Sharif Digital Repository

We are interested in the relation between entanglement and quantum phase transitions. We have presented a formulation on how the entanglement of a very large the system is related to the entanglement of a small part of system. The renormalization of entanglement is an indicator of the critical behavior of the models. The framework of our approach is introduced about the Ising model. In particular, we show that derivative of entanglement developes a minimum close to the critical point. We further clarify that this minimum point approaches to the exact critical point of the system as the system becomes thermodynamic. This phenomenon is governed by an exponent that is closely related to the... 

One of the problems that we are deal with in quantum information is secure transition of information. In Quantum State Sharing, we can encode our data on the quantum state and share the quantum state among some people such that none of the can be able to retrieve the state without collaboration with others. In this thesis, we will be concerned with reviewing of classical and quantum secret sharing and finally introduce another method in order to share a two qubit secret between N parties such any of the members can retrieve the state only with collaboration with other parties. It will be shown that using only Bell Pairs as recourse and measurement basis makes this model more efficient than... 

Density matrix of graphs as defined -for the first time- in [S. Braunstein and et al. The laplacian of a graph as a density matrix, Annals of Combinatorics, (2006)], is obtained through dividing the Laplacian matrix by the degree sum. This matrix is also semi-positive and trace one. Therefore one may talk about the Von Neumann entropy of this matrix. In [F. Passerini, S. Severini. Quantifying complexity in networks: The Von Neumann entropy. IJATS, (2009)], authors have claimed that this quantity can be consisered as a measure of regularity. Here, using a geometric interpretation of Von Neumann entropy, expresed in [G. Mitchison, R. Jozsa, Towards a geometrical interpretation of quantum... 

Coupling of optical and mechanical degrees of freedom through radiation pressure was, first, observed in the experiments for detection of gravitational waves. Recent experimental advances have granted reality to the long-hankered-after coupling between microscopic and macroscopic systems. In the first step, this thesis will review different aspects of cavity Optomechanics, such as: cooling, side-band formation and dynamics of an Optomechanical cavity. Then we will go further and study the Optomechanical dynamics of a cavity in the presence of an atom. To do so, we will analyze the entanglement, created between the center of mass motion of the atom and the mirror and will probe into its... 

Nitrogen-Vacancy (NV) consists of a substitutional nitrogen located at a lattice site adjacent to a carbon vacancy. It has attracted much attention due to its application in solid state quantum information processing. Its long coherence time and ability to be used in ambient temperature made it a major candidate for quantum bit (Qbit). Due to its importance, a lot of theoretical studies for the NV’s properties based on the DFT and other single particle approaches have been done, while many-body approaches are not expanded so. For the first time, a generalized Hubbard Hamiltonian was used as a many-body approach to study NV defects. The obtained wavefunctions and states result to good... 

Entanglement is the most important quantity in quantum information processing tasks. However the provision of the required entanglement between various quantum parts is one of the most challenging breakthroughs in realization of quantum processors. One of the scenarios for entangling various parts is use of naturally interacting qubits, called “spin chains”. The low level of required control and the ability of entangling mesoscopicaly separated parts cause such systems to be attractive for using in future low dimensional silicone compatible technology quantum processors. Accordingly in this thesis by considering a ferromagnetic spin system in which is conserved and by initializing the... 

Since concepts in quantum information theory has recently been applied to characterize states in many-body systems, we survey entanglement of the ground states in the S = 1/2 antiferromagnetic Heisenberg ladder systems and 2D square lattices. The Lancz¨os method for exact diagonalization of quantum spin models are used to find the ground states. Moreover, effect of defect in ladder on entanglement and ground state energy are studied  

In this thesis, we have implemented the quantum renormalization group method to study quantum spin systems. We reviewed the recent progress to calculate the quantum information properties of these models. The recent investigations show that concurrence and entanglement are two quantities which can show quantum critical properties of the quantum systems. More specifically, the derivative of entanglement and the second derivative of ground state energy show divergent behavior at the quantum critical point. Moreover, the ground state fidelity for two different parameters close to quantum critical point shows a drop which leads to a divergent behavior in its corresponding susceptibility. We have... 

In this M.Sc. thesis the behavior patterns of the cavity quantum electrodynamics of complex systems are analyzed, such systems are quantum optic multi-partite systems and consist of an arbitrary number of quantum dots in interaction with arbitrary number of cavity modes.
First, the coefficients matrix of the ket sate of the system was measured, in order to achieve it; the general time-dependent state of the most general possible system was specified. Its related Hamiltonian was written, and finally the Schrodinger equation was measured in different coupling regimes. It was done in Schrodinger picture without any approximation.Second, the presence probabilities of the quantum dots of any... 

