Quantum entanglement occurs when two particles share properties in such a way that measurements of one can reveal information about the other, even when the particles are separated. The connection is one of the most counterintuitive features of quantum mechanics and has challenged physicists' understanding of reality for decades.

Albert Einstein famously called entanglement "spooky action at a distance," and scientists have previously observed the effect in systems involving photons, electrons and trapped ions. Entanglement has also become central to several emerging technologies, including quantum computers, ultra-secure quantum communication networks and advanced sensors. In quantum computing, for example, entanglement allows multiple qubits to be manipulated together rather than one at a time, making it possible to carry out multiple calculations simultaneously.

Testing Quantum Entanglement at Extreme Energies

What remained less clear was whether entanglement could survive under far more extreme conditions, including the violent, high-energy particle collisions produced at CERN.

To investigate that question, an international team used the ATLAS experiment at CERN's Large Hadron Collider (LHC) near Geneva, Switzerland. Instead of studying photons or other relatively long-lived quantum systems, the researchers 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 in the experiment came from the decay of a Higgs boson, the particle discovered at the LHC in 2012. A Higgs boson can briefly decay into two Z bosons, which then decay into pairs of electrons or muons. The Higgs bosons themselves are produced when protons traveling at 99.99% the speed of light collide at energies reaching thirteen trillion electron volts.

Fleeting Z Bosons Leave Quantum Clues Behind

Although Z bosons disappear almost immediately after they are created, the ATLAS detector can accurately measure the electrons and muons produced by their decay.

Researchers analyzed the angles at which those particles emerged and used that information to reconstruct the spins of the original Z bosons. That 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, at Oxford's Department of Physics, was among the first researchers to suggest that particle colliders could be used to investigate quantum entanglement at energies far beyond those used in traditional quantum experiments.

Barr, who was involved in building the LHC, recognized that the enormous collider could serve purposes beyond the search for new particles. His ideas helped inspire a 2023 ATLAS experiment that demonstrated entanglement between pairs of top quarks, the heaviest known elementary particle.

Professor Barr said: "We're used to thinking of entanglement as something delicate, seen in laboratory experiments with single photons. 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. It's a nice reminder that the same strange rules of quantum mechanics that may one day power quantum computers are at work everywhere in nature, even at the extreme energies of the Large Hadron Collider."

Quantum Computing Ideas Meet Particle Physics

The work is also part of a broader effort to bring concepts from quantum information science (the field behind quantum computing) into high-energy particle physics.

By applying ideas developed for quantum systems to the enormous data sets produced by particle colliders, researchers hope to create more sensitive ways of detecting subtle patterns. Those techniques could eventually reveal effects that do not fit within physicists' current understanding of the universe and potentially offer clues about physics beyond existing theories.

At Oxford University, Professor Barr co-leads a major interdisciplinary project focused on the foundations of quantum mechanics at high energies. The project is testing quantum behavior at extremely small scales and very high energies while also examining the philosophical questions raised by such experiments, including what they may reveal about the underlying nature of reality.

Project co-PI Professor Chris Timpson (Faculty of Philosophy) said: "Entanglement is both the most promising and the most puzzling aspect of quantum reality; these collider experiments detecting entanglement present a new frontier in investigations of the foundations of quantum mechanics."

CERN Prepares for Even 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 and give physicists new opportunities to investigate quantum phenomena at extreme energies.

The additional data could also allow researchers to apply more sophisticated quantum information techniques to particle physics, potentially increasing the sensitivity of future searches for previously unknown phenomena.

Professor Daniela Bortoletto (Department of Physics, University of Oxford), who is the UK coordinator for producing the modules for the upgraded ATLAS detector's pixel system, said: "This measurement demonstrates the scientific power of the ATLAS collaboration and the unique capabilities of CERN's Large Hadron Collider. Oxford researchers have played a leading role in developing these new approaches to studying quantum phenomena at the highest energies, and we are proud to contribute to an international effort that is opening new ways to explore the fundamental laws of nature."

The study "Measurements of Z-boson pair entanglement in decays of Higgs bosons at the ATLAS experiment" has been published in Physical Review Letters.