- ItemQuantum randomness(Stellenbosch : Stellenbosch University, 2023-03) Strydom, Conrad; Tame, M. S.; Bosman, G. W.; Stellenbosch University. Faculty of Science. Dept. of Physics.ENGLISH ABSTRACT: Randomness is a vital resource with many important applications in information theory. In particular, random numbers play a ubiquitous role in cryptography, simulation and coordination in computer networks. When ran- domness is generated using classical techniques, the unpredictability relies on incomplete knowledge which can introduce ordered features and compromise the application. This thesis explores the use of quantum techniques to generate true randomness and its application to quantum computing. The analogue of random numbers in quantum information are random unitary operators sampled from the uniform Haar ensemble, which are used in a number of quantum protocols. Unfortunately, these cannot be generated efficiently and so pseudorandom ensembles called unitary t-designs are frequently used as a substitute. In the first part of this thesis we investigate t-designs realised using a measurement-based approach on IBM quantum computers. In particular, we implement an exact single-qubit 3-design on IBM quantum computers by performing measurements on a 6-qubit graph state. We show that the ensemble of unitaries realised was a 1-design, but not a 2-design or a 3-design under the test conditions set, which we show to be a result of depolarising noise. We obtain improved results for the 2-design test by implementing an approximate 2-design, which uses a smaller 5-qubit graph state, but the test still does not pass for all states due to noise. To obtain a theoretical understanding of the effect of noise on t-designs, we investigate the effect of various noise channels on the quality of single-qubit t-designs. We show analytically that the 1-design is affected only by amplitude damping, while numeric results obtained for the 2-, 3-, 4- and 5-design suggest that a 2t-design is significantly more sensitive to noise than a (2t − 1)-design and that, with the exception of amplitude damping, a (2t + 1)-design is as sensitive to noise as a 2t-design. Next, we test our approximate measurement-based 2-design on an important application in quantum com- puting, namely noise estimation. For this, we propose an interleaved randomised benchmarking protocol for measurement-based quantum computers that can be used to estimate the fidelity of any single-qubit measurement- based gate. We demonstrate our protocol on IBM quantum computers by estimating the fidelity of a universal single-qubit gate set using graph states of up to 31 qubits. Estimated gate fidelities show good agreement with those calculated from process tomography, which shows that our approximate measurement-based 2-design is of sufficient quality for use in randomised benchmarking, despite not passing our test for all states. While IBM quantum computers provide a sophisticated platform for randomness generation, they are not specifically designed for this task. We therefore investigate randomness generation on custom-built hardware, by integrating an on-chip nanowire waveguide into an optical time-of-arrival based quantum random number generation setup. Despite loss, we achieve a random number generation rate of 14.4 Mbits/s. The generated bits did not require any post-processing to pass industry standard tests. Our experiment demonstrates an order of magnitude increase in generation rate and decrease in device size compared to previous studies.
- ItemDeveloping new experimental techniques to investigate the co-linear cluster tri-partitioning(Stellenbosch : Stellenbosch University, 2022-12) Korsten, Riaan Louw; Wyngaardt, Shaun M.; Malaza, Vusi D.; Pyatkov, Yu V.; Stellenbosch University. Faculty of Science. Dept. of Physics.ENGLISH ABSTRACT: Computational simulations were developed to calculate data collection efficiency, as well as data accuracy for existing experimental setups used in the study of the exotic ternary decay of low excited heavy nuclei known as Co-linear Cluster Tri- partitioning (CCT) . The most populated CCT mode containing the so called “Ni-bump”. This bump is centered at the masses associated with the magic isotopes of Ni (which include ⁶⁸Ni and ⁷²Ni). The results from these simulations give new insight into the identification capability of heavy ions using current experimental equipment and hint at some possible solutions to increase data collection efficiency and data accuracy. As a charged particle interacts with the material of a semiconductor detector and deposits energy into it, it creates high conductivity plasma along the trajectory of the particle. This disrupts the internal electric field of the detector for some time which retards data collection. This effect is known as Plasma Delay (PD). Development of a new algorithm and subsequent computational implementation of this algorithm -Paraspline algorithm-, with the goal of more accurate time-of-flight (TOF) calculations, by correcting for the PD effect in semiconductor detectors. Testing of this algorithm show promising results, potentially improving the reliability of future experimental results.
- ItemSystematics of mean resonance spacing and average radiative width from random forest regression(Stellenbosch : Stellenbosch University, 2022-12) Bormans, Jeroen Peter; Wyngaardt, Shaun M.; Wiedeking, Mathis; Malatji, K.L.; Stellenbosch University. Faculty of Science. Dept. of Physics.ENGLISH ABSTRACT: The interaction of a nucleus with γ-rays plays a key role in a theoretical understanding of competi- tion in and components of nuclear reactions. The photon strength function (PSF) gives the average response of a nucleus to an electromagnetic probe. In this thesis, a study is done of the nuclear level density (NLD) and PSF of 22 isotopes in the range (A = 46 − 90) arising from a (p,γ) reac- tion. The calculations for this analysis are done in TALYS. In addition, a machine-learning driven approach for determining experimentally obtained model parameters from the neutron/proton and mass number is given.
