Quantum entanglement, the physics phenomenon Albert Einstein famously called “spooky action at a distance,” has just turned up in an extreme new setting: among Z bosons, massive subatomic particles that exist for only a tiny fraction of a second before decaying.

Entanglement occurs when two or more particles of the same origin remain in a united quantum state — measuring one can reveal information about the other, such as spin, even when they have moved apart.

Scientists have witnessed entanglement before with quantum systems like ions, photons and the tiny building blocks of matter known as quarks. But at CERN’s underground Large Hadron Collider near Geneva, Switzerland, an international team of researchers confirmed it in pairs of Z bosons using the ATLAS detector, CERN’s largest volume particle detector.

“We’re used to thinking of entanglement as something delicate, seen in laboratory experiments with single photons,” Alan Barr, a University of Oxford physics professor, said in a statement. “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.”

Barr co-authored a study on the finding published Monday in the journal Physical Review Letters .

Why Do Quantum Mechanics Matter?

Quantum entanglement is more than a theoretical physics oddity or poetic description of human interconnectedness .

The idea that one entity “knows” about the state of the other without needing to exchange information is a fundamental pillar of quantum mechanics, the study of how matter and energy function at the smallest scale. Quantum mechanics inform our understanding of how the universe works. In a more everyday sense, the coordination of minuscule particles is key to technologies like cryptography, quantum computers and next-generation sensors that could be used in future space missions.

Scientists participating in the research — from Australia, Canada, Colombia, France, Israel, South Africa, Turkey and the U.S., among other countries — studied quantum entanglement in the decay of a Higgs boson, the particle famously discovered at the LHC in 2012. Though the Z bosons vanish almost immediately, the ATLAS detector can precisely track the electrons and muons they leave in their wake as they decay.

The Z boson entanglement is “quite significant,” said Regina Demina, a University of Rochester physics professor who was not involved with the study

“Right now it is hard to point to an application that would make unstable particles more interesting for technology than the stable ones — electrons or photons,” she said in an interview. “Yet it is a common story: Curiosity is driving the research process, and the realization on the applications comes later.”

In 2024, a team led by Demina observed spin entanglement between top quarks, another type of massive, short-lived particle produced in high-energy collisions at the LHC. She said that beyond any practical applications, the Z boson quantum entanglement discovery matters for what it contributes to our understanding of the striking physical properties of quantum entanglement.

‘Another Step Forward’ For Physics

“We need more fundamental research in this area to fully comprehend this effect,” said Demina, who as a graduate student, was on the team that discovered the top quark in 1995. “In this sense the discovery of entanglement of Z-bosons marks another step forward.”

Particle colliders like the LHC allow scientists to test quantum mechanics at vastly higher energies, giving them another opportunity to gauge whether its predictions hold up under far more intense conditions than those found in university laboratories.

“This probes some of the extreme conditions where quantum mechanics might break down,” Barr said, “which would have profound consequences for the foundations of science.”