The physical-layer security of a line-of-sight (LOS) free-space optical (FSO) link using orbital angular momentum (OAM) multiplexing is studied. We discuss the effect of atmospheric turbulence to OAM-multiplexed FSO channels. We numerically simulate the propagation of OAM-multiplexed beam and study the secrecy capacity. We show that, under certain conditions, the OAM multiplexing technique provides higher security over a single-mode transmission channel in terms of the total secrecy capacity and the probability of achieving a secure communication. We also study the power cost effect at the transmitter side for both fixed system power and equal channel power scenarios.

To address key challenges for both quantum communication and quantum computing applications in a simultaneous manner, we propose to employ the photon angular momentum approach by invoking the well-known fact that photons carry both the spin angular momentum (SAM) and the orbital angular momentum (OAM). SAM is associated with polarization, while OAM is associated with azimuthal phase dependence of the complex electric field. Given that OAM eigenstates are mutually orthogonal, in principle, an arbitrary number of bits per single photon can be transmitted. The ability to generate/analyze states with different photon angular momentum, by using either holographic or interferometric methods, allows the realization of quantum states in multidimensional Hilbert space. Because OAM states provide an infinite basis state, while SAM states are 2-D only, the OAM can also be used to increase the security for quantum key distribution (QKD) applications and improve computational power for quantum computing applications. The goal of this paper is to describe photon angular momentum based deterministic universal quantum qudit gates, namely, {generalized-X, generalized-Z, generalized-CNOT} qudit gates, and different quantum modules of importance for various applications, including (fault-tolerant) quantum computing, teleportation, QKD, and quantum error correction. For instance, the basic quantum modules for quantum teleportation applications include the generalized-Bell-state generation module and the QFT-module. The basic quantum module for quantum error correction and fault-tolerant computing is the nonbinary syndrome calculator module. The basic module for entanglement assisted QKD is either the generalized-Bell-state generation module or the Weyl-operator-module. The possibility of implementing all these modules in integrated optics is discussed as well. Finally, we provide security analysis of entanglement assisted multidimensional QKD protocols, employing the proposed qudit modules, by taking into account the imperfect generation of OAM modes.

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In this thesis, an investigation into the modal characteristics of novel beam shapes is presented. Sculpting the phase profile of a Gaussian beam can result in the generation of a beam with unique properties. Described in this thesis are Laguerre-Gaussian (LG), Hermite-Gaussian (HG) and Bessel beams (BBs). The diffraction of LG beam modes from a triangular aperture is explored and this effect can be used for the efficient measurement of the azimuthal mode index l that indicates the number of multiples of 2π of phase changes that the field displays around one circumference of the optical axis. In this study, only LG beams with zero radial mode index p, with p + 1 denoting the number of bright high intensity concentric rings around the optical axis, were considered. Then, a powerful approach to simultaneously determine both mode indices of a pure LG beam using the principal component analysis (PCA) algorithm on the observed far-field diffraction patterns was demonstrated. Owing to PCA algorithm, the shape of the diffracting element used to measure the mode indices is in fact of little importance and the crucial step is ‘training' any diffracting optical system and transforming the observed far-field diffraction patterns into the uncorrelated variables (principal components). Our PCA method is generic and it was extended to other families of light fields such as HG, Bessel and superposed beams. This reinforces the widespread applicability of this method for various applications. Finally, both theoretically and experimentally investigations using interferometry show the definitive linkage between both the radial and azimuthal mode indices of a partially coherent LG beam and the dislocation rings in the far-field cross-correlation function (CCF).