Hydrogenic impurity states in wurtzite GaN/AlGaN coupled quantum dots

The ground-state binding energy of a hydrogenic donor impurity in wurtzite (WZ) GaN/AlGaN coupled quantum dots (QDs) is calculated by means of a variational method, considering the strong built-in electric fields caused by the piezoelectricity and spontaneous polarizations. The strong built-in electric fields induce an asymmetrical distribution of the ground-state binding energy with respect to the center of the coupled QDs. If the impurity is located at the low dot, the ground-state binding energy is insensitive to the interdot barrier width of WZ GaN/AlGaN coupled QDs.     

Superluminal optical soliton via resonant tunneling in coupled quantum dots

We demonstrate the formation of superluminal optical soliton in an symmetry semiconductor double quantum dot (QD) driven coherently by a weak pulsed laser using the tunnel coupling. It is shown that the group velocity of the soliton can be larger than the vacuum light speed c, i.e., superluminal soliton can be produced. The results obtained can be used for the development of new types of nanoelectronic devices for realizing high-speed optical modulation and rapidly responding quantum switching.     

Landau levels in a novel two-dimensional electron system interacting with charged quantum dots

Landau levels have been theoretically investigated in a two-dimensional electron gas near a quantum dot (QD) layer. By a diagrammatical method, we have formulated the self-energy for the Landau level and deduced its relation to the AC conductivity σ_loc(ω) in the QD layer. As an example, we have examined the density of states in the case where σ_loc(ω) is described by Aω^S(S=0.8). It is found that the Landau levels are broadened due to the interaction with the localized electrons in the QDs.     

Quantum entanglement in elliptical quantum corrals

Quantum corrals present interesting properties due to the combination of confinement and, in the case of elliptical corrals, to their focalizing properties. We study the case when two magnetic impurities are added to the non-interacting corral, where they interact via a superexchange AF interaction J with the surface electrons in the ellipse. Previous results showed that, when both impurities are located at the foci of the system, they experience an enhanced magnetic interaction, as compared to the one they would have in an open surface. For small J and even filling, they are locked in a singlet state, which weakens for larger values of this parameter. When J is much larger than the hopping parameter of the electrons in the ellipse, both spins decorrelate while forming a local singlet with the electrons of the ellipse, thus presenting a confined RKKY–Kondo transition.We interpret this behaviour by means of the von Neumann entropy between the localized impurities and the itinerant electrons of the ellipse: for small J the entropy is nearly zero while for large J it is maximum. In addition, the local density of states provides us with a concrete experimental tool for detecting the Kondo regime.     

Control of the entanglement between triple quantum dot molecule and its spontaneous emission fields via quantum entropy

The time evolution of the quantum entropy in a coherently driven triple quantum dot molecule is investigated. The entanglement of the quantum dot molecule and its spontaneous emission field is coherently controlled by the gate voltage and the rate of an incoherent pump field. The degree of entanglement between a triple quantum dot molecule and its spontaneous emission fields is decreased by increasing the tunneling parameter.     

Entangled Bell states of two electrons in coupled quantum dots—phonon decoherence

We demonstrate coupling and entangling of quantum states in a pair of vertically aligned self assembled quantum dots by studying the dynamics of two interacting electrons driven by external electric field. The present entanglement involves the spatial degree of freedom for the two electrons system. We show that system of two interacting electrons initially delocalized (localized each in one dot) oscillate slowly in response to electric field, since the strong Coulomb repulsion makes them behaving so. We use an explicit formula for the entanglement of formation of two qubit in terms of the concurrence of the density operator. In ideal situations, entangled quantum states would not decohere during processing and transmission of quantum information. However, real quantum systems will inevitably be influenced by surrounding environments. We discuss the degree of entanglement of this system in which we introduce the decoherence effect caused by the acoustic phonon. In this entangled states proposal, the decohering time depends on the external parameters.     

Entanglement in S states of two-electron quantum dots with Coulomb impurities at the center

We study a system of two Coulombically interacting electrons in an external harmonic potential in the presence of an on-centre Coulomb impurity. Detailed results for the dependencies of the reduced von Neumann entropy on the control parameters of the system are provided for both the ground state and the triplet S states with the lowest energy. Among other features, it is found that in the weak confinement regime the entanglement is strongly affected by the presence of an acceptor impurity.     

Magneto-optical study of nonmagnetic quantum dots coupled to a magnetic semiconductor quantum well

We have investigated a series of double-layer structures consisting of a layer of self-assembled non-magnetic CdSe quantum dots (QDs) separated by a thin ZnSe barrier from a ZnCdMnSe diluted magnetic semiconductor (DMSs) quantum well (QW). In the series, the thickness of the ZnSe barrier ranged between 12 and 40 nm. We observe two clearly defined photoluminescence (PL) peaks in all samples, corresponding to the CdSe QDs and the ZnCdMnSe QW, respectively. The PL intensity of the QW peak is observed to decrease systematically relative to the QD peak as the thickness of the ZnSe barrier decreases, indicating a corresponding increase in carrier tunneling from the QW to the QDs. Furthermore, polarization-selective PL measurements reveal that the degree of polarization of the PL emitted by the CdSe QDs increases with decreasing thickness of the ZnSe barriers. The observed behavior is discussed in terms of anti-parallel spin interaction between carriers localized in the non-magnetic QDs and in the magnetic QWs.     

与腔场耦合的两个量子点中激子的纠缠动力学

利用共生纠缠度研究了单模腔场内两个耦合量子点中激子的纠缠动力学行为.结果表明:无论腔场初始制备于奇相干态还是偶相干态,两个量子点间直接耦合作用均能减弱激子的纠缠度.在腔场初始为奇相干态时,激子的纠缠度随场模强度的增加而减小;偶相干态时,激子的纠缠度呈现一个转折变化.此外,也研究了单模腔场内平均光子数与激子准最大相干纠缠态的关联.     

