Two spinning tops are set off simultaneously in opposite directions. If one spins left, the other always spins right — no matter how far apart they fly. That's roughly how quantum entanglement works, except instead of tops, we have top quarks, the heaviest known particles. At the Large Hadron Collider, they are produced in pairs by accelerating protons to near-light speeds.
A top quark lives for less time than it takes to reach a neighboring atom. It decays on the fly, and from the debris you can tell which way its spin, the particle's intrinsic rotation, was pointing. Physicists studied thousands of such decays in the ATLAS and CMS detectors. It turned out that the spins of two top quarks from the same pair are linked more strongly than ordinary logic allows: knowing the spin of one, you know exactly the spin of the other, even if they are already far apart. That is quantum entanglement.
This behavior is not a trick but a direct consequence of quantum mechanics. It means that nature, at its deepest level, is not a collection of separate objects but a single web of connections. And the heavier the particles, the more astonishing it is to see these connections in action.
Now the two spinning tops from the opening picture are no longer just a pair of toys. They are two top quarks that were born together and remain linked forever — even when they fly off to opposite ends of the detector.
as written by the authors arXiv:2601.04649
The Large Hadron Collider has delivered exceptionally large proton-proton collision datasets at centre-of-mass energies of up to 13.6 TeV. These datasets, collected by the ATLAS and CMS detectors, enable precision tests of theoretical predictions using extensive samples of top-quark events. A key example is the study of top-quark pair spin correlations, which can be accessed through the angular distributions of the top-quark decay products owing to the unique property that the top quark decays before it can hadronise. This wealth of data has also enabled new measurements of top-quark pair properties, particularly those with enhanced sensitivity in the threshold region, such as quantum entanglement, that were previously out of reach. In this contribution, the latest highlights in this area from the ATLAS and CMS experiments, are presented.
The top quark is so heavy that its mass is roughly equal to that of a gold atom, even though it is billions of times smaller than an atom.