1 
State preparation and some applications in quantum optics within the context of quantum information theoryKok, Pieter January 2001 (has links)
No description available.

2 
Local Entanglement Generation in TwoQubit SystemsPerez Veitia, Andrzej 22 September 2010 (has links)
We study the entanglement of twoqubit systems resulting from local interactions with spatially extended bosonic systems. Our results apply to the case where the initial state of the bosonic system is represented by a statistical mixture of states with fixed particle number. In particular, we derive and discuss necessary conditions to generate entanglement in the twoqubit system. We also study the scenario where the joint system is initially in its ground state and the interaction is switched on adiabatically. Using time independent perturbation theory and the adiabatic theorem, we show conditions under which the qubits become entangled as the joint system evolves into the ground state of the interacting theory

3 
Sudden death of entanglement and nonlocality in two and threecomponent quantum systemsAnn, Kevin January 2011 (has links)
Thesis (Ph.D.)Boston University / Quantum entanglement and nonlocality are nonclassical characteristics of quantum states with phase coherence that are of central importance to physics, and relevant to the foundations of quantum mechanics and quantum information science. This thesis examines quantum entanglement and nonlocality in two and threecomponent quantum states with phase coherence when they are subject to statistically independent, classical, Markovian, phase noise in various combinations at the local and collective level. Because this noise reduces phase coherence, it can also reduce quantum entanglement and Bell nonlocality. After introducing and contextualizing the research, the results are presented in three broad areas.
The first area characterizes the relative time scales of decoherence and disentanglement in 2 x 2 and 3 x 3 quantum states, as well as the various subsystems of the two classes of entangled tripartite twolevel quantum states. In all cases, it was found that disentanglement time scales are less than or equal to decoherence time scales. The second area examines the finitetime loss of entanglement, even as quantum state coherence is lost only asymptotically in time due to local dephasing noise, a phenomenon entitled "Entanglement Sudden Death" (ESD). Extending the initial discovery in the simplest 2 x 2 case, ESD is shown to exist in all other systems where mixedstate entanglement measures exist, the 2 x 3 and d x d systems, for finite d > 2. The third area concerns nonlocality, which is a physical phenomenon independent of quantum mechanics and related to, though fundamentally different from, entanglement. Nonlocality, as quantified by classes of Bell inequalities, is shown to be lost in finite time, even when decoherence occurs only asymptotically. This phenomenon was named "Bell Nonlocality Sudden Death" (BNSD).

4 
Correlated Electronic Structure of Materials : Development and Application of Dynamical Mean Field TheoryThunström, Patrik January 2012 (has links)
This thesis is dedicated to the development, implementation and application of a combination of Density Functional Theory and Dynamical Mean Field Theory. The resulting program is shown through several examples to be a powerful and flexible tool for calculating the electronic structure of strongly correlated materials. The main part of this work is focused on the development and implementation of three methods for solving the effective impurity model arising in the Dynamical Mean Field Theory: HubbardI approximation (HIA), Exact Diagonalization (ED), and SpinPolarized Tmatrix Fluctuationexchange (SPTF). The HubbardI approximation is limited to systems where the hybridization between the 4forbitals and the rest of the material can be completely neglected, and can therefore not capture any Kondo physics. It has been used to study the atomiclike multiplet spectrum of the strongly localized 4felectrons in the Lanthanide compounds YbInCu4, YbB12, Yb2Pd2Sn, YbPd2Sn, SmB6, SmSn3, and SmCo5. The calculated spectral properties are shown to be in excellent agreement with experimental direct and inverse photoemission data, clearly affirming the applicability of the HubbardI approximation for this class of systems if we are not focusing on Kondo physics. Full selfconsistence in both selfenergy and electron density is shown to be of key importance in the extraction of the magnetic properties of the hard permanent magnet SmCo5. The Exact Diagonalization solver is implemented as an extension of the HubbardI approximation. It takes into account a significant part of the hybridization between the correlated atom and the host through the use of a few effective bath orbitals. This approach has been applied to the longstanding problem of the electronic structure of NiO, CoO, FeO, and MnO. The resulting spectral densities are favorably compared to photoemission spectroscopy. Apart from predicting the correct spectral properties, the Exact Diagonalization solver also provides full access to the manybody density operator. This feature is used to make an indepth investigation of the correlations in the electronic structure, and two measures of the quantum entanglement of the manybody groundstates are presented. It is shown that CoO possesses the most intricate entanglement properties, due to a competition between crystal field effects and Coulomb interaction, and such a mechanism likely carries over to several classes of correlated electron systems. The Exact Diagonalization solver has also been applied to the prototypical dilute magnetic semiconductor Mn doped GaAs, a material of great importance in the study of future spintronics applications. The problem of Fe impurities in Cs has been used to study the dependence of the spectral properties on the local environment. Finally, the Spinpolarized Tmatrix Fluctuationexchange solver has been implemented and applied to more delocalized electron systems where the effective impurity problem can be solved as a perturbation with respect to the strength of the local Coulomb interaction. This approach has been used to study the magnetic and spectral properties of the late transition metals, Fe, Co and Ni, and NiS.

