Decoherence in Josephson qubits due to charge and spin fluctuators

量子电路的相干控制水平，特别是约瑟夫森结，在过去十年中有了很大的进步。退相干的主要来源已被识别和/或抑制通过仔细的电路工程手段。因此，约瑟夫森量子比特（量子位）的相干时间通常达到数十甚至数百微秒。其余的噪声源很细微。它们大多与用于器件的材料的性质或电路环境中无法达到热平衡有关。研究这些影响是本提案的主要主题。我们的目标是双重的。一方面，我们希望为进一步提高量子电路的相干时间的目标做出贡献。另一方面，我们的目标是更好地理解材料特性，由于量子比特技术的进步，这变得容易实现。具体来说，我们将研究：(i) Josephson结中两能级系统（TLS）的性质及其在介电损耗、电荷噪声和临界电流噪声中的特征。我们将研究由于与TLS耦合而引起的量子比特弛豫机制以及TLS在固态环境下的本征耗散机制。我们还将研究TSL之间相互作用的影响。结合约瑟夫森量子比特在噪声光谱方面的最新进展，我们希望能够促进对TLS微观性质的理解。（ii）顺磁自旋对表面和界面的影响。它们似乎是造成观测到的磁通量1/f噪声的原因。我们将研究自旋扩散、自旋阻尼和自旋玻璃物理的相互作用。目的是了解磁通噪声的统计特性，即1/f类功率谱的频率和温度依赖性以及非高斯特性（噪声的噪声）。

The level of coherent control of quantum circuits in general and Josephson junctions in particular has advanced greatly in the last decade. Major sources of decoherence have been identified and/or suppressed by means of careful circuit engineering. As a consequence, the coherence times of Josephson quantum bits (qubits) reach routinely tens to even hundreds of microseconds. The remaining sources of noise are subtle. They are mostly related to properties of the materials used for the devices or to the failure to reach thermal equilibrium in the circuit surroundings. Investigation of these effects is the main subject of the present proposal. Our aims are twofold. On one hand, we want to contribute to the goal of further enhancing the coherence times of quantum circuits. On the other hand, we aim at a better understanding of material properties, which became accessible due to the advances in qubit technology. Specifically we are going to investigate: (i) The properties of two-level systems (TLS) in Josephson junctions and their signatures in dielectric losses, charge noise, and critical current noise. We will study the mechanisms of qubit relaxation due to the coupling to the TLS and those of the intrinsic dissipation of the TLS in the solid state environment. We will also investigate the effects of interactions between the TSL. In combination with the recent advances in noise spectroscopy with Josephson qubits we hope to be able to advance the understanding of the microscopic nature of the TLS.(ii) The effects of paramagnetic spins on surfaces and interfaces. They appear to be responsible for the observed 1/f noise of the magnetic flux. We will study the interplay of spin-diffusion, spin damping, and spin glass physics. The aim is to understand the statistical properties of the magnetic flux noise, i.e., the frequency and temperature dependence of the 1/f-like power spectrum as well as the non-Gaussian properties (noise of noise).