The ATLAS Collaboration has recently announced the first observation of quantum entanglement in top quark pair production at the Large Hadron Collider (LHC). This work briefly reviews the experimental status of top quark pair spin correlations and highlights the distinctions between classical and quantum correlations. A proposed criterion for detecting spin entanglement in top quark pairs is outlined, along with the experimental strategy to observe this effect, culminating in the positive results of the ATLAS measurement.
Entanglement is a key feature of quantum mechanics1,2,3, with applications in fields such as metrology, cryptography, quantum information and quantum computation4,5,6,7,8. It has been observed in a wide variety of systems and length scales, ranging from the microscopic9,10,11,12,13 to the macroscopic14,15,16. 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 centre-of-mass energy of s=13 TeV and an integrated luminosity of 140 inverse femtobarns (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, at which 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 the limitations of the Monte Carlo event generators and the parton shower model in modelling top-quark pair production. The entanglement marker is measured to be D=−0.537±0.002 (stat.)±0.019 (syst.) for . The observed result is more than five standard deviations from a scenario without entanglement and hence constitutes the first observation of entanglement in a pair of quarks and the highest-energy observation of entanglement so far.