Tunable Charge Detectors for Semiconductor Quantum Circuits

Nanostructures defined in high-mobility two-dimensional electron systems offer a unique way of controlling the microscopic details of the investigated device. Quantum point contacts play a key role in these investigations, since they are not only a research topic themselves, but turn out to serve as convenient and powerful detectors for their electrostatic environment. We investigate how the sensitivity of charge detectors can be further improved by reducing screening, increasing the capacitive coupling between charge and detector and by tuning the quantum point contacts' confinement potential into the shape of a localized state. We demonstrate the benefits of utilizing a localized state by performing fast and well-resolved charge detection of a large quantum dot in the quantum Hall regime

Increasing the {\nu} = 5 / 2 gap energy: an analysis of MBE growth parameters

The fractional quantized Hall state (FQHS) at the filling factor {\nu} = 5/2 is of special interest due to its possible application for quantum computing. Here we report on the optimization of growth parameters that allowed us to produce two-dimensional electron gases (2DEGs) with a 5/2 gap energy up to 135 mK. We concentrated on optimizing the MBE growth to provide high 5/2 gap energies in "as-grown" samples, without the need to enhance the 2DEGs properties by illumination or gating techniques. Our findings allow us to analyse the impact of doping in narrow quantum wells with respect to conventional DX-doping in AlxGa1-xAs. The impact of the setback distance between doping layer and 2DEG was investigated as well. Additionally, we found a considerable increase in gap energy by reducing the amount of background impurities. To this end growth techniques like temperature reductions for substrate and effusion cells and the reduction of the Al mole fraction in the 2DEG region were applied

Equilibrium free energy measurement of a confined electron driven out of equilibrium

We study out-of equilibrium properties of a quantum dot in a GaAs/AlGaAs two-dimensional electron gas. By means of single electron counting experiments, we measure the distribution of work and dissipated heat of the driven quantum dot and relate these quantities to the equilibrium free energy change, as it has been proposed by C. Jarzynski [Phys. Rev. Lett. {\bf78}, 2690 (1997)]. We discuss the influence of the degeneracy of the quantized energy state on the free energy change as well as its relation to the tunnel rates between the dot and the reservoir.Comment: 5 pages, 4 figure

Transport Spectroscopy of a Spin-Coherent Dot-Cavity System

Quantum engineering requires controllable artificial systems with quantum coherence exceeding the device size and operation time. This can be achieved with geometrically confined low-dimensional electronic structures embedded within ultraclean materials, with prominent examples being artificial atoms (quantum dots) and quantum corrals (electronic cavities). Combining the two structures, we implement a mesoscopic coupled dot-cavity system in a high-mobility two-dimensional electron gas, and obtain an extended spin-singlet state in the regime of strong dot-cavity coupling. Engineering such extended quantum states presents a viable route for nonlocal spin coupling that is applicable for quantum information processing