Observation of Quantum Entanglement With Top Quarks at the Atlas Detector

Abstract

Entanglement is a key feature of quantum mechanics(1-3), with applications in fields such as metrology, cryptography, quantum information and quantum computation(4-8). It has been observed in a wide variety of systems and length scales, ranging from the microscopic(9-13) to the macroscopic(14-16). 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 root s=13TeV and an integrated luminosity of 140inverse 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=-0.5370.002 (stat.)+/- 0.019 (syst.) for 340 GeV < m(t<(t)over bar>) 380 GeV. 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.