Research

Research topics and projects

We do experimental research that focuses on the quantum physics of electron spin ensembles in semiconductor devices (in GaAs-based materials, ZnSe, SiC, and organic semiconductors). Current projects study electron spin manipulation, transport and dephasing for ensembles, electron many-body effects, and quantum state transfer between spin states and optical pulses. We use electron transport methods, optical pump-probe techniques and quantum optical techniques. Our work contributes to fundamental research on quantum theory for complex systems, and to the development of new applications in the area of quantum information technology and spintronics.
In recent years our most important collaboration for obtaining MBE grown GaAs/AlGaAs heterostructure wafer materials and high-purity n-GaAs epi-layers was with the group of Andreas Wieck in Bochum (Germany). For MBE grown materials we will also continue to collaborate with Dirk Reuter, who started a new group in Paderborn (Germany) (after a position in Bochum till 2012).
For SiC materials, our main collaboration is the group of Nguyen T. Son and Erik Janzen (Linköping, Sweden).

Scroll down to find more information on facilities and the research projects:

1. Electron many-body effects in semiconductor nanodevices

This project studies electron many-body effects in GaAs quantum point contacts (QPCs). These are short one-dimensional transport channels in which the electron transport is ballistic. Transport measurements on such devices reveal signatures of electron many-body effects, which include the so-called 0.7 anomaly, enhancement of the electron g-factor, and the Kondo effect. These phenomena are not yet fully understood, but very interesting and important for further developments in nano-electronics and quantum devices. By only making a narrow constriction in a clean non-magnetic semiconductor, a state is formed that shows signatures of emergent localization via the Kondo effect. We currently investigate how the constriction geometry and Coulomb effects influence the signatures of these many-body phenomena. In strong magnetic fields, electron emission from QPCs can be spin-polarized, and we also investigate how the appearance of this can be optimized.

2. Optical pump-probe studies of transport and dephasing
of electron spin ensembles in low-dimensional devices

Time-resolved Kerr rotation measurements allow for preparing and detecting the spin states of electron ensembles in GaAs with very high (subpicosecond) time resolution. At the same time, a reasonable (around micronscale) spatial resolution can be achieved. We use this in this project to study how electron spin states evolve and dephase while they are localized or transported in diffusive micronscale devices. The electron ensembles in these devices can be low-dimensional with respect to quantum confinement, the spin precession length or the spin diffusion length. Interactions between electron spins and the environment are typically dominated by spin-orbit effects, or coupling to nuclei. We study how a lower dimensionality for an ensemble can influence spin dephasing, or can result in spin manipulation during transport. A new effort here aims at linking these techniques to spin-pumping with microwaves, and spin-injection from ferromagnets.

3. Quantum optics with electron spin ensembles in semiconductors

This project explores how one can map a quantum state carried by a light pulse onto a degree of freedom in solid state material, and vice versa. This may provide a new way for preparing entangled states in solid state. The approach aims at realizing an ensemble of quantum systems in a semiconductor, that have a set of energy levels similar to that of alkali atoms, and similar selection rules for the optical transitions. In these systems, long-lived quantum superpositions of low-energy states can be controlled by optical control fields (addressing high-energy transitions), by exploiting a quantum interference between two transitions that are driven at the same time (techniques based on so-called Electromagnetically Induced Transparency, EIT). At this stage, we can realize EIT in GaAs and similar materials, and this provides evidence that electrons in a semiconductor can form a medium for quantum optics. The experimental work concerns experiments that study the propagation of optical fields through optcial waveguides of semiconductor material, with the quantum systems (localized donor electrons or electronic defects) inside this wave guide.

4. Spin dynamics in organic molecules and organic semiconductors

This is project a new project per 2013. The goal is to explore how the physics that underlies the spin control and optical transitions in GaAs can also occur in organic materials. Success in this direction will provide new methods for exploring the charge dynamics in organic devices, such as the organic solar cell materials that are widely studied at the Zernike Institute for Advanced Materials.

5. Measurement facilities Van der Wal Labs

One setup (for visitors: building 13, room 237) is around a large dilution refridgerator (Leiden Cryogenics DRS1000, high cooling power, large sample space, 9 T field). Linked to this setup is the optical table for quantum optics experiments, where the essential part is a set of three CW Ti:Sapph lasers (Coherent MBR-110). The essential parts of the other setup (for visitors: building 13, room 144) are an optical cryostat (Oxford Spectromag, 8 T field) and a pulsed Ti:Sapph laser system (Coherent Chameleon Ultra). This room also has the new facilities for the projects with ZnSe and SiC, which are based on CW diode lasers around 443 nm, 1078 nm and 1130 nm.