Quantum correlations in spin chains

In this work, we address the question of how quantum is a collection of qubits, such as a chain of spins, a two-mode Bose-Einstein condensate, or a multiphoton state. We demonstrate that a single element of the density matrix carries the answer. Properly analyzed it brings information about the extent of the many-body entanglement and the nonlocality. This method can be used to tailor and witness highly nonclassical effects in many-body systems with possible applications to quantum computing, ultraprecise metrology, or large-scale tests of quantum mechanics. As a proof of principle, we investigate the extent of nonlocality and entanglement in ground states and thermal states of experimentally accessible interacting spin chains.