Double Quantum Dot Structure Research Articles

Identifying Pauli blockade regimes in bilayer graphene double quantum dots

Recent experimental observations of current blockades in 2D material quantum-dot platforms have opened new avenues for spin and valley-qubit processing. Motivated by experimental results, we construct a model capturing the delicate interplay of Coulomb interactions, inter-dot tunneling, Zeeman splittings, and intrinsic spin–orbit coupling in a double quantum dot (DQD) structure to simulate the Pauli blockades. Analyzing the relevant Fock-subspaces of the generalized Hamiltonian, coupled with the density matrix master equation technique for transport across the setup, we identify the generic class of blockade mechanisms. Most importantly, and contrary to what is widely recognized, we show that conducting and blocking states responsible for the Pauli-blockades are a result of the coupled effect of all degrees of freedom and cannot be explained using the spin or the valley pseudo-spin only. We then numerically predict the regimes where Pauli blockades might occur, and, to this end, we verify our model against actual experimental data and propose that our model can be used to generate data sets for different values of parameters with the ultimate goal of training on a machine learning algorithm. Our work provides an enabling platform for a predictable theory-aided experimental realization of single-shot readout of the spin and valley states on DQDs based on 2D-material platforms.

Tunability of solar cell with double quantum dot structure

This work introduces a double quantum dot (QD) structure as an intermediate band for high-performance solar cells. The density matrix equations, coupled with the continuity-current equation, are solved numerically to address the interaction between all the states, which is impossible with the rate equation modeling. This modeling allowed us to obtain quantum efficiency (QE). Using the orthogonalized plane wave assumption for WL-QD, the momentum matrix elements of QD-QD, QD-wetting layer (WL), WL-barrier transitions are calculated. Results are simulated in both the excitonic and non-excitonic (electron-hole, eh) cases and exhibit the importance of adding the QD layer.The results confirm the importance of adding the intermediate band (QD layer). Both band-to-band recombination rates and relaxation rates in the DQD structure are modulated according to the energy difference between DQD states. Compared to the eh model, the excitonic model reduces the QD-QD relaxation rates and all band-to-band recombination rates. The quantum efficiency (QE) is attaining at longer recombination and relaxation times that are confirmed depending on the wider energy difference by manipulating the transitions of the DQD structure.

Recombination rates of the double quantum dot solar cell structure

This work proposes a double quantum dot structure as an intermediate-band layer to developing solar cell performance for the first time. The continuity-current equation is coupling with the density matrix equations, which are solved numerically to obtain the quantum efficiency. This modeling will calculate the momentum matrix elements of transitions, consider the orthogonalized plane wave for wetting layer- quantum dot transitions, and covers more characteristics than the rate equations by considering all the possible interactions between the states. The results simulate both the excitonic and nonexcitonic (electron-hole) cases. The work emphasizes adding the double quantum dot layer, which gives flexibility in choosing the energy difference between states controlling the recombination rates. The work refers to the importance of orthogonalized plane-wave in solar cell work. In both models, the band-band recombination is high for slight energy differences between the states, confirms the importance of the double quantum dot system to manipulate transitions between states and obtain higher rates. For the electron-hole model, the leakage is high due to the fast recombination of holes. The discrimination between occupations of states is increasing under the excitonic model due to growing hole occupation. In both models, reducing the quantum dot - quantum dot recombinations and increasing all other recombinations will increase the quantum efficiency. The high quantum dot band-to-band rate increases the quantum efficiency. In the excitonic model, lower rates than in the electron-hole model are enough for high quantum efficiency.

