Quantum entanglement observed in top quarks — published in Nature¶
Our paper on the first observation of quantum entanglement with top quarks has been published in Nature. This is the highest-energy observation of entanglement to date.
The key result: using 140 fb\(^{-1}\) of 13 TeV ATLAS data, we measured the entanglement marker \(D = -0.537 \pm 0.002\,(\text{stat.}) \pm 0.019\,(\text{syst.})\) in the \(t\bar{t}\) threshold region (\(340 < m_{t\bar{t}} < 380\) GeV). This is more than five standard deviations from a scenario without entanglement.
The measurement uses the angle between the charged leptons in their parent top- and antitop-quark rest frames — a single observable that is sensitive to the spin entanglement of the \(t\bar{t}\) pair.
Why does this matter?¶
Top quarks are unique: they are the only quarks that decay before hadronisation, so their spin information is directly accessible. This makes \(t\bar{t}\) production a natural laboratory for testing quantum mechanics at the TeV scale.
Our result demonstrates that quantum correlations survive at energies far above the scale of everyday quantum experiments. It opens a new programme of quantum information measurements at the LHC, including tests of Bell inequalities and studies of entanglement in the Higgs sector.
What's next?¶
We are now extending this work in two directions:
- Bell inequalities in \(t\bar{t}\) — can we rule out local hidden-variable theories at collider energies?
- Higgs entanglement — the \(H^* \to ZZ\) decay channel provides access to entanglement in the bosonic sector.
Both are part of the quantumTANGO project, a collaboration between CERN, Glasgow, and Göttingen.
Links: arXiv:2311.07288 · ATLAS Briefing · CERN Courier