R. Schleser

Counting statistics of single electron transport in a quantum dot.

We have measured the full counting statistics of current fluctuations in a semiconductor quantum dot (QD) by real-time detection of single electron tunneling with a quantum point contact. This method gives direct access to the distribution function of current fluctuations. Suppression of the second moment (related to the shot noise) and the third moment (related to the asymmetry of the distribution) in a tunable semiconductor QD is demonstrated experimentally. With this method we demonstrate the ability to measure very low current and noise levels.

Time-resolved detection of individual electrons in a quantum dot

We present measurements on a quantum dot and a nearby, capacitively coupled, quantum point contact used as a charge detector. With the dot being weakly coupled to only a single reservoir, the transfer of individual electrons onto and off the dot can be observed in real time in the current signal from the quantum point contact. From these time-dependent traces, the quantum mechanical coupling between dot and reservoir can be extracted quantitatively. A similar analysis allows the determination of the occupation probability of the dot states.

Cotunneling-Mediated Transport through Excited States in the Coulomb-Blockade Regime

We present finite-bias transport measurements on a few-electron quantum dot. In the Coulomb-blockade regime, strong signatures of inelastic cotunneling occur which can directly be assigned to excited states observed in the nonblockaded regime. In addition, we observe structures related to sequential tunneling through the dot, occurring after it has been excited by an inelastic cotunneling process. We explain our findings using transport calculations within the real-time Greens function approach, including diagrams up to fourth order in the tunneling matrix elements.

Counting statistics and super-Poissonian noise in a quantum dot : Time-resolved measurements of electron transport

We present time-resolved measurements of electron transport through a quantum dot. The measurements were performed using a nearby quantum point contact as a charge detector. The rates for tunneling through the two barriers connecting the dot to source and drain contacts could be determined individually. In the high bias regime, the method was used to probe excited states of the dot. Furthermore, we have detected bunching of electrons, leading to super-Poissonian noise. We have used the framework of the full counting statistics (FCS) to model the experimental data. The existence of super-Poissonian noise suggests a long relaxation time for the involved excited state, which could be related to the spin relaxation time.

A Radio Frequency Quantum Point Contact Charge Read‐Out

A quantum point contact, capacitively coupled to a quantum dot, has been used to detect single electron charging inside the dot. Embedding the quantum point contact in a radio frequency matching circuit and measuring the reflection coefficient of the circuit, rather than the conductance of the point contact, allowed for real time measurements with a time resolution of 50 ns.

Finite-bias charge detection in a quantum dot

We present measurements on a quantum dot coupled capacitively to a quantum point contact used as a charge detector. The transport current through the dot and the time-averaged charge on the dot are measured simultaneously. At finite bias voltage through the dot, the differential charge signal coincides with some of the Coulomb diamond boundaries and with the signatures from excited states in the dot current. Combining the resulting integrated charge data with the simultaneous measurements of the dot current allows us to estimate the coupling of different energy levels inside the dot to both leads individually.

Influence of HCl etching on the electronic properties of LAO-defined nanostructures

We present an investigation of the influence of etching in hydrochloric acid on the topography and electronic properties of nanostructures defined by local anodic oxidation on shallow two-dimensional electron gases. The acid concentration and etching time necessary to remove the oxide lines were determined, yielding a process to create a flat surface suitable for scanning probe experiments. Changes in the low-temperature electronic properties induced by surface oxide removal include a moderate decrease of the total electron density and an increase of the breakdown voltages of the lines defined by local anodic oxidation. The obtained data indicate that the electronic width of the nanostructures decreases after the etching process.

Scanning Gate Measurements on a Coupled Quantum Dot — Quantum Point Contact System

We use the metallic tip of a low‐temperature scanning force microscope as a movable gate to study a quantum dot and an adjacent quantum point contact. By scanning the tip we can add single electrons to the dot and detect them with the quantum point contact. Additionally, we detect other charging events which we attribute to charge traps.

Counting statistics of single electron transport in a quantum dot

We have measured the full counting statistics of current fluctuations in a semiconductor quantum dot (QD) by real-time detection of single electron tunneling with a quantum point contact. This method gives direct access to the distribution function of current fluctuations. Suppression of the second moment (related to the shot noise) and the third moment (related to the asymmetry of the distribution) in a tunable semiconductor QD is demonstrated experimentally. With this method we demonstrate the ability to measure very low current and noise levels.

Time resolved single electron detection in a quantum dot

We have performed transport measurements on a composite nanostructure, consisting of a quantum dot and a nearby quantum point contact (QPC) used as a charge detector. For very low coupling of the dot to only one of the reservoirs and no coupling to the other one, we observe time‐dependent features related to electron tunneling onto the dot and back to the reservoir. The transition frequencies visible in the QPC signal are of the order of only a few Hz. From these time‐dependent data, the Fermi distribution in the lead and the temperature can be extracted, as well as the coupling to the reservoir.