In this thesis we review two methods for studying the non-pertubative region of QCD: the effective field theory, chiral perturbation theory (χPT), and the cloudy bag model, a successful chiral quark model of hadron structure. We use information from both of these sources to construct a simple extrapolation formula in the pion mass, mπ, for the nucleon electromagnetic form factors, which combines the correct non-analytic chiral behaviour predicted by (χPT), with the correct large mπ behaviour. This formula is applied to recent quenched lattice QCD results to extrapolate to the physical regime. Given the simple nature of the extrapolation scheme, our results compare surprisingly well with experiment. We also employ a simple chiral quark model (the hedgehog) to examine the volume and pion mass dependence of the axial coupling constant, ga, along with the hedgehog baryon mass. Our results for ga reveal large volume dependence at low pion masses. / Thesis (M.Sc.)--School of Chemistry and Physics, 2004.

Quantum optics provides powerful means to probe quantum mechanics. In this thesis, we study various aspects of quantum phenomena arising in quantum optical systems. Part I studies broadband quantum teleportation. After presenting three different methods of analyzing the standard teleportation protocol, we study the interplay between various bandwidths in determining the fidelity of a broadband quantum field teleportation. Explicit formulae for the degrees of first- and secondorder coherence for the teleportation of resonance fluorescence are derived for this purpose. Part II studies entanglement arising in cascaded open quantum (optical) systems. First, a detailed laser model is produced within quantum trajectory theory to study the total decoherence rate of a laser-driven qubit. Second, using this model, we address the issue of laser quantum state, viewed in connection with separability of the laser-driven-qubit system. Third, a measure of entanglement within quantum trajectory theory called ‘Contextual Entanglement’ is calculated for a few simple systems and compared with the ‘Entanglement of Formation’. Lastly, we introduce a method to quantify entanglement (based on the contextual entanglement) between a source and the field it emits, which we call the ‘Entanglement Spectrum’. It is applied to study the entanglement between a laser-driven qubit and the field the qubit scatters.

We study the entanglement of two-qubit 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 two-qubit 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

Thesis (Ph.D.) in Physics and Astronomy--University of Maine, 2006. / Includes vita. Includes bibliographical references (leaves 288-292).

No description available.

This thesis describes a linear-optical device for performing generalized quantum measurements on quantum bits (qubits) encoded in photon polarization, the implementation of said device, and its use in two diff erent but related experiments. The device works by coupling the polarization degree of freedom of a single photon to a `mode' or `path' degree of freedom, and performing a projective measurement in this enlarged state space in order to implement a tunable four-outcome positive operator-valued measure (POVM) on the initial quantum bit. In both experiments, this POVM is performed on one photon from a two-photon entangled state created through spontaneous parametric down-conversion. In the fi rst experiment, this entangled state is viewed as a two-qubit photonic cluster state, and the POVM as a means of increasing the computational power of a given resource state in the cluster-state model of quantum computing. This model traditionally achieves deterministic outputs to quantum computations via successive projective measurements, along with classical feedforward to choose measurement bases, on qubits in a highly entangled resource called a cluster state; we show that `virtual qubits' can be appended to a given cluster by replacing some projective measurements with POVMs. Our experimental demonstration fully realizes an arbitrary three-qubit cluster computation by implementing the POVM, as well as fast active feed-forward, on our two-qubit photonic cluster state. Over 206 diff erent computations, the average output delity is 0.9832 +/- 0.0002; furthermore the error contribution from our POVM device and feedforward is only of order 10^-3, less than some recent thresholds for fault-tolerant cluster computing. In the second experiment, the POVM device is used to implement a deterministic protocol for remote state preparation (RSP) of arbitrary photon polarization qubits. RSP is the act of preparing a quantum state at a remote location without actually transmitting the state itself. We are able to remotely prepare 178 diff erent pure and mixed qubit states with an average delity of 0.995. Furthermore, we study the the fidelity achievable by RSP protocols permitting only classical communication, without shared entanglement, and compare the resulting benchmarks for average fidelity against our experimental results. Our experimentally-achieved average fi delities surpass the classical thresholds whenever classical communication alone does not trivially allow for perfect RSP.

