Amazing stuff! It seems this discovery is largely based on a retrospective study of historical data.
"... Entanglement has been observed in electrons, streams of photons, molecules and even across many atoms.
The new study, published in Nature, reports the highest energy observation of entanglement.
It uses data from the Large Hadron Collider (LHC) ATLAS collaboration. The results were later confirmed by another experiment using the CMS (Compact Muon Solenoid) detector at the LHC. ..."
"... The ATLAS and CMS teams observed quantum entanglement between a top quark and its antimatter counterpart. The observations are based on a recently proposed method to use pairs of top quarks produced at the LHC as a new system to study entanglement.
The top quark is the heaviest known fundamental particle. It normally decays into other particles before it has time to combine with other quarks, transferring its spin and other quantum traits to its decay particles. Physicists observe and use these decay products to infer the top quark’s spin orientation.
To observe entanglement between top quarks, the ATLAS and CMS collaborations selected pairs of top quarks from data from proton–proton collisions that took place at an energy of 13 teraelectronvolts during the second run of the LHC, between 2015 and 2018. In particular, they looked for pairs in which the two quarks are simultaneously produced with low particle momentum relative to each other. This is where the spins of the two quarks are expected to be strongly entangled.
The existence and degree of spin entanglement can be inferred from the angle between the directions in which the electrically charged decay products of the two quarks are emitted. By measuring these angular separations and correcting for experimental effects that could alter the measured values, the ATLAS and CMS teams each observed spin entanglement between top quarks with a statistical significance larger than five standard deviations. ..."
From the abstract:
"Entanglement is a key feature of quantum mechanics, with applications in fields such as metrology, cryptography, quantum information and quantum computation4. 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 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 \(340\,{\rm{GeV}} < {m}_{t\bar{t}} < 380\,{\rm{GeV}}\). 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."
LHC experiments at CERN observe quantum entanglement at the highest energy yet (original news release) "The results open up a new perspective on the complex world of quantum physics"
Fig. 1: Detector-level results.
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