Dynamic Decoupling and Noise Characterization in Superconducting Qubits

超导量子比特（量子比特）是固态人造原子，由约瑟夫森隧道结和超导互连和微波谐振器组成。当冷却到毫开尔文温度时，这些超导电路表现出量子力学行为，例如取决于设计参数的通量、电荷或结相的量子化状态。这种超导人造原子已经被证明是一种有用的工具，可以促进科学界对量子力学系统中的相干性的普遍理解，特别是在自然原子和分子不容易达到的制度中。此外，超导量子比特是量子信息科学和技术应用的有希望的候选者，包括量子计算。使用超导量子比特的主要限制因素是噪声，这些系统中的噪声源将被研究和表征。NMR相关领域的既定技术（如磁共振成像中使用的技术），例如，被称为动态解耦和二维NMR光谱的目标控制序列，将被调整和扩展。破坏这些超导量子比特的量子性质的潜在微观来源将被识别和减轻。虽然量子信息科学和技术的未来应用仍在被认可，但技术本身带来的广泛社会效益（例如，量子传感器，模拟机），以及将出现的辅助附带技术（例如，材料，制造，控制模式），和年轻的研究人员谁的工作，使他们成为现实。这项工作的一个教育特点将是学生在学术（麻省理工学院，美国）的访问和参与。东京，查尔默斯），企业（NEC）和政府（林肯实验室，理研）在美国，日本和瑞典的研究环境。通过共享研究和学生实习，这将提供一种文化，培养具有全球研究视角的年轻科学家，能够跨越机构和国际边界开发和领导跨学科团队。这项工作解决了先进的高相干超导量子比特中噪声源的表征、识别和缓解问题。一个目标是使用诸如动态解耦的控制技术来评估和减轻新一代这些先进量子比特（具有高Q材料的2D和3D transmons，亚稳态通量量子比特）中的噪声。通常，这是使用基于NMR的技术来实现的，该技术已知用于基准测试和阐明微观噪声发生器，以便识别和减轻退相干的潜在来源。我们的目标是了解是什么限制了它们的相干时间。然后，这些信息可以用于改进制造工艺和材料。第二个目标是进一步推进噪声表征和缓解工具集。该研究将利用正交幅度调制器和序列器产生任意幅度和相位的微波脉冲序列，将其应用于稀释制冷机中的transmons和亚稳态通量量子比特。相干表征，包括标准相干时间的测量，将作为量子比特量化轴的函数进行。随机基准测试，状态断层扫描和过程断层扫描将用于表征门保真度。退相干将通过应用动态去耦脉冲序列来减轻。这些技术也将用于测量噪声功率谱密度。二维核磁共振光谱技术将被用来评估微观性质和起源的退相干。

Superconducting qubits (quantum bits) are solid-state artificial atoms, comprised of Josephson tunnel junctions and superconducting interconnects and microwave resonators. When cooled to milli-Kelvin temperatures, these superconducting circuits exhibit quantum mechanical behavior, such as quantized states of flux, charge, or junction phase depending on design parameters. Such superconducting artificial atoms have already proven a useful vehicle for advancing the scientific community's general understanding of coherence in quantum mechanical systems, particularly in regimes not easily accessible with natural atoms and molecules. Moreover, superconducting qubits are promising candidates for quantum information science and technology applications, including quantum computing. The main limiting factor in using superconducting qubits is noise, The sources of noise in these systems will be studied and characterized. Established techniques from the related field of NMR (such as those used in Magnetic Resonance Imaging), for example, the targeted control sequences known as dynamical decoupling and two-dimensional NMR spectroscopy, will be adapted and extended. The underlying microscopic sources that destroy the quantum nature of these superconducting qubits will be identified and mitigated. While the future applications of quantum information science and technology are still being recognized, a broad social benefit from the technology itself (e.g., quantum sensors, simulation machines) is anticipated, as well as from the the ancillary spin-off technologies that will arise (e.g., materials, fabrication, control schema), and the young researchers who work to make them a reality. An educational feature of this work will be the access and participation by students in academic (MIT, U. Tokyo, Chalmers), corporate (NEC), and government (Lincoln Laboratory, RIKEN) research environments in the US, Japan, and Sweden. Via shared research and student internships, this will provide a culture that fosters young scientists with a global research perspective, capable of developing and leading interdisciplinary teams across institutional and international boundaries. This work addresses the characterization, identification, and mitigation of noise sources in advanced, high-coherence superconducting qubits. One objective is to use control techniques such as dynamical decoupling to assess and mitigate noise in a new generation of these advanced qubits (2D and 3D transmons with high-Q materials, metastable flux qubit). In general, this is achieved using NMR-based techniques that are known to benchmark and elucidate microscopic noise generators in order to identify and mitigate the underlying sources of decoherence. The goal is to understand what is limiting their coherence times. The information can then be used to improve fabrication processes and materials. A second objective is further advance the noise characterization and mitigation toolset. The research will utilize a quadrature amplitude modulator and sequencer with arbitrary amplitude and phase to generate microwave pulse sequences, which will be applied to transmons and metastable flux qubits in a dilution refrigerator. Coherence characterization, including the measurement of standard coherence times, will be performed as a function of the qubit quantization axis. Randomized benchmarking, state tomography, and process tomography will be used to characterize gate fidelity. Decoherence will be mitigated through the application of dynamical decoupling pulse sequences. These techniques will also be used to measure the noise power spectral density. Two-dimensional NMR spectroscopy techniques will be used to assess the microscopic nature and origin of decoherence.