Electron transport in T-shaped double quantum dot structure

Electron transport in T-shaped double quantum dot structure

Tuning Optoelectronic Properties of Double Quantum Dot Structure Using Tight-Binding Model for Photo-Electric Applications

In electronics field, researchers and industries have been working to fabricate low-cost optoelectronic devices by using less materials in the fabrication process. Thus, miniaturization concept has been used in the design and synthesis of the fabricated materials and devices based on nano-dimension. However, reducing the used materials would affects the overall performance of the devices and new building blocks materials are needed to pass the performance capacity limits of current silicon-based materials. Quantum Dots DQs have been introduced and attracted much attention among researchers because of their unique characteristics that are necessary for many potential applications by using nano-dimensions structures. These properties include size-tunable optical and other electronic characteristics that are not found in the current bulk materials. Although there has been interested QDs experimental studied that have been already carried out, different theoretical efforts must be introduced in order to provide good understanding of the possible and different QDs applications. In this work, therefore, optoelectronic properties of Double Quantum Dots DQDs system were studied theoretically to provide important information about the possibility of using this system in photoelectric applications. A tight-binding framework was adopted to describe the system, and all the calculations were carried out based on the steady state formalism. The proposed DQD structure was connected to metallic leads and studied to investigate the QD size dependent. The transmission calculation presented first through the electron transport mechanism. Tunneling current and conductance were then presented to provide general understanding about the behavior of the proposed system. A correlation of transmission, current and conductance results with QD radius R, incident photon energy Eph, Temperature and bias voltage have been identified. Therefore, this correlation is strongly supporting the proposal of using DQD system in fabricating nanoscale photovoltaic devices, particularly, solar cells.

Kerr effect in ladder-plus-Y-configuration in double quantum dot structure

This work models the Kerr effect in double quantum dot (DQD) ladder-plus-Y system using the density matrix theory. The QD inhomogeneity and wetting layer effect are considered. The Kerr effect is shown to be increased at small pumping where the linear absorption is inhibited. Under very weak probe field in comparison to the pump field, no electromagnetically induced transparency window is shown.

Electron transfer properties of double quantum dot system in a fluctuating environment* *Project supported by the Natural Science Foundation of Shandong Province, China (Grant No. ZR2014AM030).

Using the innovative method of the additional Bloch vector, the electron transfer properties of a double quantum dot (DQD) system measured by a quantum point contact (QPC) in a fluctuating environment are investigated. The results show that the environmental noises in transverse and longitudinal directions play different roles in the dynamical evolution of the open quantum systems. Considering the DQD with symmetric energy level, the Fano factor exhibits a slight peak with the increase of transverse noise amplitude σ T, which provides a basis for distinguishing dynamical phenomena caused by different directional fluctuation noises in symmetric DQD structures by studying the detector output. In the case of asymmetric DQD, the dependence of a detector current involving the level displacement is distinct when increasing the transverse noise damping coefficient τ T and the longitudinal noise damping coefficient τε respectively. Meanwhile, the transverse noise damping coefficient τ T could significantly reduce the Fano factor and enhance the stability of the quantum system compared with the longitudinal one. The Fano factors with stable values as the enhancement of noise amplitudes show different external influences from the detector measurement, and provide a numerical reference for adjusting the noise amplitudes in both transverse and longitudinal directions appropriately in a microscopic experimental process to offset the decoherence effect caused by the measurements. Finally, the research of average waiting time provides unique insights to the development of single electron transfer theory in the short-time limit.

Quantum Entropy in Ladder-Plus-Y Double Quantum Dot System using Spontaneously Generated Coherence

Quantum entropy under spontaneously generated coherence (SGC) was modeled and studied in a double quantum dot (DQD) structure. This system becomes after including the wetting layer (WL) as a ladder-plus-Y- system. The results show that the high probe field under high SGC can give good entanglement between states of the DQD system. Under the high SGC component, the optical pump was more efficient than the probe signal in increasing entanglement. The application of optical fields into WL reduces quantum entropy.

Geometrical Effects on Exchange Coupling in System of Near-Surface Donors and Quantum Dots

The value of exchange energy for near-surface double-donor and double-quantum dot structures under the effect of external electric field has been calculated using unrestricted Hartree–Fock method. The dependences of exchange energy on geometric parameters have been obtained. It has been found that the location of donor centers near the surface of the semiconductor leads to an increase in the exchange energy. Approximate formulas for the dependence of the exchange energy on the distance between the centers of quantum dots are suggested. It has been shown that exchange coupling control is less sensitive to electric field as compared to magnetic field, and electron density relocation at large electric fields should be taken into account when designing devices based on the structures under study.

