Albert Einstein skeptically described quantum entanglement as “spooky action at a distance,” but today it is indispensable for almost all quantum technologies – from quantum computers to sensors and highly precise measurement methods to encrypted quantum communication. The entanglement of particles causes a quantum physical coupling of their state: If a particle is measured, this also changes the quantum state of the partner particle instantaneously and over any distance.
But can such quantum entanglement arise and remain intact even under extreme conditions? Until now, this quantum physical coupling was considered rather fragile and prone to failure, for example with qubits in quantum computers.
Are the decay products of the Higgs boson entangled?
But the world’s most powerful particle accelerator, the Large Hadron Collider (LHC) at the CERN research center near Geneva, is now teaching us otherwise. There, physicists from the ATLAS collaboration investigated whether Z bosons formed during the decay of the Higgs boson – carrier particles of the weak nuclear force – are entangled in quantum physics. “Measurements with such bosonic particles allow us to test the entanglement of massive particles under extreme conditions,” explains the team.
In the particle accelerator, Higgs bosons and their decay products are only created in extremely high-energy proton collisions. The particles collide with each other at 99.99 percent of the speed of light and energies of more than 13 teraelectron volts. “Because the LHC works at these energies, it enables us to test entanglement even for massive, short-lived particles,” the physicists write. The Z boson pairs released by the Higgs boson decay after just a few fractions of a second.
Trajectory reveals entanglement
In order to measure the quantum entanglement of the Z bosons, physicists must use indirect methods. To do this, they used the ATLAS detector to analyze the angles at which the decay products of the Z bosons fly apart. These trajectories allow conclusions to be drawn about the particle spin of the Z bosons and also whether they were created as entangled pairs or not. For their measurements, the researchers evaluated data from the entire second and part of the third runtime of the accelerator – this corresponds to millions of Z boson events.
And indeed: The analyzes showed that the Z bosons in the particle accelerator do not arise independently of one another, but are entangled with each other in pairs. “Our data refute the null hypothesis of an unentangled quantum state of these particles with a significance of 4.7 standard deviations,” report the physicists. “These results represent the first evidence of quantum entanglement between two massive vector bosons on the electroweak scale.”
Values measured in the LHC (vertical black line) and the expected values for non-entangled (orange) and entangled Z bosons (blue). — © ATLAS Collaboration/Physical Review Letters, CC by 4.0
“Shows how fundamental and robust this quantum effect is”
This suggests that quantum entanglement must be less fragile than commonly thought. “The fact that this works even with heavy, short-lived particles like the Z bosons shows how fundamental and robust this quantum effect really is,” says co-author Alan Barr from the University of Oxford. “This reminds us that the exotic rules of quantum mechanics that enable our quantum computers also apply elsewhere in nature – even at energies as extreme as those in the Large Hadron Collider.”
At the same time, this experiment underlines how closely quantum physics concepts and particle physics are linked. “Entanglement is both the most promising and most puzzling aspect of quantum reality,” says Barr’s colleague Chris Timpson. “Such tests in particle accelerators open up new opportunities for us to research the fundamentals of quantum mechanics.”
Source: ATLAS Collaboration, Physical Review Letters, 2026; doi: 10.1103/y1nh-1b82