Thermally activated charge fluctuations in GaAs double quantum dots

In this thesis, laterally defined GaAs quantum dots and double quantum dots are investigated by means of real-time charge sensing. A quantum dot charge sensor, adjacent to the investigated double dot system, detects single electron tunneling with sensitivities exceeding that of QPC-based charge sensing systems by far. The experimental setup is characterized in detail and optimized with respect to bandwidth and signal-to-noise ratio. Software tools required for data analysis are developed in combination with a simulation program, capable of creating artificial real-time data for test and characterization purposes.
The experiments described in this work led to the discovery of an intrinsic effect in double quantum dots - thermally activated, meastable charge state switching. A new feature arises in the charge stability diagram of a double quantum dot due to tunneling processes between double dot system and leads. These processes occur in the region between two associated triple points of the charge stability diagram, where originally stable charge configurations are expected. This effect is characterized in detail with respect to coupling to electron reservoirs, as well as inter-dot coupling and temperature dependence. An extension of the orthodox model of double quantum dots is developed, capturing nearly all experimentally observed features. A concrete prediction of this model is the presence of four different charge states that participate in the process of metastable charge state switching. The experimental observation of switching between four different charge configurations is presented and the implications and possible influences of this effect (e.g. loss of coherence) on other experiments are discussed.
Furthermore, this work treats the implementation of spin relaxation time measurements on single electrons, similar to previously performed experiments. This work is motivated by the expected anisotropy of spin relaxation with respect to an external magnetic field. In addition, these experiments are motivated by the previously observed effect of spin-dependent tunneling into a empty quantum dot, where tunneling into the excited spin state is suppressed considerably. These experiments involve gate pulsing techniques, which are required to load and unload electrons from a quantum dot on a millisecond timescale. The realization of such pulsing sequences is demonstrated in several experiments. The Zeeman splitting of the quantum dot ground state is resolved over a wide range of magnetic fields, which is an important ingredient for spin relaxation time measurements and allows for extracting the electron g-factor in GaAs, as well as investigating the effect of spin-dependent tunneling. Finally, first spin relaxation time measurements are presented, demonstrating the successful execution of this experiments and paving the way for further experiments to study the anisotropy of spin relaxation in GaAs quantum dots.