of the top quark’s spin.

To observe the 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. In particular, they looked for pairs in which the two quarks are produced simultaneously with a small particle momentum relative to each other. 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 distort the measured values, the ATLAS and CMS teams each observed spin entanglement between the top quarks with statistical significance greater than five standard deviations.

In his second study, currently available on the arXiv Using the preprint server, the CMS collaboration also searched for top quark pairs in which both quarks are produced simultaneously with high momentum relative to each other. In this area, 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 information exchange by particles moving at no faster than the speed of light is excluded, and CMS also observed spin entanglement between the top quarks in this case.

“By measuring 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 might lie beyond it,” said Patricia McBride, a spokesperson for the CMS.