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... 

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... 

Entanglement is an immediate consequence of quantum mechanical pos- tulates and is thus an indispensable part of studying quantum systems. A very useful quantity which can appropriately quantify entanglement between the degrees of freedom of a composite quantum system is En- tanglement Entropy (EE).
Over the last few years, this subject has attracted a lot of interest both in QFT and in the context of holography. On the one hand, an intu- itive geometrical Holographic model for entanglement entropy has been proposed in the context of gauge/gravity duality, and on the other, var- ious issues concerning entanglement have been explored, e.g. in quan- tum computation, quantum information... 

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... 

Two major category of protocols suggested for quantum key distribution (QKD) are prepare and measure protocols and entanglement based protocols. One of the most challenging problem of these two categories is that they are not immune against attacks to measurement devices. For addressing this problem, measurement device independent protocol (MDI-QKD) was proposed in 2012; in which measurement device can be in hands of untrusted party. The idea of MDI-QKD is based on time-reversed EPR protocol. Still low secure key exchange rate has provoked many efforts, one of these efforts was to improve the key rate by means of quantum memories. In this thesis different proposed structures for memory... 

In this investigation has been tried to indagate the Bell’s inequality both theoretically and experimentally. This inequality published by John Bell in 1964 on the EPR argument. In the EPR argument they reasonably deduced that quantum mechanics with the wave function interpretation is incomplete (but proposing correct results), so we must looking for a deterministic and more fundamental and underlying theory than common quantum mechanics, the alternative theory shall contain Reality and Locality assumptions and also could produce current quantum mechanics results. Afterward, scientific probation started for finding a deterministic theory that including hidden variables, infact supplement... 

Entanglement plays a major role in quantum communication as the main resource of the network and makes it possible to perform protocols such as quantum teleportation and quantum cryptography which are not achievable in classical networks. Yet, being a very fragile resource against noise, which inevitably is more destructive as the distance increases and as time passes, makes it a very hard to share entangled pairs between two points at a long distance. This discouraging result for linear networks, has prompted investigation of entanglement percolation and entanglement distribution in regular one- and two-dimensional networks with various (i.e rectangular, triangular and hexagonal)... 

To answer the explanabilty of consciousness, we have first clarified the terms “consciousness”, “explanation”, and “physic”, concluding that consciousness and physics should be integrated in a single unified explanatory system. The common problem we have targeted in this work is “direct observation”. Then, inspired by Bergson, we have suggested that ,in direct observation, there is a unity between observer and the observed, out of which the qualitative characteristic of experience or qualia is constructed. Following computer graphic courses, this unity is called “rendering unity”. In the next scientific step, by analyzing the Wigner’s friend paradox, we have stipulated that, in direct... 

In this thesis, we study two important quantum effects at deSitter spacetime and using the Schrödinger field theory, the power spectrum of massive non-interacting, minimally coupled scalars in a fixed de Sitter background would be obtained. To calculate the N-point function in Schrödinger field theory, the ”in-in” formalism is extended in the Friedmann- Lemaître-Robertson-Walker (FLRW) universe. This formalism in such picture would have a crucial role for evaluating the correlation functions of those quantum states naturally written in Schrödinger picture. We compute the three-point function for primordial scalar field fluctuation in the single field inflation by this in-in formalism. The... 

Behind almost all of the communication protocols and quantum information processing tasks, there is the assumption of existence of a shared reference frame between the parties. However, it is interesting to ask whether it is really necessitated to have a shared reference frame in order to succeed in these tasks. In this thesis, after investigating the effect of the absence of shared reference frame, classical and quantum communication in the absence of shared frame via spin ./5 particles are reviewed.Then, the concepts of relative and collective parameters of composite systems are discussed and the relative parameters of a general pure two qubit state are characterized.Finally, certain... 

Entanglement is a type of quantum correlations which due to its unique features plays a crucial role in quantum information processing. Hence trying to find new approaches in entanglement generation, distribution and also presenting strategies to optimize these processes by considering the fragility of entanglement, is always one of the most important initial steps in many quantum information processes. In this thesis we provide a systematic method to distribute entanglement by separable states and by means of separable ancilla during the process. It is worth mentioning that having separable ancilla during the process of entanglement distribution leads to more robustness against noise to...