  The present thesis shows that Quantum Information concepts can be used to
better understand the quantum-to-classical boundary in mesoscopic and
macroscopic systems. Our findings suggest a way to push this boundary towards
the macroscopic domain by coupling a moveable mirror to a confined
quasi-classical electromagnetic field (Chapters 2 and 3), and opens new
possibilities towards quantum computation and information processing with
strongly-correlated systems at realistic temperatures by demonstrating the
opening of robust gaps in 2D antiferromagnetic lattices due to the presence of
additional spin-1/2 probes (Chapters 4 and 5).
