Researchers using the ATLAS detector at CERN's Large Hadron Collider (LHC) have achieved a breakthrough: the first observation of quantum entanglement in top quark pairs. This discovery, analyzing proton-proton collisions at incredibly high energies, marks a significant step forward in understanding the quantum world at high energies.

Unveiling the Entangled Dance

By examining collisions with an energy of 13 TeV, with an integrated luminosity of 140 fb⁻¹, the team measured the spin correlation of the top quarks' decay products (charged leptons). The results demonstrated significant entanglement, particularly at high ttˉ masses, validating quantum mechanical predictions. This correlation reveals the entangled nature of these particles, where their properties are linked even when separated by vast distances. The results were statistically significant, exceeding five standard deviations, solidifying the groundbreaking discovery.

Why Top Quarks are Special

The unique properties of top quarks were crucial. Their immense mass and short lifespan prevent their quantum properties from being obscured by strong interactions. This makes them ideal candidates for studying entanglement in high-energy collisions, free from the influence of colour confinement (a fundamental property of quarks). The analysis focused on top quark decays involving bottom quarks and W bosons, with W bosons further decaying into leptons. The angle between these charged leptons, when viewed from the perspective of their parent top quarks, served as a key signature of entanglement. Additionally, researchers employed a specialized approach (fiducial phase space) to minimize uncertainties arising from simulations and models.

Concurrent Research from CMS

At the LHCP conference two weeks ago, CMS also presented a result related to this topic, documented in the CMS PAS note released on June 13th. The CMS Collaboration at CERN measured polarization, spin correlations, and entanglement in top quark pairs using proton-proton collision data at 13 TeV, with 138 fb⁻¹ of integrated luminosity. They simultaneously extracted all coefficients of polarization vectors and the spin correlation matrix via a binned likelihood fit. The study found significant evidence for entanglement in top quark pairs, particularly at high masses of the ttˉ system, with observed significances exceeding five standard deviations, confirming quantum mechanical predictions of entanglement in high-energy particle collisions. This highlights the collaborative and competitive nature of research in particle physics, where multiple teams often work on similar groundbreaking discoveries.

A Quantum Future

This observation opens doors to exploring more complex quantum phenomena using high-energy colliders. Phenomena like quantum discord and steering ellipsoids can now be investigated with greater precision. This achievement highlights the potential of particle colliders as powerful tools for advancing our knowledge of quantum mechanics and information theory. The successful observation of entanglement in top quarks underscores the increasingly intertwined nature of particle physics and quantum information science. This discovery demonstrates the immense contribution of high-energy physics experiments to our fundamental understanding of the quantum world.

For further details, refer to the ATLAS paper here and the CMS note here