Three dancers move as one: each feels all the others, and the overall harmony cannot be reduced to individual interactions. This is roughly how three particles behave, born from an electron and positron collision at the speed of light. They are a top quark, its antiparticle, and a Z boson. Their spins, the quantum analog of rotation, become entangled in a trio, forming a single system predicted by the Standard Model.
Usually, quantum entanglement is studied in pairs, as in the experiments of John Bell. But here, physicists calculated that pairwise connection is suppressed, while the collective one, each particle with the other two, is much stronger. For evaluation, they used measures akin to entropy, but for quantum information. The results showed that the triple pattern can be reconstructed from future collider data.
Such experiments are not just fun. They test quantum mechanics at extreme energies, where hints of new physics might be hiding. Reliable measurement requires large statistics, but even the first step promises a sensation.
The top quark is the heaviest elementary particle; due to its mass, its spin hardly loses quantum information, making it an ideal candidate for entanglement.