Quantum entanglement persists in fleeting Z bosons at CERN

Physicists have detected strong evidence that heavy, fleeting Z bosons can become quantum entangled during Higgs boson decays at CERN's Large Hadron Collider. The result shows that Einstein's "spooky action at a distance" survives even under some of the most extreme conditions ever created in a laboratory.

An international team used the ATLAS experiment at the Large Hadron Collider near Geneva, Switzerland. They searched for entanglement between pairs of Z bosons, massive particles that exist for only a tiny fraction of a second before decaying. The Z bosons examined came from the decay of a Higgs boson, the particle discovered at the LHC in 2012.

Testing entanglement at extreme energies

Quantum entanglement occurs when two particles share properties so that measuring one reveals information about the other, even when separated. Scientists have previously observed it in systems involving photons, electrons, and trapped ions. What remained less clear was whether entanglement could survive the violent, high-energy particle collisions at CERN.

Higgs bosons are produced when protons traveling at 99.99% the speed of light collide at energies reaching thirteen trillion electron volts. A Higgs boson can briefly decay into two Z bosons, which then decay into pairs of electrons or muons.

Reconstructing spins from decay products

Although Z bosons disappear almost immediately, the ATLAS detector can accurately measure the electrons and muons from their decay. Researchers analyzed the angles at which those particles emerged. They used that information to reconstruct the spins of the original Z bosons. This allowed the team to determine whether the two Z bosons displayed the correlations expected from quantum entanglement.

The measurements provided strong evidence that they did. The result represents one of the highest energy confirmations of quantum entanglement ever achieved.

Study co-author Professor Alan Barr of the University of Oxford's Department of Physics was among the first to suggest particle colliders could investigate quantum entanglement at far higher energies than traditional experiments. His ideas helped inspire a 2023 ATLAS experiment that demonstrated entanglement between pairs of top quarks, the heaviest known elementary particle.

Professor Barr said: Finding it alive and well among particles as heavy and short-lived as Z bosons, created in some of the most violent collisions we can produce on Earth, shows just how fundamental and robust this quantum effect really is.

Bridging quantum information and particle physics

The work is part of a broader effort to bring concepts from quantum information science into high-energy particle physics. By applying ideas developed for quantum systems to the enormous data sets from particle colliders, researchers hope to create more sensitive ways of detecting subtle patterns. These techniques could eventually reveal effects beyond current theories.

At Oxford University, Professor Barr co-leads a major interdisciplinary project focused on the foundations of quantum mechanics at high energies. Project co-lead Professor Chris Timpson of the Faculty of Philosophy said the collider experiments detecting entanglement present a new frontier in investigations of the foundations of quantum mechanics.

Preparing for deeper quantum tests

Oxford University scientists are also contributing to the ongoing upgrade of the ATLAS detector. Together with the upgraded High-Luminosity Large Hadron Collider, these improvements are expected to provide vastly larger data sets. This will give physicists new opportunities to investigate quantum phenomena at extreme energies.

The additional data could allow researchers to apply more sophisticated quantum information techniques to particle physics. Professor Daniela Bortoletto, the UK coordinator for producing modules for the upgraded ATLAS detector's pixel system, said Oxford researchers have played a leading role in developing these new approaches to studying quantum phenomena at the highest energies.