Researchers at the ATLAS Collaboration at CERN have detected the first direct evidence of quantum entanglement between qutrits—three-state quantum systems—according to a paper published in Physical Review Letters on September 11. The measurement comes from examining the rare decay of a Higgs boson into two Z bosons and tracking the four leptons subsequently produced.
Unlike previous entanglement observations in particle physics, which involved qubits (two-state systems), this result involves Z bosons, which are spin-1 particles with three possible spin orientations. “The spins of the two Z bosons are extremely entangled, considerably more so than in the top-antitop case that was measured previously,” said physicist Juan Antonio Aguilar-Saavedra of Spain’s Institute of Theoretical Physics, who developed the theoretical framework underlying the measurement.
The dimensional difference is significant. A pair of qubits exists in a four-dimensional quantum state space, while a pair of qutrits inhabits a nine-dimensional one. The constraint that the Higgs boson—which has no spin—cannot transfer spins that sum to anything other than zero forces the Z bosons into a shared quantum state, producing the entanglement.
The result carries a philosophically unsettling implication: at least one of the two Z bosons must be “virtual.” The Higgs boson weighs approximately 125 GeV, while a Z boson weighs approximately 91 GeV. Producing two fully real Z bosons would require at least 182 GeV of energy—more than available. Virtual particles exist only transiently and cannot be directly observed as free particles, with their physical status long debated among physicists.
The ATLAS team’s measurement shows the entanglement signal involves the joint spin state of both bosons—real and virtual—as a single correlated system. “If it walks like a duck and quacks like a duck, then what we’re looking at must be a duck,” Aguilar-Saavedra said. “In this case, a virtual duck.” He cautioned the result does not fully resolve whether virtual particles genuinely exist or are merely computational constructs, but may “shed a little light” on the question.
The measurement was extraordinarily demanding. Z bosons decay in approximately 3 × 10⁻²⁵ seconds. The ATLAS team searched for Higgs decays to two Z bosons, each decaying to a lepton pair, yielding four charged leptons—a channel representing only about 3 percent of Higgs decays but fully reconstructible by detectors. From hundreds of quadrillions of proton-proton collisions across multiple datasets, only approximately 400 events met all selection criteria.
Using angular distributions of the four leptons, researchers reconstructed the spin-density matrix and compared it against entangled and non-entangled hypotheses. The non-entangled state was disfavored with statistical significance of 4.7 standard deviations—strong evidence but below the 5 sigma threshold conventionally required for formal discovery claims. The result matched predictions of 4.9 sigma given the dataset size.
Key facts
- First observation of quantum entanglement between qutrits (three-state quantum systems) in particle physics
- Measurement involved Z bosons from Higgs decay, representing higher-dimensional quantum state space (nine dimensions) than previous qubit entanglement observations
- At least one Z boson involved was virtual—an off-shell particle existing only transiently and not directly observable
- Result achieved 4.7 sigma statistical significance, just below the 5 sigma threshold for formal discovery
- Analysis used approximately 400 usable events selected from hundreds of quadrillions of proton-proton collisions