超导电路量子电动力学系统的调控与读取

发布时间：2018-10-10 11:23

【摘要】：超导量子比特作为人工原子,不但是实现量子计算的热门方案,而且是研究量子力学本质问题的有力工具。近年来在腔量子电动力学系统基础上发展起来的电路量子电动力学系统,是一种全新的量子比特。由于在退相干时间等参数上远远超出之前的超导量子比特,它受到了极大的关注。和”传统”的超导量子比特不同,电路量子电动力学系统使用微波谐振腔作为系统的读出机构。这样的读出机构一方面可以减小超导量子比特和环境的耦合,提高退相干时间；另一方面给我们提供了全新的量子比特信息读取方案一一量子非破坏性测量。此外,该结构在系统集成等方面也具有显著的优势。所以在目前的超导量子比特中,该系统的性能是最优异的。本文首先介绍了量子计算,量子测量的基本概念以及三种“传统”的量子比特：电荷比特,磁通比特和相位比特。然后系统的介绍电路量子电动力学系统。该系统主要包括了谐振腔,人工原子,以及它们之间的耦合三大部分。本文中,作为人工原子的量子比特总共有三个,分别是三维Transmon,二维Transmon和三维磁通比特；使用的谐振腔有两种,一种是矩形波导谐振腔,由铝或者无氧铜材料制作而成,另一种是共面波导谐振腔。当把人工原子放置在谐振腔中使它们耦合在一起时,就构成了电路量子电动力学系统。接着,论文着重介绍了在稀释制冷机平台上搭建的测量系统,并介绍了两种测量方法：亮态测量和量子非破坏性测量。在本文的最后,论文介绍了利用该测量系统测量电路量子电动力学系统的结果。根据使用的能级数目不同分为二能级系统和三能级系统两章。在二能级系统中,主要介绍了拉比振荡,拉姆齐干涉,自旋回波,量子层析等。通过这些测量结果,获取了二能级系统的退相干参数。在三能级系统中,主要介绍了暗态,相干粒子数囚禁,奥特勒-汤尼斯劈裂和电磁感应的透明等实验结果。在暗态和相干粒子数囚禁实验中,我们在实验上通过两束微波成功的把三能级系统的态冻结,使其不再随时间演化。控制两束微波的强度和相位,我们可以冻结任意的态,所以该方法可以作为量子存储的方案,在量子计算中有着重要作用。在奥特勒-汤尼斯劈裂实验中,我们测量了共振情况下的能级劈裂,该能级劈裂的大小和理论模拟完全吻合。为了观察三能级的电磁感应的透明,我们尝试在三能级中引入噪声,使得三能级系统满足电磁感应的透明的条件。虽然最后没能看到预期的效果,但是根据能量弛豫时间的估算,该系统已经进入电磁感应的透明的临界条件。
[Abstract]:As an artificial atom, superconducting quantum bit is not only a popular scheme to realize quantum computation, but also a powerful tool to study the essential problems of quantum mechanics. In recent years, the circuit quantum electrodynamics system developed on the basis of cavity quantum electrodynamics system is a new kind of quantum bit. It has attracted much attention because it is far beyond the previous superconducting quantum bits in terms of decoherence time and other parameters. Unlike the "traditional" superconducting quantum bits, the circuit quantum electrodynamics system uses a microwave resonator as the readout mechanism of the system. On the one hand, the readout mechanism can reduce the coupling between the superconducting quantum bit and the environment and improve the decoherence time; on the other hand, it provides us with a new quantum bit information reading scheme, quantum nondestructive measurement. In addition, the structure also has significant advantages in system integration. Therefore, the performance of the system is the best among the current superconducting quantum bits. This paper first introduces the basic concepts of quantum computation, quantum measurement and three kinds of "traditional" quantum bits: charge bit, magnetic flux bit and phase bit. Then the circuit quantum electrodynamics system is introduced systematically. The system consists of three parts: resonator, artificial atom and coupling between them. In this paper, there are three quantum bits as artificial atoms, which are three-dimensional Transmon, two-dimensional Transmon and three-dimensional flux bits. There are two kinds of resonators used, one is rectangular waveguide resonator, which is made of aluminum or oxygen free copper. The other is coplanar waveguide resonator. When artificial atoms are placed in the resonator to couple them together, a circuit quantum electrodynamics system is constructed. Then, the paper mainly introduces the measurement system built on the platform of dilution refrigerator, and introduces two measuring methods: bright state measurement and quantum nondestructive measurement. At the end of this paper, the results of quantum electrodynamics system are introduced. According to the number of energy levels used, it can be divided into two chapters: the two-level system and the three-level system. In the two-level system, the Rabi oscillation, Ramsay interference, spin echo, quantum chromatography and so on are introduced. Through these measurements, the decoherence parameters of the two-level system are obtained. In the three-level system, the experimental results of dark state, coherent particle number trapping, Otell-Tonys splitting and the transparency of electromagnetic induction are introduced. In the experiments of dark state and coherent particle number trapping, we have successfully frozen the state of the three-level system by two microwave beams, so that it no longer evolves with time. We can freeze arbitrary states by controlling the intensity and phase of two microwave beams, so this method can be used as a quantum storage scheme and plays an important role in quantum computation. In the Otleer-Tonys splitting experiment, we measured the energy level splitting in the resonance case, and the size of the energy level splitting is in good agreement with the theoretical simulation. In order to observe the transparency of the three-level electromagnetic induction, we try to introduce noise into the three-level system so that the three-level system satisfies the condition of the transparency of the electromagnetic induction. Although the desired effect was not observed in the end the system has entered the critical condition of electromagnetically induced transparency according to the estimation of the energy relaxation time.
【学位授予单位】：南京大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：O413.2

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