CERN Physicists Find Quantum Entanglement in Heavy Z Bosons

Physicists at CERN's Large Hadron Collider have found strong evidence that pairs of Z bosons, among the heaviest and shortest-lived particles ever produced in a laboratory, can become quantum entangled, extending one of physics' strangest phenomena to a new class of particles at record energy. The ATLAS and CMS collaborations analyzed decays in which a Higgs boson produces two Z bosons, which in turn each decay into a pair of electrons or muons, and found the spin states of the resulting Z boson pairs were correlated in a way consistent with entanglement rather than chance.

The ATLAS measurement, published in Physical Review Letters on September 20, drew on proton-proton collisions recorded at center-of-mass energies of 13 and 13.6 trillion electron volts, with protons traveling at 99.99 percent the speed of light before impact. Researchers rejected the non-entangled hypothesis at 4.7 standard deviations, just shy of the 5-sigma threshold conventionally used to claim a formal discovery, but described as strong evidence nonetheless. Unlike the top quarks in which ATLAS and CMS first observed collider entanglement in 2023, Z bosons can occupy three spin states rather than two, making the measurement technically more demanding and the result, researchers say, a more stringent test of quantum mechanics at the electroweak energy scale.

"Finding it alive and well among particles as heavy and short-lived as Z bosons ... shows just how fundamental and robust this quantum effect really is," said Alan Barr, a physics professor at the University of Oxford who helped design the analysis technique. Fellow Oxford physicist Chris Timpson, who studies the philosophy of quantum mechanics, called entanglement "both the most promising and the most puzzling aspect of quantum reality." Researchers say the findings, reported by ScienceDaily and other outlets, have no immediate practical application but extend collider tests of Einstein's "spooky action at a distance" into an energy regime relevant to future quantum-computing and secure-communications research, which depend on entanglement persisting under extreme conditions.

To detect the effect, ATLAS physicists reconstructed the angles at which the four leptons emerged from each collision and used them to infer the spin orientation of the parent Z bosons, which exist for only a fraction of a trillionth of a second before decaying. The team measured two angular coefficients that describe correlations between the bosons' spins; both were statistically consistent with the entangled values the Standard Model predicts and inconsistent with a scenario in which the particles' spins were independent of one another. The measurement combined data collected during the LHC's second operating run, when collisions reached 13 trillion electron volts, with three additional years of higher-energy 13.6 TeV data gathered between 2022 and 2024.

The result builds directly on a 2023 finding, also from ATLAS and CMS, that top quarks produced in pairs at the LHC exhibit entanglement — at the time the highest-energy system in which the phenomenon had been confirmed. Z bosons, which along with the W boson carry the weak nuclear force responsible for radioactive decay, presented a harder test because their three possible spin states make the relevant calculations more complex than the two-state system of the top quark. CERN said the CMS collaboration independently corroborated the ATLAS result using its own dataset, giving physicists two separate measurements pointing to the same conclusion.