This thesis describes a linear-optical device for performing generalized quantum measurements on quantum bits (qubits) encoded in photon polarization, the implementation of said device, and its use in two diff erent but related experiments. The device works by coupling the polarization degree of freedom of a single photon to a `mode' or `path' degree of freedom, and performing a projective measurement in this enlarged state space in order to implement a tunable four-outcome positive operator-valued measure (POVM) on the initial quantum bit. In both experiments, this POVM is performed on one photon from a two-photon entangled state created through spontaneous parametric down-conversion. In the fi rst experiment, this entangled state is viewed as a two-qubit photonic cluster state, and the POVM as a means of increasing the computational power of a given resource state in the cluster-state model of quantum computing. This model traditionally achieves deterministic outputs to quantum computations via successive projective measurements, along with classical feedforward to choose measurement bases, on qubits in a highly entangled resource called a cluster state; we show that `virtual qubits' can be appended to a given cluster by replacing some projective measurements with POVMs. Our experimental demonstration fully realizes an arbitrary three-qubit cluster computation by implementing the POVM, as well as fast active feed-forward, on our two-qubit photonic cluster state. Over 206 diff erent computations, the average output delity is 0.9832 +/- 0.0002; furthermore the error contribution from our POVM device and feedforward is only of order 10^-3, less than some recent thresholds for fault-tolerant cluster computing. In the second experiment, the POVM device is used to implement a deterministic protocol for remote state preparation (RSP) of arbitrary photon polarization qubits. RSP is the act of preparing a quantum state at a remote location without actually transmitting the state itself. We are able to remotely prepare 178 diff erent pure and mixed qubit states with an average delity of 0.995. Furthermore, we study the the fidelity achievable by RSP protocols permitting only classical communication, without shared entanglement, and compare the resulting benchmarks for average fidelity against our experimental results. Our experimentally-achieved average fi delities surpass the classical thresholds whenever classical communication alone does not trivially allow for perfect RSP.

In this thesis, we develop a number of operational tests and tools for benchmarking the quantum nature of optical quantum communication devices. Using the laws of quantum physics, ideal quantum devices can fundamentally outperform their classical counterparts, or even achieve objectives which are classically impossible. Actual devices will not be ideal, but they may still be capable of facilitating quantum communication. Benchmarking tests, based on the presence of entanglement, can be used to verify whether or not imperfect quantum devices offer any advantage over their classical analogs. The general goal in this thesis is to provide strong benchmarking tools which simultaneously require minimal experimental resources but also offer a wide range of applicability. Another major component is the extension of existing qualitative benchmarks (`Is it quantum or classical?') to more quantitative forms (`How quantum is it?'). We provide a number of benchmarking results applicable to two main situations, namely discrete remote state preparation protocols and continuous-variable quantum device testing. The theoretical tools derived throughout this thesis are also applied to the tasks of certifying a remote state preparation experiment and a continuous-variable quantum memory.

Quantum Cryptography enables secret distribution between remotes parties where classical communications fail. The proposed technique uses optical signal modulation to encode information with relative phase difference between frequency separated signals. The single side band detection scheme (SSB) enables efficient secret key distribution. The system security is guaranteed with a strong reference protocol. One can use a fainted laser source without security breach for any distance. A second proposed technique uses relative phase difference between time separated pulses. The single side pulse detection scheme (SSP) enables efficient secret key distribution and benefits the same security features as the SSB system. Both SSP and SSB may be multiplexed to increase the secure bit rate. The maximizing initial average energy is then one photon per pulse. The implemented SSB protocol includes an autocompensation system for the optical path fluctuations that make the system robust over long time periods.

In this thesis, we explore the quantum dynamics of light interactions with optically thick collections of atoms. We provide a theoretical description of several recent experiments in which some key operations necessary for the implementation of quantum communication networks are demonstrated. Collective Raman scattering from an atomic ensemble is shown to produce probabilistic entanglement between the polarization of a scattered photon and an associated collective atomic excitation. The predicted correlations agree with experimental observations. We also propose a method to use cascade transitions to produce entanglement between a photon with a frequency in the telecom range (ideal for transmission over optical fibers) and a near infrared photon (ideal for storage in an atomic ensemble), and a description of the experimental demonstration is provided. We also propose and describe the implementation of a deterministic source of single photons. In addition, we generalize the theory of dark-state polaritons in ensembles of three level Lambda atoms to account for the nuclear spin degeneracy of alkali atoms. This generalized theory provides a description of the first demonstration of single photon storage and retrieval from atomic ensembles. Additionally, in the presence of a uniform magnetic field, we predict the occurrence of collapses and revivals of the photon retrieval efficiency as a function of storage time within the ensemble. These predictions are in very good agreement with subsequent experimental observations. We also exploit the ability of photon storage to entangle remote atomic qubits.