Encoding information into spatial modes of light

A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of requirements for the degree of Master of Science. Johannesburg, May 3, 2016.Spatial modes of light hold the possibility to power the next leap in classical and quantum communications. They provide the ability to pack more information into light, even into single photons themselves, while increasing the level of information security. In this quest, spatial modes carrying orbital angular momentum (OAM) have come under the spotlight due to their discrete in nite dimensional Hilbert space allowing, in theory, for an in nite amount of information to be carried by a photon. Here we study, theoretically and experimentally, spatial modes of two avours: scalar and vector modes. the dichotomy between the two avours is in their polarisation characteristics: scalar modes have spatially homogeneous polarisation elds, while vector modes do not. One facet of our work focusses on scalar mode carrying OAM; using digital holographic methods, we demonstrate the techniques used to tailor and analyse scalar optical elds. We discuss principles of generation and detection for scalar modes based on manipulations of the dynamic phase of light with spatial light modulators. We apply these techniques to characterise free-space and optical bre links, and demonstrate an increase in bandwidth with the additional modal channels. In the other facet of our work, we study vector vortex modes. A particular property exhibited by these modes is the non-separability of their degrees of freedom, a property traditionally associated with entangled quantum states. This raises the question: could quantum entangled systems be modelled with bright sources of vector vortex modes? We answer this question by applying vector vortex modes to the study of quantum transport of entangled states. We borrow techniques from quantum mechanics to evaluate the degree of non-separability of vector vortex modes, using the concurrence as our measure. By determining the evolution of the concurrence, and therefore the entanglement, of vector vortex modes in bres and free-space turbulent channels, we show that indeed, bright classical sources can be used to model the evolution of entangled quantum states in these channels.TG201

Erasing the orbital angular momentum information of a photon

Quantum erasers with paths in the form of physical slits have been studied extensively and proven instrumental in probing wave-particle duality in quantum mechanics. Here we replace physical paths (slits) with abstract paths of orbital angular momentum (OAM). Using spin-orbit hybrid entanglement of photons we show that the OAM content of a photon can be erased with a complimentary polarization projection of one of the entangled pair. The result is the (dis)appearance of azimuthal fringes based on whether the \which-OAM" information was erased. We extend this concept to a delayed measurement scheme and show that the OAM information and fringe visibility are complimentary

Fiber propagation of vector modes

Here we employ both dynamic and geometric phase control of light to produce radially modulated vector-vortex modes, the natural modes of optical fibers. We then measure these modes using a vector modal decomposition set-up as well as a tomography measurement, the latter providing a degree of the non-separability of the vector states, akin to an entanglement measure for quantum states. We demonstrate the versatility of the approach by creating the natural modes of a step-index fiber, which are known to exhibit strong mode coupling, and measure the modal cross-talk and non-separability decay during propagation. Our approach will be useful in mode division multiplexing schemes for transport of classical and quantum states.Comment: 6 pages, 4 figures, 1 tabl

Creation and characterization of vector vortex modes for classical and quantum communication

Vector vortex beams are structured states of light that are non-separable in their polarisation and spatial mode, they are eigenmodes of free-space and many fibre systems, and have the capacity to be used as a modal basis for both classical and quantum communication. Here we outline recent progress in our understanding of these modes, from their creation to their characterization and detection. We then use these tools to study the propagation behaviour of such modes in free-space and optical fibre and show that modal cross-talk results in a decay of vector states into separable scalar modes, with a concomitant loss of information. We present a comparison between probabilistic and deterministic detection schemes showing that the former, while ubiquitous, negates the very benefit of increased dimensionality in quantum communication while reducing signal in classical communication links. This work provides a useful introduction to the field as well as presenting new findings and perspectives to advance it further

Vector Quality Measure for Vector Beams

Vector beams are spatial modes of light with spatially variant polarization states in the transverse profile. Over the years, vector beams have found their way into plenty of applications ranging from material processing and lithography to electron acceleration and particle trapping. Though qualitative measurements are routinely used to analyse vector beams, there is currently no quantitative measure for vector beam purity. Here, we introduce a new measure, the vector quality factor (VQF), that maps the purity of vector beams to a scale ranging from 0 to 1. We demonstrate a simple optical setup to generate and detect vector beams using a birefringent phase plate known as a q-plate. Tomographic measurements are performed by decomposing the vector beam into its circular basis states, and measuring the expectation values of the Pauli matrices as intensity measurements which, are used to evaluate the VQF of vector beams

Entanglement beating in free space through spin–orbit coupling

It is well known that the entanglement of a quantum state is invariant under local unitary transformations. This rule dictates, for example, that the entanglement of internal degrees of freedom of a photon remains invariant during free-space propagation. Here, we outline a scenario in which this paradigm does not hold. Using local Bell states engineered from classical vector vortex beams with non-separable degrees of freedom, the so-called classically entangled states, we demonstrate that the entanglement evolves during propagation, oscillating between maximally entangled (purely vector) and product states (purely scalar). We outline the spin–orbit interaction behind these novel propagation dynamics and confirm the results experimentally, demonstrating spin–orbit coupling in paraxial beams. This demonstration highlights a hitherto unnoticed property of classical entanglement and simultaneously offers a device for the on-demand delivery of vector states to targets, for example, for dynamic laser materials processing, switchable resolution within stimulated emission depletion (STED) systems, and a tractor beam for entanglement

Encoding Information Using Laguerre Gaussian Modes

We experimentally demonstrate an information encoding protocol using the two degrees of freedom of Laguerre Gaussian modes having different radial and azimuthal components. A novel method, based on digital holography, for information encoding and decoding using different data transmission scenarios is presented. The effects of the atmospheric turbulence introduced in free space communication is discussed as well