No abstract

Three different quantifiers, entanglement of formation (ENT), quantum discord (QD), and measurement-induced disturbance (MID), are used to measure the quantum correlations of two qubits in a common squeezed bath. A subspace was found for initial conditions in a squeezed bath, where the system experiences no decoherence. We relate the three measurements with the "distance" from the initial condition to the decoherence free subspace, in order to study the effect of the decoherence in the quantum correlations. We show examples of a system with quantum correlations even when entanglement is null. Furthermore, we study the necessary conditions for the system to become truly classical. We found that, under certain initial conditions and at specific times, the system becomes classical and both the QD and the MID vanish, thus observing the phenomena of sudden death and revival of the quantum correlations. Finally, we observe discontinuities in the QD.

It is usually believed that, in the macroscopic limit, coarse-graining of measurements leads to classical behavior and, in particular, that coarse-graining of quantum correlations leads to local realistic correlations, a principle known as macroscopic locality. We show that, in the most general case, the Hilbert space structure of quantum theory can be preserved in the macroscopic limit. This leads to a violation of a Bell inequality for coarse-grained collective measurements, thus breaking the principle of macroscopic locality.