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Investigation of Quantum Correlations of Coupled Superconducting Qubits in Circuit Cavity QED

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Tutor: ZuoYingHua
School: Jiangxi Normal University
Course: Theoretical Physics
Keywords: Circuit Cavity QED,Coupled Superconducting Qubits,Concurrence,Quantum Discord,De
CLC: O413.2
Type: Master's thesis
Year:  2013
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Abstract:
With the rapid development of quantum computing and quantum informationtechnology, superconducting qubits shows enormous potential in it, andthe quantum correlation is considered to be an essential resource to investigate thequantum information process. Therefore, we compared and investigated the quantumcorrelations of coupled superconducting qubits in circuit cavity quantumelectrodynamics, and analyzed the effects of decoherence on the system. The researchresults may be benefit for completing quantum information processing, and theresearch content mainly includes the following points.1. Investigated and analyzed the effects of the interaction betweensuperconducting qubits and circuit cavity on the entanglement and geometric quantumdiscord evolutions when the superconducting qubits initially in different states. Theresults show that,(1) When the coupled qubits initially in|ψ_Q(0)〉_Ⅰ=sinα|↓↑〉+cosα|↑↓〉, through enhancing the interaction between coupledqubits can increase its robustness in evolution process, and the quantum discordbetween qubits can be promoted in whole. Regulating the appropriate parameters caneffectively avoid the death of concurrence and geometric quantum discord. Theinfluence of interactions between qubits on entanglement is nearly the same as ongeometric quantum discord.(2) When the coupled qubits initiallyinψ_Q (0)〉_Ⅱ=sinα|↓↓+cosα|↑↑〉, the entanglement and geometric quantumdiscord of superconducting qubits also does not exist the gradual stable value. Butthrough weakening the interaction between coupled qubits can enhance its robustnessin evolution process. It should be pointed out that we can conveniently realize thecoupling of multiple superconducting qubits. At this time, the distance between twosuperconducting qubits is far away, so the interaction between them can be neglected.2. Investigated the effects of the cavity state on quantum correlations dynamicevolution behaviors of coupled superconducting qubits in circuit QED system.When the cavity field in coherent state, with the average photon number increasing,the quantum correlations for zero including entanglement death can be more difficult to appear, that is to say prolonging the survival time of quantum correlations will bebenefit for keeping the quantum correlations. When the cavity field is in squeezedstate, the squeezed amplitude parameters too big or too small are all not available tokeep the system quantum correlations. The further study results show that with theincrease of the initial relative phase of coupling superconducting qubits can also keepthe quantum correlations.3. Investigated the effects of decoherence on the quantum entanglement andquantum discord of coupled superconducting qubits in circuit cavity, and analyzedthe effects of parameters (reflects the initial entanglement of quantum state),(reflects the relaxation rate of qubits), and (reflects the dephasing rate of qubits) onsystem quantum entanglement and quantum discord evolutions. We found that in theprocess of decoherence, the quantum discord showed more stable properties thanentanglement. The entanglement and quantum discord are all decreasing to deadevolve with time. However, the survival time of quantum discord was obviouslylonger than of entanglement with the change of parameters. In conclusion, quantumdiscord was more robust than entanglement in the process of decoherence, and it is amore suitable method for the measure of quantum correlations to complete quantuminformation processing.
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