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In most of quantum information protocols, entanglement or, in general expression, quantum correlation, plays the major role. So it is very important to know the ways of generating entanglement and protecting it against noise.
In a general view, environment is usually known as a destructive agent for entangled states. Being exposed to environment, non-diagonal elements of an entangled state’s density matrix would tend to zero, because of decoherence effect. Thus, the entanglement would be disappear. But, recent studies have shown that, this destructive agent can be convert to a useful agent for generating entanglement between system components.
In this thesis, at first, the... 

Composite quantum systems are generally correlated and these correlations may have classical or quantum nature. Entanglement is only a special type of quantum correlations and separable states may be quantum correlated. Quantum discord, local quantum uncertainty and the rank of the correlation matrix are the most important measures of quantum correlations beyond entanglement. Regarding the various applications of discordant separable states in the field of quantum information, the important problem is how to produce discordant separable states and what is the resource which should be used for this preparation. In this thesis we present a method of preparation for such states in two qubit... 

In recent studies, it has been shown that entanglement is not the only kind of genuinely quantum correlation. There exist models of quantum computation that provide exponential speed up over the best known classical algorithms and don’t have any quantum entanglement. These models show that in some cases separable state can be very useful in quantum computing models. Look for a property that causes the performance of these scenarios, quantum correlation being one of the most important concepts, including but not limited to entanglement. Local noise can produce quantum correlations on an initially classically correlated state, provided that it is not represented by a unital or semiclassical... 

Entanglement is a type of quantum correlation that plays a crucial role at the heart of almost every quantum algorithm which has led them to outperform their classical counterparts. Recently, a new type of correlation has been introduced. Namely, ”Quantum Discord”. Which is different from the entanglement in the essence, nonetheless it has caused some quantum algorithms to outperform their classical counterparts. Since, creating and maintaining this states is easier than entangled states, they have draw many interests. In this thesis, we review and investigate the use of ”Quantum Discord” for transferring quantum information, in particular remote preparation of quantum states and Remote... 

Two qubit density matrices which are of X-shape, are a natural generalization of Bell Diagonal States recently simulated on the IBM quantum device. We propose a quantum circuit for simulation of a general X-state on the same quantum device and study its properties for several values of the extended parameter space. We also show by specific measurements, that their X-shape is robust against noisy quantum gates. To further physically motivate this study, we invoke the two-spin Heisenberg XYZ system and show that for a wide class of initial states, it leads to dynamical density matrices which are X-states. Due to the symmetries of this Hamiltonian, we show that by only two qubits, one can... 

Quantum thermodynamics is a new field of study in physics, which by considering the fundamentals of statistical mechanics, open quantum system, quantum information and mesoscopic system aims at the investigation of quantum systems from a thermodynamical point of view. To achieve these aims, a better understanding of the relations between quantum and thermodynamics fundamentals is essential. Energy and correlation are two key concepts in statistical mechanics and quantum thermodynamics (and information), respectively, that studying the relation between them will shed light on the fundamentals of quantum thermodynamics. As generating any correlation in quantum systems with initial heat state... 

Communication is nothing but making correlation between sent and received data via mediating particles and in quantum communications these are the quantum carriers that create these correlations. Since quantum carriers can be correlated in ways inaccessible to classical carriers, quantum communication protocols can perform tasks that are classically impossible or less efficient. Therefore, it is expected that in a suitable model, the quantum advantage of any quantum communication protocol can be expressed in terms of the quantum correlations of its carriers. Here we consider two quantum communication protocols, namely remote state preparation (RSP) and quantum locking of classical... 

In this thesis, first we study superconductivity and its mechanisms then classify different types of that based on symmetry of gap function. After that, we shall discuss metallic systems that have impurities in them and propose two successful model describing this phenomenon, Anderson’s model and Kondo’s model. In the next step, by introducing Kondo lattice model which is an ordered array of impurities in system, try to solve it by mean-field approximation and will be seen a phase transition in a characteristic temperature known as Kondo’s temperature. Then generalize these models to situation when background system is a superconductor and study it’s behavior and quantum phases. Although...