Browsing by Author "Forbes, Andrew"

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    Entanglement beating in free space through spin–orbit coupling
    (Springer Nature, 2018) Rosales-Guzmán, Carmelo; Denz, Cornelia; Otte, Eileen; Ndagano, Bienvenu; Forbes, Andrew
    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.
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    The Eigenmodes of Complex Media
    (University of the Witwatersrand, Johannesburg, 2024) Peters, Cade Ribeiro; Forbes, Andrew
    Structured light refers to the tailoring of light in all of its degrees of freedom. This includes amplitude, phase, wavelength and polarisation. Structuring light allows us to create complex optical fields with many interesting and useful properties. These fields have allowed us to ask deeper and more fundamental questions about Physics and have revealed new avenues for investigating aspects of the world around us. They have allowed us to significantly increase the speed at which we communicate and make information more accessible. Additionally, they allow for increased resolution and precision in imaging and measurements, both classical and quantum. One of the primary limitations when using structured light are the effects of perturbations. Many complex media, such as the atmosphere, underwater or biological specimens have a non-uniform refractive index (varying dielectric constant). This distorts most structured light beams, limiting its performance and possible uses. This works seeks to investigate this problem and offer a solution. Much attention has been given to finding which forms of structured light perform best in certain systems or scenarios. This work focuses on offering a potential solution to this problem. We begin with a discussion on common forms of structured light and models of light propagation. We then move onto methods for generating structured light experimentally. We then propose the concept of an eigenmode: modes that are perfectly invariant through such systems. They are structured light fields that are specially tailored, using our knowledge and understanding of the Physics of the system, to ensure that they propagate through the system and exit unchanged. We achieve this by modelling our system as a linear operator and then using this to find the eigenstates of this operator. We do this for two highly topical aberrations, providing approaches that can be generalised to almost any optical system. We end off this work with a discussion on important considerations when using eigenmodes for real world applications