What is quantum entanglement?

Quantum entanglement occurs when two particles share linked properties, meaning that measurements of one particle can reveal information about the other, even when the particles are separated. It is one of the most counterintuitive features of quantum mechanics and has challenged physicists’ understanding of reality for decades.

Albert Einstein famously described entanglement as a “distant and mysterious effect.” Scientists have previously observed it in systems involving photons, electrons and trapped ions. The phenomenon is also central to emerging technologies such as quantum computers, quantum communication networks and advanced sensors.

In quantum computing, for example, entanglement allows multiple qubits to be manipulated together, supporting calculations that cannot be performed in the same way using independent classical bits.

Testing quantum entanglement at extreme energies

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

To investigate, an international team used the ATLAS experiment at the Large Hadron Collider near Geneva, Switzerland. Rather than studying photons or other relatively long-lived quantum systems, the researchers examined entanglement between pairs of Z bosons—massive particles that exist for only a fraction of a second before decaying.

The Z bosons studied in the experiment were produced when a Higgs boson decayed. The Higgs boson briefly decays into two Z bosons, which then decay into pairs of electrons or muons. The Higgs boson itself is created when protons traveling at 99.99% of the speed of light collide at energies reaching 13 trillion electron volts.

How researchers detected entanglement in Z bosons

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

The researchers analyzed the angles at which those particles emerged and used the measurements to reconstruct the spin 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 achieved to date.

Professor Alan Barr of the Oxford Department of Physics, a co-author of the study, was among the first researchers to suggest that particle colliders could be used to investigate quantum entanglement at energies far beyond those used in conventional quantum experiments.

Barr, who helped build the LHC, recognized that the collider could be used for purposes beyond the search for new particles. His ideas helped inspire the 2023 ATLAS experiment demonstrating entanglement between pairs of top quarks, the heaviest elementary particles known.

“We’re used to thinking of entanglement as delicate, as seen in laboratory experiments with single photons,” Barr said. “We found that entanglement is alive and well in heavy, short-lived particles like Z particles, produced in some of the most violent collisions possible on Earth.”

“What we did shows how fundamental and robust this quantum effect really is. It’s a good reminder that the same strange laws of quantum mechanics that may one day power quantum computers are at work everywhere in nature.”

Connecting quantum computing and particle physics

The research is 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 vast datasets produced by particle colliders, researchers hope to develop more sensitive ways to identify subtle patterns. These techniques could eventually reveal effects beyond physicists’ current understanding of the universe and provide clues about physics beyond existing theory.

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

Project co-lead Professor Chris Timpson of the School of Philosophy said:

“Entanglement is one of the most promising aspects of quantum reality, but also one of the most puzzling. These collision-type experiments that detect entanglement represent a new frontier in the study of the fundamentals of quantum mechanics.”

CERN prepares more advanced quantum tests

Scientists at the University of Oxford are also contributing to upgrades of the ATLAS detector. Together with the upgraded High-Luminosity Large Hadron Collider, these improvements are expected to produce extremely large datasets and give physicists new opportunities to study quantum phenomena at extreme energies.

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

Professor Daniela Bortlett of the Department of Physics at the University of Oxford, and UK coordinator of module manufacturing 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 be contributing to this international effort to open up new avenues to explore fundamental natural laws.”

The research is titled “Measuring the entanglement of Z particle pairs in the decay of the Higgs boson in the ATLAS experiment” and was published in Physical Review Letters.