This PhD project aims to explore the quantum nature of top-quark pair production at the Large Hadron Collider by studying spin correlations and entanglement-related observables in data recorded by the ATLAS experiment. The recent breakthrough observations of entanglement in top-antitop events have opened an entirely new window onto the quantum structure of fundamental interactions, transforming the LHC into a machine to test quantum information at the TeV scale. Building on this momentum, the thesis will focus on reconstructing the quantum state of top-quark pairs using ATLAS Run-3 data, with particular attention to the extraction of spin correlations and entanglement-sensitive observables in challenging high-momentum topologies. By improving reconstruction strategies and carefully assessing detector effects, the aim is to measure quantum properties with good precision and to contribute to understand what quantum information can bring us to our understanding of elementary particles.