Higgs-decay Z boson pairs show strong evidence of entanglement

Reconstructing Z-boson spins from about 400 events

According to the report, researchers reconstructed the spins of Z bosons from four resulting leptons in roughly 400 Higgs-to-two-Z decay events. Their most sensitive test favored an entangled state over a non-entangled alternative at 4.7 sigma, below the conventional 5-sigma discovery threshold. The article says at least one Z boson in each pair must be virtual because of the Higgs boson's mass.

At the LHC, quantum entanglement had previously been observed in pairs of top quarks, according to the article.

Strong evidence below the five-sigma threshold

The reported evidence shifts entanglement measurements to Z-boson pairs produced by Higgs-boson decay, including elementary particles with three spin states.

The report presents the first entanglement measurement involving elementary qutrits—particles with three possible spin states—rather than two-state qubits. It also suggests that a virtual Z can carry spin information participating in entanglement, without settling what virtual particles physically represent.