CERN finds quantum entanglement among heaviest particles ever tested

Physicists working with the ATLAS detector at CERN's Large Hadron Collider have found strong evidence of quantum entanglement between pairs of Z bosons produced in the decay of Higgs bosons, extending one of quantum mechanics' most puzzling phenomena to the heaviest and shortest-lived particles ever tested. The results, published in Physical Review Letters on September 11, 2026, represent one of the highest-energy confirmations of entanglement on record.phys+1

The ATLAS Collaboration analyzed proton-proton collision data at energies of 13 and 13.6 trillion electron volts, hunting for the rare decay chain in which a Higgs boson splits into two Z bosons, each of which then decays into a pair of electrons or muons. Because Z bosons vanish in roughly 3 × 10⁻²⁵ seconds, physicists could not measure their spins directly. Instead, they reconstructed the angles at which the four resulting leptons traveled through the detector, working backward to infer the quantum states of the parent Z bosons.sciencealert+3

Qutrits, Not Qubits

Unlike the top-quark entanglement the ATLAS team demonstrated in a 2024 Nature paper, Z bosons carry three possible spin states rather than two, making each one a "qutrit" in the language of quantum information. Physicist Juan Antonio Aguilar-Saavedra of the Institute of Theoretical Physics in Spain noted that "the spins of the two Z bosons are extremely entangled, considerably more so than in the top-antitop case that was measured previously".nature+1

The team's most sensitive test favored the entangled state over the non-entangled alternative with a statistical significance of 4.7 sigma — just short of the 5-sigma threshold traditionally required to claim a discovery, but strong enough to constitute robust evidence.sciencealert+1

A 'Virtual Duck'

An unusual wrinkle emerged from the mass arithmetic. A Higgs boson weighs about 125 GeV, while each Z boson has a mass near 91 GeV, meaning there is not enough energy for two ordinary Z bosons to appear simultaneously. At least one must be "virtual" — a fleeting quantum fluctuation that cannot be directly observed. The new result suggests that even a virtual Z boson can participate in entanglement, raising philosophical questions about the nature of such particles.sciencealert+1

> "If it walks like a duck and quacks like a duck, then what we're looking at must be a duck. In this case, a virtual duck," Aguilar-Saavedra said.sciencealert

Testing Quantum Mechanics at the Extremes

With only about 400 qualifying collision events drawn from years of data, the measurement pushed the limits of what the LHC can extract from its rarest processes. Oxford physicist and study co-author Alan Barr, who first proposed using particle colliders to probe entanglement at high energies, called the finding "a nice reminder that the same strange rules of quantum mechanics that may one day power quantum computers are at work everywhere in nature, even at the extreme energies of the Large Hadron Collider".phys+1

An upgrade now underway on both the ATLAS detector and the High-Luminosity LHC is expected to deliver far larger datasets, opening the door to crossing the 5-sigma threshold and probing quantum phenomena with still greater precision.phys