The LHC Smashed Protons Together, Made a Higgs Boson, Watched It Split Into Two Z Bosons That Lived Less Than a Trillionth of a Trillionth of a Second, and Found They Were Quantum Entangled.

Quantum entanglement is usually demonstrated with the gentlest objects physics has: single photons in a darkened lab, trapped ions held still by lasers, electrons in a cooled chip. The ATLAS collaboration at CERN has now found it in some of the most violent conditions humans can produce. In a paper published in Physical Review Letters, physicists including a University of Oxford team report strong evidence that pairs of Z bosons born from the decay of a Higgs boson are entangled with one another, at collision energies a trillion times higher than any tabletop test.

The Z boson is one of the carriers of the weak nuclear force, about 97 times heavier than a proton, and it lives for roughly three times ten to the minus twenty-five seconds before decaying. There is no way to catch one. What ATLAS can do is record what a Z leaves behind: when protons collide at 13 trillion electron volts, they occasionally produce a Higgs boson, which can split into two Z bosons, each of which then decays into a pair of electrons or a pair of muons. Those four charged particles are what the detector actually sees.

The angles at which they fly out encode the spins of the Z bosons that produced them. By reconstructing those angles across a large sample of Higgs-to-four-lepton events, the team could infer the spin state of each Z pair and test whether the two were correlated in the specific way that only entanglement allows. They were. "The results showed strong evidence that they were," the Oxford summary states, making this one of the highest-energy confirmations of quantum entanglement on record.

Alan Barr, a professor in Oxford's Department of Physics and a co-author, was among the first to argue that particle colliders could serve as quantum-mechanics laboratories rather than only discovery machines. His ideas underpinned a 2023 ATLAS measurement that found entanglement between pairs of top quarks, the heaviest known elementary particles. The Z boson result extends that program to a different kind of particle produced by a different mechanism.

"We're used to thinking of entanglement as something delicate, seen in laboratory experiments with single photons," Barr 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." The stakes are larger than confirmation. "Using particle colliders allows us to test quantum mechanics at energies a trillion times higher and over distances smaller than the size of the nucleus," he said. "This probes some of the extreme conditions where quantum mechanics might break down, which would have profound consequences for the foundations of science."

The measurement also marks the maturing of a cross-disciplinary approach. Techniques borrowed from quantum information science, the field that underpins quantum computing, are now being applied to collider data as analysis tools, offering new and more sensitive ways to look for subtle deviations that might point to physics beyond the Standard Model. "It has been gratifying to see that the use of particle colliders to test quantum effects has now become a major subfield in physics, with more than a hundred researchers working on it worldwide," Barr said.

For now, quantum mechanics has passed another test at an energy where some theorists had wondered whether it might fail. Einstein's "spooky action at a distance" survived the Large Hadron Collider.