Experimental demonstration of coherent coupling of two quantum dots

During the recent years semiconductor nanostructures have attracted considerable interest with respect to potential applications in quantum information processing. In particular, quantum dot molecules have been suggested to provide the building block of a quantum computer: forming quantum gates due to coherent coupling of two dots. The characteristic dependence of the splitting of ‘bonding’ and ‘anti-bonding’ states suggests coherent coupling of two InAs/GaAs quantum dots. Anti-crossings in the fine structure of excitons due to mixing of optically bright and dark states have been observed in Faraday configuration. In Voigt configuration the diamagnetic shift of the quantum dot molecule is enhanced compared to a single quantum dot. These findings altogether demonstrate the coherent coupling of exciton states in quantum dot molecules.     

Quantum Cryptography in Spin Networks

In this paper we propose a new scheme of long-distance quantum cryptography based on spin networks with qubits stored in electron spins of quantum dots. By＂ conditional Faraday- rotation, single photon polarization measurement, and quantum state transfer, maximal-entangled Bell states for quantum cryptography between two long-distance parties are created. Meanwhile, efficient quantum state transfer over arbitrary＂ distances is obtained in a spin chain by＂ a proper choice of coupling strengths and using spin memory- technique improved. We also analyse the security＂ of the scheme against the cloning-based attack which can be also implemented in spin network and discover that this spin network cloning coincides with the optimal fidelity- achieved by＂ an eavesdropper for entanglement-based cryptography.     

Optical generation of spin coherence in single-charged (In,Ga)As/GaAs self-assembled quantum dots

The generation of electron spin coherence has been studied in n-modulation-doped (In,Ga)As/GaAs self-assembled quantum dots (QDs) which contain on average a single electron per dot. The doping has been confirmed by pump–probe Faraday rotation experiments in a magnetic field parallel to the heterostructure growth direction. For studying spin coherence, the magnetic field was rotated by 90° to the Voigt geometry, and the precession of the electron spin about the field was monitored. The coherence is generated by resonant excitation of the QDs with circularly polarized laser pulses, creating a coherent superposition of an electron, and a trion state. The efficiency of the generation can be controlled by the pulse intensity, being most efficient for (2n+1)π pulses.     

Far-infrared spectroscopy of quantum wires and dots, breaking Kohn’s theorem

We review far-infrared experiments on quantum wires and dots. In particular, we show that with tailored deviations from a parabolic external lateral confinement potential one can break Kohn’s theorem. This allows a detailed investigation of the internal relative motion in quantum dots and wires and the study of electron–electron interaction effects, for example, the formation of compressible and incompressible states in quantum dots and antidots.     

Teleportation of electronic many-qubit states encoded in the electron spin of quantum dots via single photons

We propose a teleportation scheme that relies only on single-photon measurements and Faraday rotation, for teleportation of many-qubit entangled states stored in the electron spins of a quantum dot system. The interaction between a photon and the two electron spins, via Faraday rotation in microcavities, establishes Greenberger-Horne-Zeilinger entanglement in the spin-photon-spin system. The appropriate single-qubit measurements, and the communication of two classical bits, produce teleportation. This scheme provides the essential link between spintronic and photonic quantum information devices by permitting quantum information to be exchanged between them.     

Self-assembly Drives Quantum Dot Photoluminescence

Engineering the spectral properties of quantum dots can be achieved by a control of the quantum dots organization on a substrate. Indeed, many applications of quantum dots as LEDs are based on the realization of a 3D architecture of quantum dots. In this contribution, we present a systematic study of the quantum dot organization obtained on different chemically modified substrates. By varying the chemical affinity between the quantum dots and the substrate, the quantum dot organization is strongly modified from the 2D monolayer to the 3D aggregates. Then the photoluminescence of the different obtained samples has been systematically studied and correlated with the quantum dot film organization. We clearly show that the interaction between the substrate and the quantum dot must be stronger than the quantum dot–quantum dot interaction to avoid 3D aggregation and that these organization strongly modified the photoluminescence of the film rather than intrinsic changes of the quantum dot induced by pure surface chemistry.     

Quantum computing based on space states without charge transfer

An implementation of a quantum computer based on space states in double quantum dots is discussed. There is no charge transfer in qubits during a calculation, therefore, uncontrolled entanglement between qubits due to long-range Coulomb interaction is suppressed. Encoding and processing of quantum information is merely performed on symmetric and antisymmetric states of the electron in double quantum dots. Other plausible sources of decoherence caused by interaction with phonons and gates could be substantially suppressed in the structure as well. We also demonstrate how all necessary quantum logic operations, initialization, writing, and read-out could be carried out in the computer.     

