ATLAS detects quantum entanglement between particle pairs from Higgs decays

The ATLAS experiment at CERN’s Large Hadron Collider has found strong evidence that pairs of Z bosons produced in Higgs boson decays are quantum-entangled, extending tests of the unusual phenomenon to one of the highest-energy settings yet studied. Oxford physicists contributed to the analysis, which was published in Physical Review Letters. The result shows that entanglement can be detected not only in controlled laboratory systems involving photons, electrons or trapped ions, but also between massive, extremely short-lived particles created in high-energy proton collisions.

Quantum entanglement describes a connection between particles that share a common origin. In an entangled state, the properties of the particles cannot be described independently, even when they are separated by substantial distances or time. Measuring a property of one particle can therefore reveal information about its partner. Albert Einstein famously referred to this aspect of quantum mechanics as “spooky action at a distance.” The effect is a cornerstone of proposed quantum technologies, including processors that manipulate multiple qubits together and communication systems designed to use quantum states to protect information.

The new measurement focused on a specific particle-physics process. When a Higgs boson decays, it can briefly produce two Z bosons, each of which subsequently decays into a pair of electrons or muons. The researchers examined the angular relationships between the visible particles left behind by those decays. Because the Z bosons exist for only a fraction of a second, their properties cannot be measured directly with a detector. Instead, the ATLAS collaboration reconstructed the directions in which the decay products were emitted and used those angles to infer information about the spins of the original Z bosons.

That reconstruction allowed the team to test whether the two parent Z bosons possessed the joint quantum state required for entanglement. The analysis produced strong evidence that they did. The experiment therefore demonstrates that a quantum connection between the decay products is present in the selected Higgs decay channel. It is not simply a comparison of separate Z bosons, nor does it rely on their spatial separation after leaving the collision point. The test concerns correlations encoded in the spins of two particles that originated together in a highly energetic collision.

The setting is among the most demanding in which entanglement has been sought. Protons were accelerated to 99.99 percent of the speed of light and brought together in collisions with energies of 13 trillion electron volts. Their collisions can create the Higgs boson, which is itself short-lived and decays almost immediately. The subsequent Z bosons are also fleeting, but they leave detectable electrons or muons whose momentum and emission angles can be measured with high precision by the ATLAS detector. This chain of short-lived intermediate particles links the quantum state under examination to observable final-state particles.

Oxford’s Alan Barr, a co-author of the study and a professor in the Department of Physics, had previously proposed that particle colliders could investigate entanglement at energies far beyond those accessible in more conventional laboratory experiments. He also contributed to the construction of the Large Hadron Collider and recognized that its collecting system could serve purposes beyond the discovery of new particles. His earlier ideas informed a 2023 ATLAS experiment that demonstrated entanglement between pairs of top quarks, the heaviest known elementary particles. The Z-boson result builds on that work by examining a different pair of particles and a different production process.

Barr said the new finding showed how robust the effect can be. “We’re used to thinking of entanglement as something delicate, seen in laboratory experiments with single photons,” he 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.” He added that the result was a reminder that the rules of quantum mechanics underlying proposed quantum computers operate at the extreme energies reached by the collider as well as in carefully controlled laboratories.

The study also illustrates an effort to bring tools and concepts from quantum information science into particle physics. Collider physicists are using quantum-state measurements to extract information that may be difficult to recognize through conventional event analyses alone. The release said this cross-pollination could support more sensitive methods for analyzing collider data and potentially reveal subtle effects associated with physics beyond the current understanding of the universe. That possibility remains a prospect of the research program rather than a demonstrated result of the present measurement, which concerns the observation of entanglement itself.

At Oxford, Barr co-leads an interdisciplinary project examining the foundations of quantum mechanics at high energies. The project combines empirical measurements with questions about what those measurements imply for the fundamental nature of reality. Oxford philosopher Chris Timpson, a project co-principal investigator, 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.” The involvement of both physics and philosophy reflects the unusually broad interpretation of the work, which treats a collider result as both a precise particle-physics measurement and an opportunity to probe the conceptual structure of quantum theory.

Oxford researchers are also contributing to the ongoing upgrade of the ATLAS detector, including work on modules for its pixel system. The detector upgrade, together with the planned High-Luminosity Large Hadron Collider, is intended to provide larger data sets and improved capabilities for studying quantum phenomena in collider events. The present paper is a peer-reviewed measurement rather than a demonstration of a practical quantum-computing or communication system, and it does not report entanglement between arbitrary Z bosons. Its scope is the specified Higgs-boson decay channel studied by ATLAS. Even so, by finding a strong quantum signature among particles created at 13 teraelectron-volts, the experiment establishes that entanglement remains observable in an exceptionally energetic and fleeting environment.

Subject of Research: Experimental evidence of quantum entanglement between Z-boson pairs produced in Higgs boson decays at the ATLAS experiment

Article Title: Oxford physicists help uncover ‘spooky’ quantum effect in the Large Hadron Collider

Article References: Oxford physicists help uncover ‘spooky’ quantum effect in the Large Hadron Collider. Original publication (Bibliographic details unavailable.)

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Keywords: Quantum entanglement, ATLAS experiment, Z bosons, Higgs boson, Large Hadron Collider, CERN, Quantum information, Particle physics, Quantum foundations, Oxford, physicists, help