Our group pursues two main research directions. The larger effort is dedicated to the development and study of electron spin qubits in quantum dots. The long term goal is to establish a semiconductor based platform for scalable quantum information processing, which promises an exponential speedup for certain computational problems by taking advantage of quantum mechanical superposition and entanglement. Our approach is based on confining individual electrons using electrostatic gates and manipulating them via electric fields. We are investigating the fundamental physics of these devices and at the same time advancing their technological development. As with nearly all types of qubits, major topics are decoherence, i.e. the loss of quantum mechanical behavior, accurate manipulation, and coupling multiple qubits.
We are pursuing qubits fabricated in both the GaAs/AlGaAs and Si/SiGe material systems. The former host material is more mature and the required device functionality is well established, but has the drawback of strong coupling between electrons and nuclear spins, which leads to strong decoherence. Si-based qubits could largely avoid this problem by isotopic purification, but are less well developed in terms of fabrication yield and material quality.
The second research area is the development and use of scanning SQUID microscopy for imaging and measuring magnetic effects in micro- and nanoscale samples at temperatures as low as 20 mK. The basic principle of scanning SQUID microscopy is to scan a magnetic field sensor over the sample surface, thus obtaining information about local magnetic properties. SQUIDs (superconducting quantum interference devices) are highly sensitive magnetic flux sensors. We are developing a novel microwave based SQUID readout technique that promises a significantly better sensitivity and bandwidth compared to conventional DC-based readout. Our scanning SQUID microscope will be used to study spin phenomena such as flux noise in superconducting qubits due to impurity spins, magnetic molecules, and scanning based resonance imaging techniques. We are also interested in quantum effects such as persistent currents or tunneling of superconducting vortices.