超导量子比特测量链路的优化

  量子计算机由于具有超强算力，逐渐成为如今热门的研究领域。在众多备选的实验方案中，超导量子计算体系目前是最有可能实现通用量子计算的路径之一。然而，由于环境噪声的影响，超导量子比特不可避免的发生退相干，导致量子计算过程的失真。因此，在测量控制系统的线路上加入合适的滤波和衰减器件，从而降低量子比特感受到的噪音强度，显得十分关键。低温环行器是超导量子比特测控系统中的一个关键低温微波器件，主要有两方面的应用，其一是作为隔离器降低稀释制冷机的高温盘区的热噪音通过测控线对超导量子芯片的影响，其二是作为环行器对超导量子比特读取信号进行特定方向性的传导。目前实用于超导量子计算工作频率的微波波段的商用低温环行器，主要都是国外公司的产品，价格昂贵且存在禁运风险，而国内则鲜有对这类低温微波器件的研究。因此，本论文对高隔离度、适用于4K低温下的C波段环行器展开研究。低温环行器与室温环行器最主要的区别在于构成环行器的永磁体、铁氧体和介质等材料的选取，因此研制低温环行器需要充分考虑这些材料在极低温环境下的性质。

  本文总结了带线环行器的设计流程，包括材料选择和设计电路两部分。基于环行器中使用的磁性材料特性以及由于铁氧体材料旋磁特性产生的环行器非互易性、非线性效应和损耗，通过测试实验选择具有合适的磁矩的磁性材料，并对带线环行器中不同结构的尺寸计算，完成了一个适用于7-9GHz的带线双Y结环行器的设计。

  本文对设计的环行器进行了实物的常温、低温测试，通过实验和仿真模拟结果比较，得出该环行器设计在磁性材料选取、填充介质介电常数和内导体形状中的优化方案。实验结果表明，低温下的环行器在7-9GHz具有较好的环行性能，基本达到隔离度、插入损耗的指标。

  Quantum computers are gradually becoming a popular research area today due to their super computing power. Among the different implementation methods, superconducting quantum computers are recognized as the most promising quantum computers for pervasiveness. The development of superconducting quantum computing faces the problem of decoherence of quantum bits due to the effect of noise in the experimental environment, therefore, it is crucial to study the devices that reduce noise in the system. The cryogenic circulator is a key cryogenic microwave device in the superconducting quantum bit measurement and control system, which has two main roles: as an isolator to isolate the noise brought to the superconducting quantum chip by the cryogenic measurement and control line inside the dilution chiller; as a circulator to separate the superconducting quantum bit reading input and output signals. At present, there is little research on such cryogenic microwave devices in China, and the commercially available foreign-made cryogenic circulators are expensive and subject to embargo risks. Therefore, in this thesis, a C-band circulator with high isolation and suitable for 4K cryogenic temperature is investigated. The main difference between cryogenic and room temperature circulators lies in the selection of materials such as permanent magnets, ferrites and dielectrics that make up the circulator, so the development of cryogenic circulators needs to fully consider the properties of these materials at very low temperatures.

  This paper summarizes the design process for a wire-banded circulator, including both material selection and design of the circuit. Based on the characteristics of the magnetic materials used in the circulator and the non-reciprocity, non-linear effects and losses of the circulator due to the spinning magnetic properties of the ferrite material, the design of a band-line double Y-junction circulator suitable for 7-9 GHz is completed by selecting magnetic materials with suitable magnetic moments through test experiments and calculating the dimensions of different structures in the band-line circulator.

  In this paper, the designed circulator is tested in real life at room temperature and low temperature, and the experimental and simulation results are compared to arrive at an optimized solution for this circulator design in terms of magnetic material selection, dielectric constant of the filling medium and shape of the inner conductor. The experimental results show that the circulator at low temperature has good circulating performance at 7-9 GHz, and basically achieves the index of isolation degree and insertion loss.

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