152 research outputs found
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
Strong Coupling Cavity QED with Gate-Defined Double Quantum Dots Enabled by a High Impedance Resonator
The strong coupling limit of cavity quantum electrodynamics (QED) implies the
capability of a matter-like quantum system to coherently transform an
individual excitation into a single photon within a resonant structure. This
not only enables essential processes required for quantum information
processing but also allows for fundamental studies of matter-light interaction.
In this work we demonstrate strong coupling between the charge degree of
freedom in a gate-detuned GaAs double quantum dot (DQD) and a frequency-tunable
high impedance resonator realized using an array of superconducting quantum
interference devices (SQUIDs). In the resonant regime, we resolve the vacuum
Rabi mode splitting of size MHz at a resonator linewidth
MHz and a DQD charge qubit dephasing rate of MHz extracted independently from microwave spectroscopy in the dispersive
regime. Our measurements indicate a viable path towards using circuit based
cavity QED for quantum information processing in semiconductor nano-structures
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
Universal nuclear focusing of confined electron spins
For spin-based quantum computation in semiconductors, dephasing of electron
spins by a fluctuating background of nuclear spins is a main obstacle. Here we
show that this nuclear background can be precisely controlled in generic
quantum dots by periodically exciting electron spins. We demonstrate this
universal phenomenon in many-electron GaAs/AlGaAs quantum dot ensembles using
optical pump-probe spectroscopy. A feedback mechanism between the saturable
electron spin polarization and the nuclear system focuses the electron spin
precession frequency into discrete spin modes. Employing such control of
nuclear spin polarization, the electron spin lifetime within individual dots
can surpass the limit of nuclear background fluctuations, thus substantially
enhancing the spin coherence time. This opens the door to achieve long electron
spin coherence times also in lithographically-defined many-electron systems
that can be controlled in shape, size and position
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