Physicists at the University of Oxford, working alongside an international collaboration at CERN, have helped demonstrate that one of quantum mechanics’ strangest and most counterintuitive phenomena—quantum entanglement—can persist even among some of the heaviest and shortest-lived fundamental particles ever produced. The breakthrough finding, achieved using data from the powerful Large Hadron Collider (LHC), has been successfully published in the prestigious scientific journal Physical Review Letters.
Quantum entanglement occurs when two or more particles become fundamentally interconnected, sharing physical properties in such a way that the measurement of one particle instantly reveals information about the state of the other, regardless of the physical distance separating them. This profound and non-intuitive connection has challenged physicists’ understanding of reality for nearly a century.
Albert Einstein famously expressed deep skepticism about the concept, referring to it dismissively as "spooky action at a distance." Despite its initial conceptual hurdles, scientists have extensively observed the entanglement effect over the years in controlled laboratory systems involving relatively stable and delicate entities such as photons, electrons, and trapped ions.
In recent decades, entanglement has also transitioned from a purely theoretical curiosity into a cornerstone of several emerging technologies. It now serves as the driving principle behind cutting-edge innovations including quantum computers, ultra-secure quantum communication networks, and advanced precision sensors. In the realm of quantum computing, for instance, entanglement permits multiple quantum bits—or qubits—to be manipulated collectively and simultaneously, rather than sequentially one at a time, exponentially expanding processing capabilities and making it possible to carry out complex calculations at unprecedented speeds.
Testing Quantum Entanglement at Extreme Energies
While the applications and behaviors of entanglement in low-energy, highly controlled laboratory environments have become increasingly well understood, what remained a compelling mystery for the scientific community was whether this delicate quantum bond could survive under far more extreme conditions. Specifically, physicists questioned whether entanglement could endure amidst the violent, high-energy particle collisions routinely produced at modern particle accelerators like CERN.
To investigate this profound question, an international research team utilized the ATLAS experiment, one of the massive detectors situated along CERN’s Large Hadron Collider near Geneva, Switzerland. Rather than focusing on familiar, relatively long-lived quantum systems like photons or isolated electrons, the researchers set out to search for evidence of entanglement between pairs of Z bosons. Z bosons are massive elementary particles that govern the weak nuclear force and exist for only a microscopic, nearly incomprehensible fraction of a second before decaying into other particles.
The specific Z bosons examined during the course of the experiment originated from the decay of a Higgs boson, the landmark particle famously discovered at the LHC back in 2012. Under the extreme conditions of the collider, a Higgs boson can briefly materialize before decaying into two Z bosons, which in turn rapidly decay into pairs of secondary particles such as electrons or muons. These Higgs bosons themselves are brought into existence when intense beams of protons, traveling at roughly 99.99% the speed of light, are smashed together head-on at colossal energy levels reaching thirteen trillion electron volts.
Fleeting Z Bosons Leave Quantum Clues Behind
Although Z bosons vanish almost instantaneously after they are created in these high-energy cataclysms, the sophisticated ATLAS detector is engineered to track and accurately measure the secondary electrons and muons produced as a result of their decay.
By analyzing the precise spatial angles and trajectories at which these resulting particles emerged from the collision point, researchers were able to work backward and reconstruct the intrinsic spins of the original, parent Z bosons. This meticulous reconstruction process allowed the research team to determine whether the two fleeting Z bosons displayed the specific mathematical correlations and directional dependencies that are uniquely expected from quantum entanglement.
The resulting measurements provided remarkably strong evidence that they did indeed display these correlations. This successful detection represents one of the highest-energy confirmations of quantum entanglement ever achieved in the history of experimental physics.
Study co-author Professor Alan Barr, a prominent researcher at Oxford’s Department of Physics, was among the very first scientists to publicly suggest that large-scale particle colliders could be repurposed and utilized to investigate quantum entanglement at energy scales vastly exceeding those used in traditional, low-energy quantum optics experiments.
