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

Hybrid entanglement for quantum communication

A dissertation submitted to the Faculty of Science in partial fulfillment of the requirements for the Degree of Master of Science School of Physics University of Witwatersrand November 1, 2017The generation and detection of entangled photons is a topic of interest in quantum communication. With current state-of-the-art methods it is possible to manipulate any degree of freedom (DoF) of photons, e.g, polarisation, transverse momentum, orbital angular momentum and energy. Furthermore, it is possible to combine these DoF to realise hybrid entanglement { entanglement between the DoF of photons. In this dissertation we focus on hybrid entanglement between photon states of coupled orbital angular momentum and polarisation. We engineer hybrid-entanglement using geometric phase control between spatially separated photons produced from spontaneous parametric down conversion. We present a new type of quantum eraser that does not rely on physical path interference. We show that in principle any other degree of freedom can be used and demonstrate this e ectively through polarisation control. The use of high dimensional hybrid photon states in quantum communication, particularly in quantum cryptography, is still in its infancy. Here we tailor photon states that are coupled in their polarisation and spatial DoF (orbital angular momentum) to realise high dimensional encoding alphabets. We show how photons entangled in their internal DoF can be generated and deterministically detected. We exploit them in a demonstration of a high dimensional quantum key distribution protocol and show that our scheme generates secure keys at high rates.MT 201

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

Self-healing high-dimensional quantum key distribution using hybrid spin-orbit Bessel states

Using spatial modes for quantum key distribution (QKD) has become highly topical due to their infinite dimensionality, promising high information capacity per photon. However, spatial distortions reduce the feasible secret key rates and compromise the security of a quantum channel. In an extreme form such a distortion might be a physical obstacle, impeding line-of-sight for free-space channels. Here, by controlling the radial degree of freedom of a photon's spatial mode, we are able to demonstrate hybrid high-dimensional QKD through obstacles with self-reconstructing single photons. We construct high-dimensional mutually unbiased bases using spin-orbit hybrid states that are radially modulated with a non-diffracting Bessel-Gaussian (BG) profile, and show secure transmission through partially obstructed quantum links. Using a prepare-measure protocol we report higher quantum state self-reconstruction and information retention for the non-diffracting BG modes as compared to Laguerre-Gaussian modes, obtaining a quantum bit error rate (QBER) that is up to 3 times lower. This work highlights the importance of controlling the radial mode of single photons in quantum information processing and communication as well as the advantages of QKD with hybrid states.Comment: Published version, 15 pages, 6 figures, 2 table

The effect of competition on the control of invading plant pathogens

1. New invading pathogen strains must compete with endemic pathogen strains to emerge and spread. As disease control measures are often non-specific, i.e. they do not distinguish between strains, applying control not only affects the invading pathogen strain but the endemic as well. We hypothesise that the control of the invasive strain could be compromised due to the non-specific nature of the control. 2. A spatially-explicit model, describing the East African cassava mosaic virus-Uganda strain (EACMV-UG) outbreak, is used to evaluate methods of controlling both disease incidence and spread of invading pathogen strains in pathosystems with and without an endemic pathogen strain present. 3. We find that while many newly introduced or intensified control measures (such as resistant cultivars or roguing) decrease the expected incidence, they have the unintended consequence of increasing, or at least not reducing, the speed with which the invasive pathogen spreads geographically. We identify which controls cause this effect and methods in which these controls may be applied to prevent it. 4. We found that the spatial spread of the invading strain is chiefly governed by the incidence at the wave front. Control can therefore be applied, or intensified, once the wave front has passed without increasing the pathogen’s rate of spread. 5. When trade of planting material occurs, it is possible that the planting material is already infected. The only forms of control in this study that reduces the speed of geographic spread, regardless of the presence of an endemic strain, are those that reduce the amount of trade and the distance over which trade takes place. 6. Synthesis and applications. Imposing trade restrictions before the epidemic has reached a given area and increasing other control methods only once the wave front has passed is the most effective way of both slowing down spread and controlling incidence when the presence of an endemic strain is unknow

Taking the Quantum Eraser to the Abstract World

Youngs double slit experiment is one of the most celebrated achievements in quantum and classical optics; it provides experimental proof of the wave-particle duality of light. When the paths of the double slit are marked with orthogonal polarizations, the path information is revealed and no interference pattern is observed. However, the path information can be erased with a complimentary analysis of the polarization. Here we use hybrid entanglement between photons carrying orbital angular momentum and polarization to show that, just as in Young's experiment, the paths (OAM) marked with polarization do not lead to interference. However, when introducing the eraser (polarizer) which projects the polarization of one of the entangled photons onto a complementary polarization basis, the OAM (paths) are allowed to interfere, leading to the formation of azimuthal fringes whose frequency is proportional to the OAM content carried by the photon

Progress in cassava technology transfer in Uganda

This publication contains the full text of papers presented at a Workshop held in Masindi, Uganda, 9-12 January 1996, and sponsored by the Gatsby Charitable Foundation. During the Workshop the need became evident for additional statistics on the multiplication, distribution and uptake of improved varieties of cassava in the six districts where activities are supported by The Gatsby Charitable Foundation and also elsewhere. The results of a subsequent survey in selected sub-counties of each of the six Gatsby districts are also presented here, together with estimates of the amount of improved material that has been distributed and the area now grown. These latest figures (Appendix 1) represent the best available estimates and in some instances differ substantially from those compiled and presented in the earlier district reports