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Design and Optimization of the Operation and Readout of Superconductor Qubits

Zandi, Hesam | 2012

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 43641 (05)
  4. University: Sharif University Of Technology
  5. Department: Electrical Engineering
  6. Advisor(s): Khorasani, Sina; Fardmanesh, Mehdi
  7. Abstract:
  8. Computational instruments such as personal computers (PC) are among the most important and developed inventions needed for the society. Nowadays, the fastest classical computing machines are based on the conventional semiconductor fabrication technology. But these computers cannot satisfy all of the requirements of today’s ongoing developments. Two important weaknesses of the undergoing technology are the speed of computing which is in the frontend of the semiconductor technology, and solving some complicated problems which may take years and even decades with these computers. This is while, based on quantum theory and related algorithms, solving of these problems would take much less time. In order to exploit quantum abilities, quantum computers need quantum bits (qubits) instead of classical ones, which among all different platforms, superconductor qubits have drawn the highest attention. All of the researches about these qubits are performed considering trivial sinusoidal relation between current density and phase difference of Josephson junctions. But this is just an approximation, and because of high sensitivity of qubits, more accurate calculations are required in those junctions. To improve the qubit’s operation, we utilized a new method for analyzing the tunneling phenomenon in Josephson junctions, in which the state tunneling probabilities of a qubit is calculated by accurate solving of the nonlinear Ginzburg–Landau equation. For the first time, the precise effects of junction and bias parameters such as thickness of the insulating barrier, cross sectional area, bias current, and magnetic field are fully investigated in this project. Defining and computing appropriate figures of merit, optimum conditions of the phase qubit operation and its state measurement are achieved. Results are in complete agreement with a reported experimental data from a distinguished group, which confirms the genuineness of the procedure. We have also studied 2D-tunneling concept in coupled two-qubits, from which a novel designed configuration came out. In coupled quantum systems, the individually correct state measurement is not possible due to the association of the states (specially the entanglement). The new configuration is designed in such a way that the crosstalk issue will be severely suppressed. This configuration has led us to design a single qubit circuit with two coupled junctions, in which the measurement fidelity reaches to 99.99%. This can be even obtained by current pulses shorter than 1 nanoseconds. Finally, the coherent time of the states of this system has been computed revealing a significant improvement
  9. Keywords:
  10. Superconductor ; Josephson Junction ; Cross Talk ; Quantum Information Theory ; Phase Qubit ; Measurement Fidelity

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