Professor Barr, who was directly involved in the construction and commissioning of the LHC, recognized early on that the enormous collider could serve vital scientific purposes well beyond its initial, primary mission of searching for new fundamental particles. His visionary proposals and theoretical groundwork directly helped inspire a prior 2023 ATLAS experiment that successfully demonstrated entanglement between pairs of top quarks, which hold the distinction of being the heaviest known elementary particles in the universe.
Reflecting on the magnitude of the latest discovery, Professor Barr noted that scientists are traditionally accustomed to thinking of entanglement as an extremely delicate phenomenon, typically observed under highly controlled laboratory conditions using single photons. Finding the effect alive and well among particles as heavy and short-lived as Z bosons—which are forged in some of the most violent collisions humanity can produce on Earth—demonstrates just how fundamental and surprisingly robust this quantum effect truly is. He added that it serves as a powerful reminder that the very same strange rules of quantum mechanics that may one day power revolutionary quantum computers are actively at work everywhere in nature, even amidst the extreme, fiery energies of the Large Hadron Collider.
Quantum Computing Ideas Meet Particle Physics
The groundbreaking work is also part of a much broader, ongoing scientific effort to bridge the gap between concepts rooted in quantum information science—the academic discipline driving the development of quantum computing—and the complex realm of high-energy particle physics.
By applying theoretical ideas and analytical frameworks originally developed for delicate quantum systems to the enormous, highly complex data sets produced by massive particle colliders, researchers hope to forge significantly more sensitive methods for detecting subtle, hidden patterns within subatomic data. These innovative analytical techniques could eventually reveal physical effects that do not neatly fit within physicists’ current standard model understanding of the universe, potentially offering vital clues and pathways toward discovering new physics that lies beyond existing theoretical frameworks.
At the University of Oxford, Professor Barr co-leads a major, highly collaborative interdisciplinary project that focuses squarely on exploring the fundamental foundations of quantum mechanics at high energies. This dedicated project is actively testing quantum behavior simultaneously at extremely small spatial scales and exceptionally high energy thresholds, while also carefully examining the deep philosophical questions raised by such experiments, including what their outcomes may ultimately reveal about the underlying, objective nature of physical reality.
Project co-principal investigator Professor Chris Timpson, a member of Oxford’s Faculty of Philosophy, emphasized the philosophical and scientific weight of the findings. He remarked that entanglement remains both the most promising and the most puzzling aspect of quantum reality, and noted that these collider experiments detecting entanglement represent an exciting new frontier in ongoing investigations into the foundations of quantum mechanics.
CERN Prepares for Even Deeper Quantum Tests
Building upon the success of these latest measurements, scientists and researchers from Oxford University are also actively contributing to the ongoing, comprehensive upgrade of the ATLAS detector infrastructure. Working in tandem with the upcoming implementation of the High-Luminosity Large Hadron Collider, these technological improvements are widely expected to provide vastly larger and more precise data sets, granting physicists unprecedented new opportunities to probe deeper into quantum phenomena at extreme energies.
The acquisition of this additional data could soon allow researchers to apply even more sophisticated quantum information processing techniques directly to particle physics data, potentially multiplying the sensitivity of future searches for previously undiscovered physical phenomena and rare particle interactions.
Professor Daniela Bortoletto, a senior member of the Department of Physics at the University of Oxford and the UK coordinator responsible for overseeing the production of the modules for the upgraded ATLAS detector’s pixel system, highlighted the collaborative nature of the achievement. She noted that this successful measurement clearly demonstrates the immense scientific power of the international ATLAS collaboration alongside the truly unique experimental capabilities of CERN’s Large Hadron Collider. Furthermore, she expressed pride in the leading role that Oxford researchers have played in developing these pioneering approaches to studying quantum phenomena at the highest attainable energies, contributing directly to an expansive global effort that continues to open up entirely new ways to explore the fundamental laws of nature.
The formal academic study detailing these findings, titled "Measurements of Z-boson pair entanglement in decays of Higgs bosons at the ATLAS experiment," has been successfully peer-reviewed and published in the scientific journal Physical Review Letters.
