Tailoring quantum entanglement of orbital angular momentum

[摘要] ENGLISH ABSTRACT: High-dimensional quantum entanglement offers an increase in information capacity perphoton; a highly desirable property for quantum information processes such as quantumcommunication, computation and teleportation. As the orbital angular momentum (OAM)modes of light span an infinite-dimensional Hilbert space, they have become frontrunnersin achieving entanglement in higher dimensions. In light of this, we investigate the potentialof OAM entanglement of photons by controlling the parameters in both the generationand measurement systems. We show the experimental procedures and apparatus involvedin generating and measuring entangled photons in two-dimensions. We verify importantquantum tests such as the Einstein, Podolsky and Rosen (EPR) paradox using OAM and anglecorrelations, as well as a violation of a Bell-type inequality. By performing a full statetomography, we characterise our quantum state and show we have a pure, highly entangledquantum state. We demonstrate that this method can be extended to higher dimensions. Theexperimental techniques used to generate and measure OAM entanglement place an upperbound on the number of accessible OAM modes. As such, we investigate new methods inwhich to increase the spiral bandwidth of our generated quantum state. We alter the shapeof the pump beam in spontaneous parametric down-conversion and demonstrate an effect on both OAM and angle correlations. We also made changes to the measurement schemeby projecting the photon pairs into the Bessel-Gaussian (BG) basis and demonstrate entanglementin this basis. We show that this method allows the measured spiral bandwidthto be optimised by simply varying the continuous radial parameter of the BG modes. Wedemonstrate that BG modes can be entangled in higher dimensions compared with the commonlyused helical modes by calculating and comparing the linear entropy and fidelity forboth modes. We also show that quantum entanglement can be accurately simulated usingclassical light using back-projection, which allows the study of projective measurementsand predicts the strength of the coincidence correlations in an entanglement experiment.Finally, we make use of each of the techniques to demonstrate the effect of a perturbationon OAM entanglement measured in the BG basis. We investigate the self-healing propertyof BG beams and show that the classical property is translated to the quantum regime. Bycalculating the concurrence, we see that measured entanglement recovers after encounteringan obstruction.

[发布机构] Stellenbosch University