WITHDRAWN: Exploring Bahaviors of Electrode-driven Si Quantum Dot Systems: From Charge Control to Qubit Operation

Abstract Charge stabilities and spin-based quantum bit (qubit) operations in Si double quantum dot (DQD) systems, whose confinement potentials are controlled with multiple gate electrodes, are theoretically studied with a multi-scale modeling approach that combines electronic structure simulations and a Thomas-Fermi method. Taking Si / SiGe heterostructures as the target of modeling, this work presents in-depth discussion on designs of electron reservoirs, electrostatic controls of quantum dot (QD) shapes and corresponding charge confinement, and spin qubit manipulations. Effects of unintentional inaccuracy in DC control biases and geometric symmetries on the Rabi cycle of spin qubits are investigated to examine the robustness of logic operations. Solid connections to the latest experimental result are also established to validate the simulation method. As a rare modeling study that explores a full-stack functionality of Si DQD structures as quantum logic gate devices, this work delivers the knowledge of engineering details that are not uncovered by the latest experimental work, and can serve as a basic but practical guideline for potential device designs.

Majorana molecules and their spectral fingerprints

We introduce the concept of a Majorana molecule, a topological bound state appearing in the geometry of a double quantum dot (QD) structure flanking a topological superconducting nanowire. We demonstrate that, if the Majorana bound states (MBSs) at opposite edges are probed nonlocally in a two probe experiment, the spectral density of the system reveals the so-called half-bowtie profiles, while Andreev bound states (ABSs) become resolved into bonding and antibonding molecular configurations. We reveal that this effect is due to the Fano interference between pseudospin superconducting pairing channels and propose that it can be catched by a pseudospin resolved Scanning Tunneling Microscope (STM)-tip.

Spontaneously generated coherence in ladder-plus-Y double quantum dot system

A model was presented for linear susceptibility in ladder-plus-Y configuration of double quantum dot system using density matrix theory and including spontaneously generated coherence of Λ-type system. Wetting layer and quantum dot inhomogeneity were considered in the calculations, which gives a practical description of double quantum dot structures well. With increasing spontaneously generated coherence from Λ-component, the dispersion was increased and shifted under spontaneously generated coherence. The inclusion of wetting layer under spontaneously generated coherence increases gain which coincides with the published results. A possibility of slow light was predicted.

Asymmetric Double Quantum Dot Structure as Nanoscale Diode

In this studying, a nanoscale diode was proposing by using asymmetric double quantum dot structure attached to metallic electrodes. An asymmetric double quantum dot (ADQD) is used as a diode for rectification. Tight-binding model was adopting to describe our specimen, and our results depend upon steady-state forming to show charge transport. The theoretical computations for current and conductance as a function of the time, that encouraging common traits of the rectifications, were presented. The analysis may be helpful in fabricating nanoscale electronic devices.

Nonlocal pairing as a source of spin exchange and Kondo screening

We report on a novel exchange mechanism, mediating the Kondo screening, in a correlated double quantum dot structure in the proximity of superconductor, stemming from nonlocal Andreev reflection processes. We estimate its strength perturbatively and corroborate analytical predictions with accurate numerical renormalization group calculations using an effective model for the superconductor-proximized nanostructure. We demonstrate that superconductor-induced exchange leads to a two-stage Kondo screening. We determine the dependence of the Kondo temperature on the coupling to superconductor and predict a characteristic modification of conventional low-temperature transport behavior, which can be used to experimentally distinguish this phenomenon from other Kondo effects.

