Manipulation And Decoherence Of Superconducting Qubits

Superconducting quantum computation has made great progress in the last decade. Su-perconducting quantum bit is becoming one of the most hopeful realizations of future quantum computer. This kind of qubit is made up of superconduting circuit, which includes a few of Josephson junctions. Unlike microscopic entities-electrons, atoms and ions-on which other qubits are based, superconducting quantum circuits have a mesoscopic scale. The advantage of superconducting qubits compared to the natural qubit is that they could be designed on purpose and be easily manipulated. What is more, they can strongly interact with each other and with other quantum systems, such as superconducting transmission line, which makes them more suitable to scale up. However, the other side of the coin is that superconduting qubits have very short co-herence time. The reason is that the qubit interact with many degree of freedom of its environment both inside and outside the superconducting circuit. Therefore, it is very important for prolonging the coherence time of the qubit to explore the mechanism of the noises of the qubit. Microscopic two-level defects in Josephson junction is one kind of intrinsic noise source in superconducting Josephson circuit. Although they were ob-served in superconducting qubit almost ten years ago, neither their microscopic source nor the coupling mechanism between them and qubit are uncovered without doubt. Another study line of superconducting quantum computation is manipulating qubits with less operation errors. Landau-Zener interference could be applied to quantum information processing. In conventional Landau-Zener interference, the interference result is affected by dynamical phase accumulated between two successive Landau-Zener transitions。Hence the conventional interference is not robust to local noises and parameter fluctuations. Geometric phase is immune to certain parameter fluctua-tions. However, till now there was no experimental report of Landau-Zener interfer-ence based on geometric phase. Fault-tolerant quantum computation can be achieved by using error correcting code if the error rate of single quantum operation is below the threshold10-4. Unfortunately, the error threshold is unreachable for state-of-the-art superconducting qubit. The other way to realize fault-tolerant quantum computation is topological quantum computation. The drawback of topological quantum computation is that the usually-used topological qubits can not realize universal quantum logical gates. Nevertheless, we could combine superconducting qubit with topological qubit to take both advantages of them. Below we would introduce our work:1. We have provide an experimentally feasible scheme to clarify the coupling mechanism of the two-level defect system and superconducting qubit. We found that in a three-junction flux qubit the relative magnitudes of the transverse and longitudinal coupling factors are largely model-dependent and very sensitive to the external flux bias. We propose that these features can be used to clarify the microscopic model of the two-level system.2. We have constructed a theoretical scheme using phase qubit and demonstrated in experiment geometric Landau-Zener interference using phase qubit. The interfer-ence result depends on non-adiabatic geometric phase instead of dynamical phase. In our scheme, the anti-crossing is generated by microwave coupled to the phase qubit, and the dynamical phase is offset by using spin-echo microwave pulse. Due to its geo-metric nature, this new interference is robust to some noises, and is more suitable than its conventional analog for quantum manipulation.3. We have developed a hybrid topological qubit and superconducting flux qubit system. A composite system of Majorana-hosted semiconductor nanowire and super-conducting flux qubits, named top-flux-flux, is presented to process quantum informa-tion. We can electrically control the coupling between the Majorana-based topological qubit and the readout flux qubit, supplying a convenient method to implement a π/8phase gate of the topological qubit. In addition, we design a scheme to transfer quan-tum information back and forth between the topological qubit and the flux qubit by employing the Landau-Zener transition. With the demonstration of the entanglement of two topological qubits, it is very promising to use this hybrid system for quantum-information processing.