A particle that exists for less than a heartbeat has provided physicists with a new way to test one of quantum theory’s most perplexing predictions.
Physicists at the University of Oxford and an international research team have found strong evidence of quantum entanglement between pairs of Z bosons produced at CERN’s Large Hadron Collider (LHC).
The result extends experimental tests of one of quantum physics’ most counterintuitive effects to particles that are both extremely massive and extraordinarily short-lived. The study used data from the ATLAS experiment to examine Z bosons created when Higgs bosons decay.
Each Z boson survives for only a tiny fraction of a second before breaking down into other particles, yet measurements of those decay products revealed the characteristic correlations expected from quantum entanglement.
The finding, published in Physical Review Letters, provides one of the highest-energy demonstrations of quantum entanglement to date.
It also strengthens the case for using particle colliders as laboratories for investigating quantum mechanics under conditions far removed from conventional quantum experiments.
Testing entanglement at extreme energies
Quantum entanglement describes a relationship between particles whose quantum states remain correlated, even when the particles are separated.
Measuring one particle can provide information about its entangled partner in a way that cannot be explained by treating the two particles as entirely independent systems.
The phenomenon has been observed in systems including photons, electrons and trapped ions. It is also central to technologies being developed around quantum computing, communication and sensing.
Until now, however, an important question remained: does entanglement survive when particles are produced in the extraordinarily energetic and chaotic environment of a particle collider?
The ATLAS researchers investigated this question by studying pairs of Z bosons produced through the decay of Higgs bosons. The Higgs particles themselves are created when protons collide inside the LHC at energies reaching 13 trillion electron volts.
Z bosons provide a new quantum test
Z bosons are among the fundamental particles responsible for the weak nuclear force. They are also extremely unstable, disappearing almost immediately after being produced.
That fleeting existence does not prevent ATLAS from studying them. Instead, researchers reconstruct what happened by analysing the particles produced when the Z bosons decay.
In this case, the Z bosons ultimately produced pairs of electrons or muons. By examining the directions in which these particles emerged from the collisions, physicists could reconstruct information about the spins of the original Z bosons.
The resulting measurements showed strong evidence that the Z bosons were entangled.
This is significant because it demonstrates that quantum entanglement is not confined to relatively isolated and carefully controlled laboratory systems. The effect can also be observed among heavy particles created amid some of the most energetic collisions humans can produce.
Building on the top-quark experiment
The Oxford contribution builds on earlier work exploring whether particle accelerators can be repurposed as enormous quantum experiments.
Alan Barr, a professor in Oxford’s Department of Physics, was among the researchers who proposed using high-energy particle collisions to investigate quantum entanglement. That approach led to an ATLAS measurement in 2023 that demonstrated entanglement between pairs of top quarks.
Top quarks are the heaviest known elementary particles. The latest study extends the approach to Z bosons, providing another opportunity to examine quantum behaviour in a fundamentally different particle system.
The work also highlights a growing overlap between particle physics and quantum information science. Techniques developed for understanding quantum information can provide new methods for extracting subtle correlations from enormous collider datasets.
Could quantum entanglement help find new physics?
The immediate importance of the result is fundamental rather than technological. The experiment does not mean that an entangled Z-boson system could directly be turned into a quantum computer or communications device.
Instead, it gives physicists another way to test whether quantum mechanics continues to behave as expected under extreme conditions.
That could become particularly valuable when researchers search for evidence of physics beyond the Standard Model, the framework currently used to describe fundamental particles and their interactions.
Extremely precise measurements of quantum correlations could potentially expose deviations that conventional analyses might miss.
Oxford researchers are also involved in a broader interdisciplinary programme examining quantum mechanics, measurement and the nature of reality at high energies.
More powerful tests ahead
The ATLAS detector is currently undergoing major upgrades, while the LHC is being developed towards its High-Luminosity phase.
The resulting increase in collision data should give physicists far more opportunities to investigate quantum phenomena at unprecedented energies.
For quantum entanglement, the latest result marks an important shift in experimental territory. What was once primarily studied using relatively delicate laboratory systems can now be investigated through the violent collisions of fundamental particles.
The findings suggest that entanglement remains remarkably robust, even when the particles involved exist for an almost immeasurably short time.
