Bell correlations of a thermal fully-connected spin chain in a vicinity of a quantum critical point

Abstract: Bell correlations are among the most exotic phenomena through which quantum mechanics manifests itself. Their presence signals that the system can violate the postulates of local realism, once believed to be the nonnegotiable property of the physical world. The importance of Bell correlations from this fundamental point of view is even straightened by their applications -- ranging from quantum cryptography through quantum metrology to quantum computing. Hence it is of growing interest to characterize the ``Bell content'' of complex, scalable many-body systems. Here we perform the detailed analysis of the character and strength of many-body Bell correlations in interacting multi-qubit systems with particle-exchange symmetry. Such configuration can be mapped onto an effective Schr\"odinger-like equation, which allows for precise analytical predictions. We show that in the vicinity of the quantum critical point, these correlations quickly become so strong that only a fraction of qubits remains uncorrelated. We also identify the threshold temperature, which, once overpassed, empowers thermal fluctuations that destroy Bell correlations in the system. We hope that the approach presented here, due to its universality, could be useful for the upcoming research on genuinely nonclassical Bell-correlated complex systems.