- ItemRadiation shielding design for sealed radioactive gamma-ray and neutron sources: measurements and modeling(Stellenbosch : Stellenbosch University, 2022-04) Mahanyapane, Motlatsi Vincent; Newman, Richard T.; Van Zyl, J. J.; Stellenbosch University. Faculty of Science. Dept. of Physics.ENGLISH ABSTRACT: Adequacy of the radiation shielding implemented for sealed radioactive sources used at the University was evaluated and improved. Three different setups were investi- gated; 1. neutron activation drum, 2. radioactive-source store, and 3. gamma-ray container. Gamma-ray and neutron dose rate measurements were performed for the 5.8 GBq 241Am-Be neutron source inside the neutron activation drum in the labora- tory, as well as in and around the radioactive-source store in the Department. The radiation shielding design of the neutron activation drum and the radioactive-source store was modeled using the Monte Carlo radiation transport code FLUKA. Fur- thermore, the shielding container for gamma-ray sources was modeled and designed. Dose rates in the areas around the laboratory were at the natural background levels (0.1 μSv/h), except in the next-door laboratory close to the wall near the neutron activation drum. Dose rates were above acceptable limits inside the laboratory. The largest contribution was due to neutrons that leak out of the drum due to inade- quate shielding while gamma rays were effectively attenuated. It is recommended that at the time of neutron activation experiments, one should reduce stay times to short times when placing the neutron source inside the drum and taking it out. These times should be less than 20 minutes stay time 5 cm away from the drum side. The measured and simulated neutron dose rates were comparable, with the simulated neutron dose rates being slightly higher than the measured dose rates by a factor of 1.3 and 1.8 on the top drum surface and 5 cm horizontally away from the drum, respectively. For the radioactive-source store, measurements and simulations suggested safe ra- diation levels around the radioactive-source store with dose rates at the natural background radiation levels (0.1 μSv/h). An implication was that the next-door offices could be accessed without restrictions. Inside the radioactive-source store, however, dose rates were above acceptable levels. Therefore, it was concluded that the implemented radiation shielding was inadequate for maintaining dose rates at safe levels. The simulated gamma-ray dose rates alone were at natural background radiation levels. However, it was found from the comparison of results that gamma- ray dose rates were mainly due to the gamma-ray sources stored in the storeroom. Significantly high neutron dose rates were also resulting from the 241Am-Be neutron sources in the soil moisture gauges inside the storeroom. It was suggested that the radiation shielding for the 5.8 GBq 241Am-Be neutron source should be improved by increasing the thickness of paraffin wax around the source to 28 cm. A new gamma-ray shielding container was simulated, showing that 7 cm-thick lead attenuated gamma rays and reduced dose rates by 97 % on the container surface.
- ItemQuantum computing on cloud-based processors(Stellenbosch : Stellenbosch University, 2022-04) Skosana, Unathi; Tame, M. S.; Stellenbosch University. Faculty of Science. Dept. of Physics.ENGLISH ABSTRACT: The noisy intermediate-scale quantum (NISQ) era refers to the current technological epoch permeated with quantum processors that are big enough (50-100 qubits) to be no longer trivially simulatable with digital computers but not yet capable of full fault-tolerant computation. Such processors provide great testbeds to understand the practical issues and resources needed to realize quantum tasks in these processors, such as quantum algorithms. Many pressing issues arise in this context that are a direct consequence of the limitations of these processors (limited number of qubits, low qubit connectivity, and limited coherence times). Hence, for near-term quantum algorithms, there is an overriding imperative to adopt an approach that takes into account, and attempts to mitigate or circumvent some of these limitations. In this thesis, we examine realizing Grover’s quantum search algorithm for four qubits on IBM Q superconducting quantum processors, and potentially scaling up to more qubits. We also investigate non-canonical forms of the quantum search algorithm that trade accuracy for speed in a way that is more suitable for near-term processors. Our contribution to this topic of research is a slight improvement in the accuracy of the solution to a graph problem, solved with a quantum search algorithm implemented on IBM Q quantum processors by Satoh et .al in IEEE Transactions on Quantum Engineering (2020). We also explore the realization of a measurement-based quantum search algorithm for three qubits. Unfortunately, the number of qubits and two-qubit gates required by such an algorithm puts it beyond the reach of current quantum processors. Based on a recently published work with Professor Mark Tame, we also report a proof-of-concept demonstration of a quantum order-finding algorithm for factor- ing the integer 21. Our demonstration builds upon a previous demonstration by Martín-López et al. in Nature Photonics 6, 773 (2012). We go beyond this work by implementing the algorithm on IBM Q quantum processors using a configuration of approximate Toffoli gates with residual phase shifts, which preserves its functional correctness and allows us to achieve a complete factoring of N D 21 using a quantum circuit with relatively fewer two-qubit gates. Lastly, we realize a small-scale three-qubit quantum processor based on a spontaneous parametric down-conversion source built to generate a polarization-entangled Bell state. The state is enlarged by using the path degree of freedom of one of the photons to make a 3-qubit GHZ state. The generated state is versatile enough to carry out quantum correlation measurements such as Bell’s inequalities and entanglement witnesses. The entire experimental setup is motorized and made automatic allowing remote control of the measurements of each of the qubits, and we design and build a mobile graphical user interface to an provide intuitive and visual way to interact with the experiment.