Observation of quantum entanglement with top quarks at the ATLAS detector

Entanglement is a key feature of quantum mechanics, with applications in fields such as metrology, cryptography, quantum information, and quantum computation. It has been observed in a wide variety of systems and length scales, ranging from the microscopic to the macroscopic. However, entanglement remains largely unexplored at the highest accessible energy scales. Here we report the highest-energy observation of entanglement, in top$-$antitop quark events produced at the Large Hadron Collider, using a proton-proton collision dataset with a center-of-mass energy of sqrt(s) = 13 TeV and an integrated luminosity of 140 fb^-1 recorded with the ATLAS experiment. Spin entanglement is detected from the measurement of a single observable D, inferred from the angle between the charged leptons in their parent top- and antitop-quark rest frames. The observable is measured in a narrow interval around the top-antitop quark production threshold, where the entanglement detection is expected to be significant. It is reported in a fiducial phase space defined with stable particles to minimize the uncertainties that stem from limitations of the Monte Carlo event generators and the parton shower model in modeling top-quark pair production. The entanglement marker is measured to be D=-0.537 +- 0.002(stat.) +- 0.019(syst.) for 340 < m_{ttbar} < 380 GeV. The observed result is more than five standard deviations from a scenario without entanglement and constitutes the first observation of entanglement in a pair of quarks and the highest-energy observation of entanglement so far.