Observation of quantum entanglement in top-quark pairs using the ATLAS detector

Abstract: Entanglement is a key feature of quantum mechanics, with applications in fields such as metrology, cryptography, quantum information, and quantum computation. 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 center-of-mass energy of $\sqrt{s}=13$ TeV and an integrated luminosity of 140 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, where 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 limitations of the Monte Carlo event generators and the parton shower model in modeling top-quark pair production. The entanglement marker is measured to be $D=-0.537 \pm 0.002~\text{(stat.)} \pm 0.019~\text{(syst.)}$ for $340 < m_{t\bar{t}} < 380$ GeV. The observed result is more than five standard deviations from a scenario without entanglement and constitutes the first observation of entanglement in a pair of quarks and the highest-energy observation of entanglement so far.

Observation of Quantum Entanglement with Top Quarks at the ATLAS Detector

The paper presents an investigation into quantum entanglement at the highest energy scales accessible through laboratory experiments. It reports the observation of quantum entanglement in top--antitop quark pairs produced at the Large Hadron Collider (LHC) using data from the ATLAS detector. This study provides a significant contribution to the understanding of quantum mechanics by exploring entanglement in a novel high-energy regime.

Measurement of Spin Entanglement

The central focus of the paper is the measurement of spin entanglement in top--antitop quark pairs. The experiment utilizes a proton-proton collision dataset at a center-of-mass energy of $\sqrt{s}=13 \text{ TeV}$ with an integrated luminosity of 140 fb$^{-1}$. An observable $D$, derived from the angular correlation between charged leptons in the rest frames of their parent quarks, serves as the marker for entanglement. Entanglement is expected to be significant near the production threshold of top--antitop quarks, specifically when the invariant mass $\mttbar$ lies within $340 < \mttbar < 380 \text{ GeV}$.

The fiducial phase space defined using stable particles minimizes uncertainties originating from the Monte Carlo simulations and parton shower models. The results demonstrate spin entanglement at more than five standard deviations from the non-entangled scenario, marking the first observation of such phenomena in quarks at this energy level. The measured entanglement marker $D$ is $-0.537 \pm 0.002 \text{ (stat.)} \pm 0.019 \text{ (syst.)}$, corroborating the theoretical predictions of the Standard Model (SM).

Implications and Future Directions

This observation posits the LHC as a unique laboratory for investigating quantum information processes intertwined with relativistic quantum mechanics, offering new possibilities for understanding quantum effects in high-energy environments. The entanglement of top-quark pairs, due to their distinct properties like extremely short decay lifetimes and lack of hadronization, provides a pseudo-bare quark scenario where quantum numbers are preserved, allowing precise measurement of spin correlations transferred to decay products.

The observed phenomenon invites further exploration into quantum information concepts at high-energy colliders, such as measuring quantum discord and reconstructing steering ellipsoids. It also opens avenues for searching physics beyond the SM by developing novel theoretical approaches using QM foundations.

Conclusion

The paper successfully establishes the presence of quantum entanglement in top-quark pairs, providing experimental confirmation at unprecedented energy levels. The research paves the way for high-energy colliders to become potent tools for studying fundamental quantum mechanics and quantum information problems, potentially influencing the future of both precision quantum measurement techniques and searches for new physics.