5 
Application and Manipulation of Bipartite and Multipartite Entangled StatesFortescue, Benjamin 24 September 2009 (has links)
The phenomenon of quantum entanglement is a fundamental feature of quantum mechanics which, as a counterintuitive and inherently ”quantum” phenomenon (with no classical analogue) has been the subject of much study, especially in quantum information theory. One fruitful approach to the description of entanglement has been in its operational description  that is, in the consideration of what can be achieved using entangled states under certain restrictions, typically the regime of local operations and classical communications.
We present results here related to the operational characterisation of entanglement in the resource model, in both bipartite and multipartite cases. First, we consider the conversion between pure bipartite entangled states in terms of an oftenignored resource  the classical communication cost. Using prior results for more specific conversions, we derive lower bounds on this cost (and the related quantity of the conversion inefficiency) for general bipartite pure states.
We also consider purestate conversions of multipartite entanglement, in particular the class of protocols in which multipartite states are converted to states shared between fewer parties. We have found a previouslyunconsidered variety of such conversions, in which the target state of the conversion is a state shared between a random subset of the parties. We find that when such postselection of parties in the protocol is permitted allows for a wider variety of achievable target states; certain states which can not be reliably obtained between predetermined parties (even some where the probability of doing so is arbitrarily small) can be obtained between random parties. We consider a variety of states in which this phenomenon occurs, as well as bounds on such protocols can achieve.
Finally we consider a practical use of entanglement as a resource, in an experimental implementation of a multipartite QKD protocol. This is based on the tripartite GHZ entangled state, but can be implemented using only bipartite entanglement. We adapt existing QKD results for both the bipartite and multipartite case to derive a secure key rate for this implementation, taking into account the ways in which it differs from the idealised theoretical case.

6 
Application and Manipulation of Bipartite and Multipartite Entangled StatesFortescue, Benjamin 24 September 2009 (has links)
The phenomenon of quantum entanglement is a fundamental feature of quantum mechanics which, as a counterintuitive and inherently ”quantum” phenomenon (with no classical analogue) has been the subject of much study, especially in quantum information theory. One fruitful approach to the description of entanglement has been in its operational description  that is, in the consideration of what can be achieved using entangled states under certain restrictions, typically the regime of local operations and classical communications.
We present results here related to the operational characterisation of entanglement in the resource model, in both bipartite and multipartite cases. First, we consider the conversion between pure bipartite entangled states in terms of an oftenignored resource  the classical communication cost. Using prior results for more specific conversions, we derive lower bounds on this cost (and the related quantity of the conversion inefficiency) for general bipartite pure states.
We also consider purestate conversions of multipartite entanglement, in particular the class of protocols in which multipartite states are converted to states shared between fewer parties. We have found a previouslyunconsidered variety of such conversions, in which the target state of the conversion is a state shared between a random subset of the parties. We find that when such postselection of parties in the protocol is permitted allows for a wider variety of achievable target states; certain states which can not be reliably obtained between predetermined parties (even some where the probability of doing so is arbitrarily small) can be obtained between random parties. We consider a variety of states in which this phenomenon occurs, as well as bounds on such protocols can achieve.
Finally we consider a practical use of entanglement as a resource, in an experimental implementation of a multipartite QKD protocol. This is based on the tripartite GHZ entangled state, but can be implemented using only bipartite entanglement. We adapt existing QKD results for both the bipartite and multipartite case to derive a secure key rate for this implementation, taking into account the ways in which it differs from the idealised theoretical case.

7 
Experimental Quantum Information Processing with PhotonsLavoie, Jonathan January 2013 (has links)
This thesis describes experimental generation, manipulation and measurement of quantum information using photon pairs emitted in bulk crystals. Multiphoton sources engineered during the course of this thesis have proven to be ideal for original contributions in the field of optical quantum information.
In the first part of this dissertation, we study nonlocality, bound entanglement and measurementbased quantum computing using entangled resources produced by our source. First, we produced and characterised threephoton GHZ polarisation states. We then experimentally violate the longstanding Svetlichny's inequality with a value of 4.51, which is greater than the classical bound by 3.6 standard deviations. Our results agree with the predictions of quantum mechanics, rule out nonlocal hiddenvariable theories and certify the genuine tripartite entanglement achievable by our source. Second, with fourphoton polarisation states, we demonstrate bound entanglement in Smolin states and realize all of their conceptually important characteristics. Our results highlight the difficulties to achieve the critical condition of undistillability without completely losing entanglement. We conclude the first part by simulating, for the first time, valencebond solid states and use them as a resource for measurementbased quantum computing. AffleckKennedyLiebTasaki states are produced with 87% fidelity and singlequbit quantum logic gates reach an average fidelity of 92% over all input states and rotations.
In the second part of this dissertation, we explore controlled waveform manipulation at the singlephoton level. Specifically, we shrink the spectral bandwidth of a single photon from 1740 GHz to 43 GHz and demonstrate tunability over a range 70 times that bandwidth. The results are a considerable addition to the field of quantum frequency conversion and have genuine potential for technological applications.

