Ultra-high sensitivity scanning SQUID microscopy with dispersive readout

• Applicant: Professor Dr. Hendrik Bluhm
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2014 to 2017
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 256185976

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.

• DFG Programme: Research Grants