Theodore Choi

Observation of excited states in a graphene double quantum dot

We study a graphene double quantum dot in different coupling regimes. Despite the strong capacitive coupling between the dots, the tunnel coupling is below the experimental resolution. We observe additional structures inside the finite-bias triangles, part of which can be attributed to electronic excited dot states, while others are probably due to modulations of the transmission of the tunnel barriers connecting the system to source and drain leads.

Coherent electron–phonon coupling in tailored quantum systems

The coupling between a two-level system and its environment leads to decoherence. Within the context of coherent manipulation of electronic or quasiparticle states in nanostructures, it is crucial to understand the sources of decoherence. Here we study the effect of electron-phonon coupling in a graphene and an InAs nanowire double quantum dot (DQD). Our measurements reveal oscillations of the DQD current periodic in energy detuning between the two levels. These periodic peaks are more pronounced in the nanowire than in graphene, and disappear when the temperature is increased. We attribute the oscillations to an interference effect between two alternative inelastic decay paths involving acoustic phonons present in these materials. This interpretation predicts the oscillations to wash out when temperature is increased, as observed experimentally.

Counting statistics in an InAs nanowire quantum dot with a vertically coupled charge detector

A gate-defined quantum dot (QD) in an InAs nanowire is fabricated on top of a quantum point contact realized in a two-dimensional electron gas. The strong coupling between these two quantum devices is used to perform time-averaged as well as time-resolved charge detection experiments for electron flow through the quantum dot. We demonstrate that the Fano factor describing shot noise or time-correlations in single-electron transport depends in the theoretically expected way on the asymmetry of the tunneling barriers even in a regime where the thermal energy kBT is comparable to the single-particle level spacing in the dot.

Correlated counting of single electrons in a nanowire double quantum dot

We report on correlated real-time detection of individual electrons in an InAs nanowire double quantum dot (DQD). Two self-aligned quantum point contacts (QPCs) in an underlying two-dimensional electron gas material serve as highly sensitive charge detectors for the DQD. Tunnel processes of individual electrons and all tunnel rates are determined by simultaneous measurements of the correlated signals of the QPCs.

Counting statistics of hole transfer in a p-type GaAs quantum dot with dense excitation spectrum

Low-temperature transport experiments on a p-type GaAs quantum dot capacitively coupled to a quantum point contact are presented. The time-averaged as well as time-resolved detection of charging events of the dot are demonstrated and they are used to extract the tunneling rates into and out of the quantum dot. The extracted rates exhibit a super-linear enhancement with the bias applied across the dot, which is interpreted in terms of a dense spectrum of excited states contributing to the transport, characteristic for heavy hole systems. The full counting statistics of charge transfer events and the effect of back action is studied. The normal cumulants as well as the recently proposed factorial cumulants are calculated and discussed in view of their importance for interacting systems. DOI: 10.1103/PhysRevB.88.035417 PACS number(s): 73.23.−b, 73.63.−b I. INTRODUCTION Quantum dots (QDs or simply dots) are small conducting islands that confine charge carriers in three dimensions, resultinginadiscretespectrumofexcitedstates.Thisspectrum is often studied in transport experiments by measuring the current, 1 which is allowing carriers to tunnel between the dot and source and drain leads (reservoirs). The capacitive coupling of QD to nearby gates enables tuning the energy of excited states with respect to electrochemical potential of the leads.Itisafascinatingexperimentalobservationthatasimilar capacitive coupling to a nearby electrical current passing through a constriction provides the possibility of measuring the charge of the dot with a precision of a small fraction of an electron’s charge. 2 The conductance of the constriction changes as a function of the average charge population of the

Highly tunable hybrid quantum dots with charge detection

In order to employ solid state quantum dots as qubits, both a high degree of control over the confinement potential as well as sensitive charge detection are essential. We demonstrate that by combining local anodic oxidation with local Schottky-gates, these criteria are nicely fulfilled in the resulting hybrid device. To this end, a quantum dot with adjacent charge detector is defined. After tuning the quantum dot to contain only a single electron, we are able to observe the charge detector signal of the quantum dot state for a wide range of tunnel couplings.

Noise-induced spectral shift measured in a double quantum dot

We measure the shot noise of a quantum point-contact using a capacitively coupled InAs double quantum dot as an on-chip sensor. Our measurement signals are the (bidirectional) interdot electronic tunneling rates which are determined by means of time-resolved charge sensing. The detector frequency is set by the relative detuning of the energy levels in the two dots. For nonzero detuning, the noise in the quantum point contact generates inelastic tunneling in the double dot and thus causes an increase in the interdot tunneling rate. Conservation of spectral weight in the dots implies that this increase must be compensated by a decrease in the rate close to zero detuning, which is quantitatively confirmed in our experiment.

Field effect in the quantum Hall regime of a high mobility graphene wire

In graphene-based electronic devices like in transistors, the field effect applied thanks to a gate electrode allows tuning the charge density in the graphene layer and passing continuously from the electron to the hole doped regime across the Dirac point. Homogeneous doping is crucial to understand electrical measurements and for the operation of future graphene-based electronic devices. However, recently theoretical and experimental studies highlighted the role of the electrostatic edge due to fringing electrostatic field lines at the graphene edges [P. Silvestrov and K. Efetov, Phys. Rev. B 77, 155436 (2008); F. T. Vasko and I. V. Zozoulenko, Appl. Phys. Lett. 97, 092115 (2010)]. This effect originates from the particular geometric design of the samples. A direct consequence is a charge accumulation at the graphene edges giving a value for the density, which deviates from the simple picture of a plate capacitor and also varies along the width of the graphene sample. Entering the quantum Hall regime would, in principle, allow probing this accumulation thanks to the extreme sensitivity of this quantum effect to charge density and the charge distribution. Moreover, the presence of an additional and counter-propagating edge channel has been predicted [P. Silvestrov and K. Efetov, Phys. Rev. B 77, 155436 (2008)] giving a fundamental aspect to this technological issue. In this article, we investigate this effect by tuning a high mobility graphene wire into the quantum Hall regime in which charge carriers probe the electrostatic potential at high magnetic field close to the edges. We observe a slight deviation to the linear shift of the quantum Hall plateaus with magnetic field and we study its evolution for different filling factors, which correspond to different probed regions in real space. We discuss the possible origins of this effect including an increase of the charge density towards the edges.

A circuit analysis of an in situ tunable radio-frequency quantum point contact

A detailed analysis of the tunability of a radio-frequency quantum point contact setup using a C - LCR circuit is presented. We calculate how the series capacitance influences resonance frequency and charge-detector resistance for which matching is achieved as well as the voltage and power delivered to the load. Furthermore, we compute the noise contributions in the system and compare our findings with measurements taken with an etched quantum point contact. While our considerations mostly focus on our specific choice of matching circuit, the discussion of the influence of source-to-load power transfer on the signal-to-noise ratio is valid generally.

Single‐Step Realization Of Charge Detectors For Nanowire Quantum Dots

We present different schemes for the realization of charge detectors for nanowire quantum dots. We use a two‐dimensional electron gas as a functional substrate for the nanowires where quantum point contacts can be fabricated using etching techniques and by the deposition of metallic gates. The sample designs ensure perfect alignment of the charge detectors to the quantum dots and offer a strong capacitive coupling of the two systems compared to conventional structures on two‐dimensional electron gases with planar geometry.