Frank Hohls

Noise Enhancement due to Quantum Coherence in Coupled Quantum Dots

We show that the intriguing observation of noise enhancement in the charge transport through two vertically coupled quantum dots can be explained by the interplay of quantum coherence and strong Coulomb blockade. We demonstrate that this novel mechanism for super-Poissonian charge transfer is very sensitive to decoherence caused by electron-phonon scattering as inferred from the measured temperature dependence.

Bimodal counting statistics in single-electron tunneling through a quantum dot

We explore the full counting statistics of single electron tunneling through a quantum dot using a quantum point contact as non-invasive high bandwidth charge detector. The distribution of counted tunneling events is measured as a function of gate and source-drain-voltage for several consecutive electron numbers on the quantum dot. For certain configurations we observe super-Poissonian statistics for bias voltages at which excited states become accessible. The associated counting distributions interestingly show a bimodal characteristic. Analyzing the time dependence of the number of electron counts we relate this to a slow switching between different electron configurations on the quantum dot.

Fabrication of quantum point contacts by engraving GaAs/AlGaAs heterostructures with a diamond tip

We use the all-diamond tip of an atomic force microscope for the direct engraving of high-quality quantum point contacts in GaAs/AlGaAs heterostructures. The processing time is shortened by two orders of magnitude compared to standard silicon tips. Together with a reduction of the line width to below 90 nm, the depletion length of insulating lines is reduced by a factor of two with the diamond probes. The such fabricated defect-free ballistic constrictions show well-resolved conductance plateaus and the 0.7 anomaly in electronic transport measurements.

Partitioning of on-demand electron pairs

The on-demand generation and separation of entangled photon pairs are key components of quantum information processing in quantum optics. In an electronic analogue, the decomposition of electron pairs represents an essential building block for using the quantum state of ballistic electrons in electron quantum optics. The scattering of electrons has been used to probe the particle statistics of stochastic sources in Hanbury Brown and Twiss experiments and the recent advent of on-demand sources further offers the possibility to achieve indistinguishability between multiple sources in Hong-Ou-Mandel experiments. Cooper pairs impinging stochastically at a mesoscopic beamsplitter have been successfully partitioned, as verified by measuring the coincidence of arrival. Here, we demonstrate the splitting of electron pairs generated on demand. Coincidence correlation measurements allow the reconstruction of the full counting statistics, revealing regimes of statistically independent, distinguishable or correlated partitioning, and have been envisioned as a source of information on the quantum state of the electron pair. The high pair-splitting fidelity opens a path to future on-demand generation of spin-entangled electron pairs from a suitably prepared two-electron quantum-dot ground state.

High-order cumulants in the counting statistics of asymmetric quantum dots

Measurements of single electron tunneling through a quantum dot (QD) using a quantum point contact as charge detector have been performed for very long time traces with very large event counts. This large statistical basis is used for a detailed examination of the counting statistics for varying symmetry of the QD system. From the measured statistics we extract high order cumulants describing the distribution. Oscillations of the high order cumulants are observed when varying the symmetry. We compare this behavior to the observed oscillation in time dependence and show that the variation in both system variables lead to the same kind of oscillating response.

Observation of an Interedge Magnetoplasmon Mode in a Degenerate Two-Dimensional Electron Gas

We study the propagation of edge magnetoplasmons by time-resolved current measurements in a sample which allows for selective detection of edge states in the quantum Hall regime. At filling factors close to nu=3 we observe two decoupled modes of edge excitations, one of which is related to the innermost compressible strip and is identified as an interedge magnetoplasmon mode. From the analysis of the propagation velocities of each mode the internal spatial parameters of the edge structure are derived.

Bilayer Graphene Quantum Dot Defined by Topgates

We investigate the application of nanoscale topgates on exfoliated bilayer graphene to define quantum dot devices. At temperatures below 500 mK, the conductance underneath the grounded gates is suppressed, which we attribute to nearest neighbour hopping and strain-induced piezoelectric fields. The gate-layout can thus be used to define resistive regions by tuning into the corresponding temperature range. We use this method to define a quantum dot structure in bilayer graphene showing Coulomb blockade oscillations consistent with the gate layout.

High cumulants in the counting statistics measured for a quantum dot

We report on measurements of single electron tunneling through a quantum dot using a quantum point contact as non-invasive charge detector with fast time response. We elaborate on the unambiguous identification of individual tunneling events and determine the distribution of transferred charges, the so-called full counting statistics. We discuss our data analysis, including the error estimates of the measurement, and show that the quality of our experimental results is sufficiently high to extract cumulants of the distribution up to the 20th order for short times.

Quantized current source with mesoscopic feedback

We study a mesoscopic circuit of two quantized current sources, realized by non-adiabatic single- electron pumps connected in series with a small micron-sized island in between. We find that quantum transport through the second pump can be locked onto the quantized current of the first one by a feedback due to charging of the mesoscopic island. This is confirmed by a measurement of the charge variation on the island using a nearby charge detector. Finally, the charge feedback signal clearly evidences loading into excited states of the dynamic quantum dot during single-electron pump operation.

High Frequency Conductivity in the Quantum Hall Regime

It is widely accepted that the understanding of the integer quantum Hall effect (QHE) is closely related to a disorder driven localization-delocalization transition occurring in two-dimensional electron systems (2DES) in high magnetic fields [1]. Many experimental and theoretical works approve the interpretation of the transition between adjacent QHE plateaus as a quantum critical phase transition. It is governed by a diverging localization length j~ jE 2 Ecj 2g which scales with the distance of the energy E from the critical energy Ec in the center of a Landau band. The exponent g 2.3 is believed to be a universal quantity independent of disorder. For finite systems with effective size Leff theory predicts that the conductivities sab follow scaling functions sab fabLeffjE resulting in a finite width DE ~ L 21g eff of the transition region. The effective system size Leff is determined by the physical sample size, the electron temperature T , or the frequency f. The most common test of scaling uses an analysis of the temperature or frequency dependence of the conductivity peak width in the QHE plateau transition. However, lacking an exact expression for LeffT , f, this method does not allow direct access to the scaling behavior of the localization length. An alternative approach to scaling was proposed by Polyakov and Shklovskii [2]. Using the fact that the conductivity sxx in the QHE plateaus at low temperatures is dominated by variable-range hopping (VRH) [3] a direct access to the localization length j can be gained from analyzing the dependence of sxx on temperature, current, and frequency. Experimentally mainly the temperature dependence of sxx in the VRH regime was investigated [4]. However, due to an unknown theoretical prefactor, j could only be estimated from these experiments. In contrast, the frequency driven variable-range hopping conductivity sxxf [in the limit sxxf ? sxx0] is given by [2] Resxxv 2p