Browsing by Subject "Quantum optics."
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Item Open Access Decoherence in open quantum systems : a realistic approach(Bilkent University, 2006) Savran, KerimDecoherence mechanisms of opcu quantum systems ın intcraction with an enviromental bath is investigated using the master equation formalism. Widely used two-level approximation is questioned.Item Open Access Entanglement : quantification via uncertainties and search among ultracold bosons in optical lattices(Bilkent University, 2009) Öztop, BarışIn the first part of the Thesis, the known measures of entanglement for finite dimensional systems are reviewed. Both the simplest case of pure states that belong to bipartite systems and more general case of mixed states are discussed. The multipartite extensions are also mentioned. In addition to the already existing ones, we propose a new measure of entanglement for pure states of bipartite systems. It is based on the dynamical symmetry group approach to quantum systems. The new measure is given in terms of the total uncertainty of basic observables for the corresponding state. Unlike conventional measures concurrence and 3-tangle, which measure the amount of entanglement of different groups of correlated parties, our measure gives the total amount of multipartite entanglement in a specific state. In the second part of the Thesis, the trapping of bosonic atoms in optical lattices is reviewed. The band structure together with Bloch functions and Wannier basis are discussed for this system. In relation with that, the corresponding Bose-Hubbard model and by the use of this model, the resulting superfluid to Mott-insulator quantum phase transition is summarized. In this regard, the Bose-Hubbard Hamiltonian of a specific system, namely ultracold spin-1 atoms with coupled ground states in an optical lattice is considered. For this system we examine particle entanglement, that is characterized by pseudo-spin squeezing both for the superfluid and Mott-insulator phases in the case of ferromagnetic and antiferromagnetic interactions. The role of a small but nonzero angle between the polarization vectors of counterpropagating lasers forming the optical lattice on quantum correlations is investigated as well.Item Open Access Intrinsic entanglement of photons(Bilkent University, 2006) Duru, AlperMultipole radiation is treated both classically and also quantum mechanically. Dipole atom as a source of radiation is investigated within the Jaynes-Cummings model. Polarization properties of quantum multipole radiation are given. It is shown that multipole photons have all three components of polarization but we can perform a local transformation of radiation frame such that the new z− axis corresponding to linear polarization becomes parallel to the Poynting vector. It is shown that the spin angular momentum and orbital angular momentum have the same operator structure, and in the far zone, they contribute equally to the total angular momentum. Hence in this regime, these two contributions are indistinguishable and they may differ from each other only by spatial dependence in the very vicinity of the source. Another aspect of the behavior in the far zone is that the longitudinal polarization of multipole photons vanish. A variational approach to entanglement which is introduced recently based on analysis of dynamic symmetry of systems and quantum uncertainties, accompanying the measurement of mean value of basic observables is applied to investigate the intrinsic entanglement of electric dipole photons. The basic observables are defined in terms of an orthogonal basis of Lie Algebra, corresponding to the dynamic symmetry group of the system of interest. It is shown that electric dipole photons can carry entanglement with respect to its intrinsic degrees of freedom, namely the spin angular momentum and orbital angular momentum, each of which may be considered as a qubit.Item Open Access Optical near field interaction of spherical quantum dots(Bilkent University, 2012) Amirahmadov, TogayNanometer-sized materials can be used to make advanced photonic devices. However, as far as the conventional far-field light is concerned, the size of these photonic devices cannot be reduced beyond the diffraction limit of light, unless emerging optical near-fields (ONF) are utilized. ONF is the localized field on the surface of nanometric particles, manifesting itself in the form of dressed photons as a result of light-matter interaction, which are bound to the material and not massless. In this thesis, we theoretically study a system composed of differentsized quantum dots involving ONF interactions to enable optical excitation transfer. Here this is explained by resonance energy transfer via an optical nearfield interaction between the lowest state of the small quantum dot and the first dipole-forbidden excited state of the large quantum dot via the dressed photon exchange for a specific ratio of quantum dot size. By using the projection operator method, we derived the formalism for the transfered energy from one state to another for strong confinement regime for the first time. We performed numerical analyses of the optical near-field energy transfer rate for spherical colloidal quantum dots made of CdSe, CdTe, CdSe/ZnS and PbSe. We estimated that the energy transfer time to the dipole forbidden states of quantum dot is sufficiently shorter than the radiative lifetime of excitons in each quantum dot. This model of ONF is essential to understanding and designing systems of such quantum dots for use in near-field photonic devices.Item Open Access Robust entanglement in atomic systems(Bilkent University, 2005) Çakır, ÖzgürVarious models for generation of robust atomic entangled states and their implementation with current accessible technologies are proposed and worked out. Deterministic creation of long living Bell states with respect to metastable states in three-level Λ type systems is studied. Strong atom-field coupling drives atoms into a transient entangled state followed by an irreversible evolution towards a long-living maximally entangled state featuring robustness against dipole-allowed transitions. First, generation of pairwise atomic entanglement in cavities in ideal case is discussed, extension to multi-party entangled states is made. Observation of photons emitted from the system signals the generation of a Bell state. The interaction of multi-level atoms with body-assisted electro-magnetic field in the presence of dispersing and absorbing media is studied and these results are applied to the description of a pair of Λ type atoms passing by a microsphere. Microspheres give rise to resonances of well defined height and width with easy access to strong and weak coupling regimes for atom-field interaction, thus enabling realization of the proposed scheme of ”robust entanglement of three-level atoms”. Even in realistic settings it is possible to obtain quite high amount of entanglement at spatially well separated distances. Then we focus on steady state entanglement between atomic dipoles. It is shown that two dipoles in free space driven by a classical driving field become entangled in the steady state. The crucial point is that, this entanglement is irrespective of the initial state and may be preserved as long as the engineered system is kept intact. Absorption effects in real cavities are studied, and an input-output relation is formulated in the presence of a source in the cavity. Extraction of non-classical photon states from a cavity is investigated.