Research

Spin qubits are created via coherent control of single spins in their environment. We build and study chip-based electronic quantum circuits to experimentally implement few-qubit configurations in silicon and in germanium. Low hyperfine interaction in isotope-purified silicon and germanium and limited spin-orbit interaction are key advantages, resulting in attractive spin qubit coherence times. The isolation and the coherent control of single spins are realized in gate-defined Coulomb blockade quantum dot configurations in two-dimensional carrier systems. Our group has a longstanding expertise in gate-controlled rectangular quantum well Si/SiGe (electron spin qubits) and Ge/GeSi (hole spin qubits) semiconductor heterostructures and has pioneered the use of isotope-purified silicon (²⁸Si) and germanium (⁷⁰Ge) in MBE-grown Si/SiGe and Ge/GeSi heterostructures for spin qubit applications.

Currently, we are particularly interested in uncovering microscopic mechanisms which set the experimental limits of the qubit coherence and different qubit fidelities (initialization, manipulation, readout). Furthermore, we participate in transmitting the knowledge developed in academic labs to industrial pilot-lines, to leverage the synergies with the current silicon chip industry, in order to launch massive upscaling of our comparably small footprint qubit unit cells into chip-based quantum computing processors.

Semiconductor crystals can be excited optically or electrically to spontaneously induce light emission. By engineering the dielectric environment of the excited semiconductor, emitted light can not only be guided into optical modes or trapped in cavities, but also made to strongly interact with the semiconductor, creating phenomena such as stimulated emission.

We build nanocavities and optical mode guiding with custom-tailored dielectric properties, integrated with semiconductor emitters which for example allow us to explore the physics of low threshold lasing and to delve into experimental cavity quantum electrodynamics (cQED).

Regarding low threshold lasing, we are currently experimentally studying the physics governing lasing of single MBE-grown GaAs nanowires, considering aspects such as spatial quantum confinement of the active lasing volume, mode guiding as a function of the nanowire geometry as well as enhanced emission via coupling to engineered plasmonic substrates.

We have realized cQED devices by confining Landau-quantized as well as magneto-plasmonic states in densely packed 2D electron systems with switchable metamaterials. Spin qubits and Josephon junctions also represent quantum mechanical matter systems which can be coupled to superconducting microwave resonators to study cQED.