Christian Fricke

Measurement of finite-frequency current statistics in a single-electron transistor

Electron transport in nanoscale structures is strongly influenced by the Coulomb interaction that gives rise to correlations in the stream of charges and leaves clear fingerprints in the fluctuations of the electrical current. A complete understanding of the underlying physical processes requires measurements of the electrical fluctuations on all time and frequency scales, but experiments have so far been restricted to fixed frequency ranges, as broadband detection of current fluctuations is an inherently difficult experimental procedure. Here we demonstrate that the electrical fluctuations in a single-electron transistor can be accurately measured on all relevant frequencies using a nearby quantum point contact for on-chip real-time detection of the current pulses in the single-electron device. We have directly measured the frequency-dependent current statistics and, hereby, fully characterized the fundamental tunnelling processes in the single-electron transistor. Our experiment paves the way for future investigations of interaction and coherence-induced correlation effects in quantum transport.

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.

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.

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.

Spin-dependent shot noise enhancement in a quantum dot

The spin-dependent dynamical blockade was investigated in a lateral quantum dot in a magnetic field. Spin-polarized edge channels in the two-dimensional leads and the spatial distribution of Landau orbitals in the dot modulate the tunnel coupling of the quantum dot level spectrum. In a measurement of the electron shot noise we observe a pattern of super-Poissonian noise which is correlated to the spin-dependent competition between different transport channels.

Coupling symmetry of quantum dot states

With noninvasive methods, we investigate ground and excited states of a lateral quantum dot. Charge detection via a quantum point contact is used to map the dot dynamics in a regime where the current through the dot is too low for transport measurements. In this way we investigate and compare the tunneling rates from the dot to source and drain. We find a symmetry line on which the tunneling rates to both leads are equal. In this situation ground states as well as excited states influence the mean charge of the dot. A detailed study in this regime reveals that the coupling symmetry depends on the number of states contributing to transport and on the spatial distribution of individual states.

Noninvasive detection of charge rearrangement in a quantum dot in high magnetic fields

We demonstrate electron redistribution caused by magnetic field on a single quantum dot measured by means of a quantum point contact as noninvasive detector. Our device, which is fabricated by local anodic oxidation, allows us to control independently the quantum point contact and all tunneling barriers of the quantum dot. Thus we are able to measure both the change of the quantum dot charge and also changes of the electron configuration at constant number of electrons on the quantum dot. We use these features to exploit the quantum dot in a high magnetic field where transport through the quantum dot displays the effects of Landau shells and spin blockade. We confirm the internal rearrangement of electrons as function of the magnetic field for a fixed number of electrons on the quantum dot.

Transport through a quantum dot analyzed by electron counting

We present measurements of single electron tunneling through a quantum dot (QD) using a quantum point contact as charge detector. The counting statistics for varying symmetry of the QD system is examined from a very large statistical basis. We extract high order cumulants describing the distribution and observe prominent oscillations 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.

Universal oscillations of high-order cumulants

We discuss our recent measurement of high‐order cumulants of charge transport through a quantum dot. The cumulants were found to oscillate as functions of measurement time before reaching their linear‐in‐time asymptotics. A theoretical analysis revealed that such oscillations in fact constitute a universal phenomenon: for a large class of stochastic processes the high‐order cumulants are predicted to oscillate as functions of basically any parameter. Here, we give an overview of these recent results, provide an outlook on future applications of our findings, and formulate a number of open questions.

Shot noise and electron counting measurements on coupled quantum dot systems

Measuring shot noise and electron counting are both methods for accessing information about the dynamics of electronic transport through a system of interest. Here we apply these tools to examine electronic transport through coupled quantum dot systems. In the first part of the paper we show temperature dependent shot noise measurements for a strongly coupled double-quantum-dot system. We observe super-Poissonian shot noise as expected for coherent inter-dot coupling and asymmetric lead tunnelling rates. In the second part we apply direct electron counting to examine a weakly coupled triple-dot system. A quantum point contact placed near the triple dot allows us to monitor electron hops between the dots and the leads. This allows us to, e.g., individually characterize different tunnelling rates relevant in the system.