Physicists detect quantum entanglement in top quarks at record energy

<p>Entanglement is a key feature of quantum mechanics^1,2,3, with applications in fields such as metrology, cryptography, quantum information and quantum computation^4,5,6,7,8. It has been observed in a wide variety of systems and length scales, ranging from the microscopic^{9,10,11,12,13} to the macroscopic^14,15,16. 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 √<em>s</em> = 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 <em>D</em>, 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 <em>D</em> = −0.537 ± 0.002 (stat.) ± 0.019 (syst.) for 340GeV&lt;mt¯t&lt;380GeV. 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.</p>