| In recent years, quantum computation has become an interdisciplinary field at the intersection of both, theory of quantum mechanics, quantum mechanics experiments, computer science, mathematics, quantum engineering, quantum metrology, materials science, and so on. Superconducting quantum bits (qubits), as a promising candidates for realizing quantum computation, has advantages of being coupled with the outside world, easy to be integrated and so on. In this paper we research the fabrication processes of superconducting flux quantum bits chip using aluminum, and have an analysis of several parameters such as sub-gap current, current density and barrier layer thickness in the Al/AlOx/Al tunnel junctions, which is the key device in the chip. At the same time, we have an electrical properties measurement of the Al/AlOx/Al tunnel junctions and the DC-SQUID in the superconducting quantum bits.Appropriate physical interpretation are given with the quantum properties in the measurement of tunnel junctions. This article includes the following aspects:1. Decreasing sub-gap leakage current ratio of the Al/AlOx/Al tunnel junction by improving the Al film flatnessWe make the flatness analysis of aluminum films with different thicknesses by using atomic force microscopy (AFM), and then choose a thinner films for improving the Al film flatness.Besides,we make advances in the flatness of Al film that are all in the same thickness by the means of increasing the distance between evaporation source to the substrate in the electron beam evaporation system. The I-V characteristics of the tunnel junction show that the tunnel junction which has better flatness of Al films has a lower leakage current ratio.2. The influence of static oxidation parameters on the barrier layer of AlOx-We control the superconducting current density of the tunnel junction by adjusting the oxygen partial pressure in static oxidation. Making analysis of tri-layer structure of aluminum tunnel junction through transmission electron microscopy (TEM), we have gotten the barrier layer thickness in the range of 10-1000 Pa oxygen partial pressure.According to the equation of jc ≈ 2ehpoe-d/(?)/m(?)d,we make numerical computation and numerical fitting. In certain parameter values, the experimental results and numerical results are in good agreement.3. The analysis of the phase diffusion phenomenon in Al/AlOx/Al tunnel junctionPlacing the Al/AlOx/Al tunnel junction in different temperature,we had the measurement of switching current distribution(SCD). The transition from macroscopic quantum tunneling mechanism(MQT) to thermal activation(TA) is clearly observed in the range of 20 mK to 50 mK. When the temperature is higher than 80 mK, the phase diffusion (PD) phenomenon,in which the width of SCD is decreased when we increase the temperature,is the main feature that the switching current is affected by phase diffusion regime.4. Design of superconducting magnetic shielding systemUsing Niobium as superconducting materials, we have designed a superconducting magnetic shielding system for testing parameters of tunnel junctions in 3He refrigerator. After adding the magnetic shielding system, the junctions’ superconducting current does not been changed. The results indicates that the magnetic shielding system we designed plays a certain magnetic shielding effect.The above research work lay the foundation for the preparation of a superconducting flux qubit chips of better performance. |
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