Ultra-high sensitivity scanning SQUID microscopy with dispersive readout

扫描SQUID显微镜采用超导干涉装置（SQUID）作为磁场传感器，对样品进行扫描，以便成像或测量其磁性。传统上，传感器通过直流测量其I-V特性来读出。我们请求支持开发一种新型的SQUID，用于采用色散读出的扫描实验。SQUID用电容器进行分流，形成一个非线性LC谐振器，其谐振频率随通过SQUID的磁通而变化，可以通过反射外部产生的微波信号来测量。仿真和初步实验表明，该方法可以显著提高灵敏度和带宽。理解和优化这类器件的噪声性能需要详细研究由约瑟夫森电感引起的非线性。我们从IBM沃森研究中心获得的设备的这种特性是这个项目的主要目标之一。其次，这些设备将被集成到一台扫描SQUID显微镜中，放在稀释冰箱中。一旦完成，该仪器将与扫描自旋共振和磁化率测量技术结合使用，研究表面自旋的起源和动力学。这种表面自旋是超导器件中通量噪声的原因。这种通量噪声是超导量子比特的主要障碍，因为它限制了消相时间，并对它们的设计和操作施加了限制。此外，了解自旋之间的相互作用如何导致1/f噪声是基本的兴趣。该项目结合了我们在高灵敏度扫描SQUID显微镜和高频技术以及自旋量子比特实验中获得的自旋物理学方面的经验。从长远来看，它将为自旋和量子现象的广泛研究提供基础。

Scanning SQUID microscopy employs superconducting interference devices (SQUIDs) as magnetic field sensors that are scanned over a sample in order to image or measure its magnetic properties. Traditionally, the sensors are read out via DC measurements of their I-V characteristics. We request support for the development of a novel type of SQUID for scanning experiments employing dispersive readout. The SQUID is shunted with a capacitor so that it forms a nonlinear LC resonator whose resonance frequency varies with the flux through the SQUID and can be measured via reflection of an externally generated microwave signal. Simulations and preliminary experiments indicate that significant improvements in sensitivity and bandwidth can be achieved with this approach. Understanding and optimizing the noise performance of such devices require a detailed study of their nonlinearity arising from the Josephson inductances. This characterization of our devices, which are being obtained from IBM Watson Research Center, is one of the primary goals of this project. Second, the devices will be integrated into a scanning SQUID microscope in a dilution refrigerator. Once completed, this instrument will be used in conjunction with scanning spin resonance and susceptibility measurement techniques to study the origin and dynamics of spins on surfaces. Such surface spins are responsible for flux noise in superconducting devices. This flux noise is a major hurdle for superconducting qubits as it limits the dephasing times and imposes restrictions on their design and operation. Furthermore, it is of fundamental interest to understand how interactions between spins can lead to 1/f noise. The project combines our experience with high sensitivity scanning SQUID microscopy and with high frequency techniques and spin physics gained from spin qubit experiments. In the longer term, it will provide the foundations for a wide range of studies of spin and quantum phenomena.