Investigation of quantum discord in qantum spin systems

Quantum mechanics is one of the most important scientific developments of the twentieth century. It constitutes a theoretical basis for a very broad and impressive application area in terms of its structure. The developing quantum information theory, on the other hand, takes its first steps in going beyond the existing technologies with the tools it puts forward today. Entanglement, a special correlation shared between quantum states, is a resource that plays a fundamental role in the realization of many information processes, such as quantum teleportation, super-dense coding, and quantum cryptology. These operations cannot be performed without entanglement. Besides, entanglement does not constitute the most general form of quantum correlations. Even the non-entangled states of combined systems can exhibit quantum properties as they share quantum correlations between their subsystems. This is an important result because entanglement is difficult to create and maintain in many applications. Indeed, entanglement deteriorates very quickly due to the destructive effects of the environment. Therefore, alternative sources are needed. Considering these, it is important to determine whether a quantum state of two or more parts is entangled, separable, or quantum correlated. Quantum discord is also a nonlocal quantum correlation different from entanglement. Heisenberg spin systems exhibit entanglement properties. For this purpose, it is important to examine also the quantum discord in these systems. Therefore, in this thesis, we analyzed the quantum discord in the four-qubit J1-J2 Heisenberg spin system according to the anisotropy parameters we determined as control parameters in the system for both the ground states and the thermal states, and the coupling constant between the next nearest neighbors. We conclude that quantum discord is an alternative source such as entanglement.