Four-wave mixing in an asymmetric double quantum dot molecule

The four-wave mixing (FWM) effect of a weak probe field, in an asymmetric semiconductor double quantum dot (QD) structure driven by a strong pump field is theoretically studied. Similarly to the case of examining several other nonlinear optical processes, the nonlinear differential equations of the density matrix elements are used, under the rotating wave approximation. By suitably tuning the intensity and the frequency of the pump field as well as by changing the value of the applied bias voltage, a procedure used to properly adjust the electron tunneling coupling, we control the FWM in the same way as several other nonlinear optical processes of the system. While in the weak electron tunneling regime, the impact of the pump field intensity on the FWM is proven to be of crucial importance, for even higher rates of the electron tunneling it is evident that the intensity of the pump field has only a slight impact on the form of the FWM spectrum. The number of the spectral peaks, depends on the relation between specific parameters of the system.

Type-II quantum-dot-in-nanowire structures with large oscillator strength for optical quantum gate applications

We present a numerical investigation of the exciton energy and oscillator strength in type-II nanowire quantum dots. For a single quantum dot, the poor overlap of the electron part and the weakly confined hole part of the excitonic wave function leads to a low oscillator strength compared to type-I systems. To increase the oscillator strength, we propose a double quantum dot structure featuring a strongly localized exciton wave function and a corresponding fourfold relative enhancement of the oscillator strength, paving the way towards efficient optically controlled quantum gate applications in the type-II nanowire system. The simulations are performed using a computationally efficient configuration-interaction method suitable for handling the relatively large nanowire structures.

Spin dynamics and magneto-optical response in charge-neutral tunnel-coupled quantum dots

We model the electron and hole spin dynamics in an undoped double quantum dot structure, considering the carrier tunneling between quantum dots. Taking the presence of an additional in-plane or tilted magnetic field into account, we enable the simulation of magneto-optical experiments, like the time-resolved Kerr rotation measurement, which are currently performed on such structures to probe the temporal spin dynamics. With our model, we reproduce the experimentally observed effect of the extension of the spin polarization lifetime caused by spatial charge separation, which may occur in structures of this type. Moreover, we provide a number of qualitative predictions concerning the necessary conditions for observation of this effect as well as about possible channels of its suppression, including the spin–orbit coupling, which leads to tunneling of carriers accompanied by a spin flip. We also consider the impact of the magnetic field tilting, which results in an interesting spin polarization dynamics.

Giant gain from spontaneously generated coherence in Y-type double quantum dot structure

A theoretical model was presented for linear susceptibility using density matrix theory for Y-configuration of double quantum dots (QDs) system including spontaneously generated coherence (SGC). Two SGC components are included for this system: V, and Λ subsystems. It is shown that at high V-component, the system have a giga gain. At low Λ-system component; it is possible to controls the light speed between superluminal and subluminal using one parameter by increasing SGC component of the V-system. This have applications in quantum information storage and spatially-varying temporal clock.

Second-order nonlinearity in ladder-plus-Y configuration in double quantum dot structure.

Second-order nonlinear susceptibility (SONS) in a ladder-plus-Y double quantum dot structure was modeled and then studied numerically under the application of an electric field. The density matrix theory was used to formulate the system while the orthogonalized plane waves for wetting layer-quantum dot (WL-QD) were considered to state the momentum matrix elements for this system. It is found that the momenta follow the smallest energy difference between states with an obvious overlap of the mediated states. Since WL-QD momenta are small, neglecting WL gives high SONS. Millimeter waves are predicted, and a huge SONS can be obtained by the application of more optical fields, which is important in medical and biological applications. The possibility of changing light speed between subluminal and superluminal was predicted here. This opens the way for many applications like multichannel waveguide-multichannel quantum information processing, real quality imaging, and temporal clock.

All-optical processes in double quantum dot structure.

The ladder-plus-Y double quantum dot structure was modeled for all-optical processing by combining the density matrix theory with the pulse width description of the applied pulse. The momentum matrix elements are calculated including the wetting layer. The ladder-plus-Y structure exhibits pattern-free output with high bit rate (50 Tbps), which is critical in optical communication applications. It is shown that very high ground-state occupation with periodic shape for state occupations is critical in obtaining a pattern-free eye diagram.