8 
Detection for Quantum EntanglementLee, KuoHao 23 July 2006 (has links)
In the 1990¡¦s, the research of quantum information attracts many people¡¦s attention. In this period of time, Shor find a new method to demonstrate that a quantum computer could factor very large numbers superefficiently. The method also shows that quantum computer has more potential than classical computer. Beside, quantum information contains many different new fields, such as quantum computation, quantum entanglement, quantum searching, etc. We believe the most fundamental physics of the applications of quantum information is quantum entanglement. In order to understand the physical meaning of entanglement, we choose entanglement as the goal of our thesis.

9 
Coherenceinduced entanglementXiong, Han 16 August 2006 (has links)
Coherence and entanglement are the two key concepts that distinguish quantum
mechanics from classical mechanics. Many novel phenomena occuring in the quantum
world are due to these two Âphysical quantitiesÂ. They also play essential roles in
quantum computation and quantum information. For example, coherence, which
says that a quantum mechanical system could be in a superposition state, makes the
quantum parallel computing scheme possible; and entanglement, which says that two
quantum systems separated in space could be in an intervened state, is the key factor
in various quantum teleportation algorithms.
We have studied entanglement generation in various systems. We found that with
atomic coherence, entanglement could be generated between two thermal fields with
arbitrarily high temperatures. We also found that temperature difference instead of
the purity of state is essential for the entanglement generation between an atom and
a thermal field. We discovered that correlated spontaneous emission lasers (CELs)
could be used to generate bright entanglement laser beams. As a special case of CEL
systems, we studied entanglement generation in Nondegenerate Optical Parametric
Amplifiers (NOPAs). We performed the inputoutput calculations for a NOPA system
and showed that the two output optical beams are still entangled. This justifies our idea that CEL (or NOPA) systems can be used as an ideal entanglement source
for various quantum information schemes. From an experimental point of view, we
considered the effects of pumping fluctuations on entanglement generation in CEL
and NOPA systems. We found that these fluctuations, especially the phase diffusion
processes, in the pump laser would greatly reduce the entanglement generated in such
systems.

10 
Multipartite Entanglement: Transformations, Quantum Secret Sharing, Quantum Error CorrectionHelwig, Wolfram Hugo 27 March 2014 (has links)
Most applications in quantum information processing make either explicit or implicit use of entanglement. It is thus important to have a good understanding of entanglement and the role it plays in these protocols. However, especially when it comes to multipartite entanglement, there still remain a lot of mysteries. This thesis is devoted to getting a better understanding of multipartite entanglement, and its role in various quantum information protocols.
First, we investigate transformations between multipartite entangled states that only use local operations and classical communication (LOCC). We mostly focus on three qubit states in the GHZ class, and derive upper and lower bounds for the successful transformation probability between two states.
We then focus on absolutely maximally entangled (AME) states, which are highly entangled multipartite states that have the property that they are maximally entangled for any bipartition. With them as a resource, we develop new parallel teleportation protocols, which can then be used to implement quantum secret sharing (QSS) schemes. We further prove the existence of AME states for any number of parties, if the dimension of the involved quantum systems is chosen appropriately. An equivalence between threshold QSS schemes and AME states shared between an even number of parties is established, and further protocols are designed, such as constructing ramp QSS schemes and opendestination teleportation protocols with AME states as a resource.
As a framework to work with AME states, graph states are explored. They allow for efficient bipartite entanglement verification, which makes them a promising candidate for the description of AME states. We show that for all currently known AME states, absolutely maximally entangled graph states can be found, and we were even able to use graph states to find a new AME state for seven threedimensional systems (qutrits). In addition, the implementation of QSS schemes from AME states can be conveniently described within the graph state formalism.
Finally, we use the insight gained from entanglement in QSS schemes to derive necessary and sufficient conditions for quantum erasure channel and quantum error correction codes that satisfy the quantum Singleton bound, as these codes are closely related to ramp QSS schemes. This provides us with a very intuitive approach to codes for the quantum erasure channel, purely based on the entanglement required to protect information against losses by use of the parallel teleportation protocol.

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