Hydrogenic impurity in double quantum dots

The ground state binding energy and the average interparticle distances for a hydrogenic impurity in double quantum dots with Gaussian confinement potential are studied by the variational method. The probability density of the electron is calculated, too. The dependence of the binding energy on the impurity position is investigated for GaAs quantum dots. The result shows that the binding energy has a minimum as a function of the distance between the two quantum dots when the impurity is located at the center of one quantum dot or at the center of the edge of one quantum dot. When the impurity is located at the center of the two dots, the binding energy decreases monotonically.     

Quantum dots in quantum well structures

Recent progress toward fabricating and characterizing quantum dots in III–V quantum well structures is reviewed. Quantum dots made by use of lithography and etching, including deep-etched, barrier-modulated, strain-induced and interdiffused quantum dots, are described. Quantum dots fabricated by growth, including natural quantum dots, dots on patterned substrates, and self-assembled dots, are discussed. Dot sizes and uniformity, energy-level splittings, and luminescence efficiencies that are now being achieved are discussed. The status of key issues, such as the energy relaxation in quantum dots, is mentioned.     

Substitutional impurity in the graphene quantum dots

The process of formation of the localized defect states due to substitutional impurity in sp²-bonded graphene quantum dot is considered using a simple tight-binding-type calculation. We took into account the interaction of the quantum dot atoms surrounding the substitutional impurity from the second row of elements. To saturate the external dangling sp² orbitals of the carbon additionally 18 hydrogen atoms were introduced. The chemical formula of the quantum dot is H₁₈C₅₁X, where X is the symbol of substitutional atom. The position of the localized levels is determined relative to the host-atoms (C) ε_p energies. We focused on the effect of substitutional doping by the B, N and O on the eigenstate energies and on the total energy change of the graphene dots including for O the effect of lattice distorsion. We conclude that B, N, and O can form stable substitutional defects in graphene quantum dot.     

The binding energy of a hydrogenic impurity in self-assembled double quantum dots

The binding energy of a hydrogenic impurity in self-assembled double quantum dots is calculated via the finite-difference method. The variation in binding energy with donor position, structure parameters and external magnetic field is studied in detail. The results found are: (i) the binding energy has a complex behaviour due to coupling between the two dots; (ii) the binding energy is much larger when the donor is placed in the centre of one dot than in other positions; and (iii) the external magnetic field has different effects on the binding energy for different quantum-dot sizes or lateral confinements.     

Enhancement of spin polarization in asymmetrically coupled CdSe and CdZnMnSe quantum dots in ZnSe matrix

An asymmetrically coupled double quantum dot (QD) system consisting of adjacent CdSe and CdZnMnSe QD layers in a ZnSe matrix was investigated using polarization-selective magneto-photoluminescence (PL). Two well-resolved PL peaks are observed corresponding, respectively, to the CdSe and the CdZnMnSe QDs. The peaks exhibit significant change in the intensity and energy position when a magnetic field is applied. The enhancement of the degree of σ⁻ circular polarization emitted by the non-magnetic CdSe QDs is observed in the double layer system, as compared to that observed in CdSe QDs without the influence of neighboring CdZnMnSe QDs. This behavior was discussed in terms of antiferromagnetic interaction between carrier spins localized in pairs of CdSe and CdZnMnSe QDs that are electronically coupled.     

Disappearance of quantum chaos in coupled chaotic quantum dots

Statistical properties of the single electron levels confined in the semiconductor (InAs/GaAs, Si/SiO₂) double quantum dots (DQDs) are considered. We demonstrate that in the electronically coupled chaotic quantum dots the chaos with its level repulsion disappears and the nearest neighbor level statistics becomes Poissonian. This result is discussed in the light of the recently predicted “huge conductance peak” by R.S. Whitney et al. [Phys. Rev. Lett. 102 (2009) 186802] in the mirror symmetric DQDs.     

Temperature-dependent photoluminescence of vertically stacked self-assembled CdSe quantum dots in ZnSe

We studied the optical properties of multiple layers of self-assembled CdSe quantum dots (QDs) embedded in ZnSe, grown by molecular beam epitaxy. The ZnSe barrier thicknesses separating the QD layers ranged from 30 to 60 monolayers (ML). For stacks with thinnest ZnSe barriers photoluminescence (PL) measurements reveal blue shifts as large as 180 meV relative to PL observed for single QD layers. The amount of blue shift decreases with increasing barrier thickness, and for the 60 ML spacer the PL energy returns to that emitted by a single layer of QDs. Temperature dependence of the integrated intensity of the emission spectra reveals that the activation energy for PL quenching is largest for barrier thicknesses of 30 and 45 ML. We tentatively attribute these effects to a decrease in the size of the vertically stacked QDs when the thickness of the barrier layers is small.     

Two-dimensional probe absorption in coupled quantum dots

We investigate the two-dimensional (2D) probe absorption in coupled quantum dots. It is found that, due to the position-dependent quantum interference effect, the 2D optical absorption spectrum can be easily controlled via adjusting the system parameters. Thus, our scheme may provide some technological applications in solid-state quantum communication.     

Quantum coherence and entanglement partitions for two driven quantum dots inside a coherent micro cavity

We explore the coherence and entanglement of two separated quantum dots placed in an open microcavity. The generated entanglement and coherence depend crucially on the two-exciton decay and the initial coherence intensity. The phenomena of sudden death and sudden birth entanglement are observed.