An impurity-induced gap system as a quantum data bus for quantum state transfer

We introduce a tight-binding chain with a single impurity to act as a quantum data bus for perfect quantum state transfer. Our proposal is based on the weak coupling limit of the two outermost quantum dots to the data bus, which is a gapped system induced by the impurity. By connecting two quantum dots to two sites of the data bus, the system can accomplish a high-fidelity and long-distance quantum state transfer. Numerical simulations for finite system show that the numerical and analytical results of the effective coupling strength agree well with each other. Moreover, we study the robustness of this quantum communication protocol in the presence of disorder in the couplings between the nearest-neighbor quantum dots. We find that the gap of the system plays an important role in robust quantum state transfer.     

Differential capacitance between circular stacked quantum dots

We have investigated the differential capacitance between two stacked, circular quantum dots. An expression defining such differential capacitance has been derived on the basis of that for the self-capacitance of a single quantum dot. By means of a self-consistent simulation we have obtained numerical results showing that the differential capacitance between the two dots is strongly influenced by shell-filling effects, and that the classical limit of the parallel-plate capacitor is retrieved when the two dots are in close proximity. Our results represent a contribution to the effort for the definition of a capacitance matrix for a complex system of quantum dots.     

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.     

Electron–phonon interaction in CdS/ZnS quantum dot

Electron–phonon effects on the two first electronic states in both CdS and GaAs quantum dots are investigated. Both confined longitudinal optical (LO) and surface optical (SO) phonons are considered. We use the intermediate-type variational approach. We find that, shifts caused by phonon contribution on electronic energies are more significant for CdS quantum dot. We find, also, that, contrary to GaAs based quantum dots, we shouldn’t neglect the SO phonon contribution for CdS based ones, especially for small dots.     

Temperature-induced broadening of the emission lines from a quantum-dot nanostructure

We report on the effect of temperature fluctuations on the midinfrared electroluminescence from a cascade of coupled AlInAs quantum dots and GaAs quantum wells. The observed line width is significantly broadened with increasing temperature. We then present our theoretical results on homogeneous line broadening due to temperature fluctuations for our experimental system. Our numerical simulations clearly indicate that, temperature fluctuations can account for the observed finite width of the emission lines at high-temperatures.     

Spectral response of the intrinsic region of a GaAs–InAs quantum dot solar cell considering the absorption spectra of ideal cubic dots

Recently, attempts have been made by some researchers to improve the efficiency of quantum dot solar cells by incorporating different types of quantum dots. In this paper, the photocurrent density has been obtained considering the absorption spectra of ideal cubic dots. The effects of quantum dot size dispersion on the spectral response of the intrinsic region of a GaAs–InAs quantum dot solar cell have been studied. The dependence of the spectral response of this region on the size of quantum dots of such solar cell has also been investigated. The investigation shows that for smaller quantum dot size dispersion, the spectral response of the intrinsic region of the cell increases significantly. It is further observed that by enlarging the quantum dot size it is possible to enhance the spectral response of such solar cells as it causes better match between absorption spectra of the quantum dots and the solar spectrum. These facts indicate the significant role of quantum dot size and size dispersion on the performance of such devices. Also, the power conversion efficiency of such solar cell has been studied under 1 sun, AM 1.5 condition.     

Dimensional effects and magnetic field influence on excitons in coupled quantum dots and coupled quantum wells

Direct and indirect excitons in coupled quantum wells and in coupled quantum dots are studied. We consider excitons with two-dimensional, quasi-two-dimensional and three-dimensional carriers. Problems were investigated for a wide range of characteristic parameters—confining to potential steepness, distances between quantum wells or dots, effective width of wells and magnetic fields. The mutual influence of the controlling parameters of the problem on exciton properties is analyzed. Energy and wave function spectra were calculated and dispersion law and effective masses were obtained.     

Optical control of spin coherence in singly charged (In,Ga)As/GaAs quantum dots

Electron spin coherence has been generated optically in n-type modulation doped (In,Ga)As/GaAs quantum dots (QDs) which contain on average a single electron per dot. The coherence arises from resonant excitation of the QDs by circularly polarized laser pulses, creating a coherent superposition of an electron and a trion. Time dependent Faraday rotation is used to probe the spin precession of the optically oriented electrons about a transverse magnetic field. The coherence generation can be controlled by pulse intensity, being most efficient for (2n+1)pi pulses.     

Study on the coupled multiple nanocrystal quantum-dot system

We have studied excess electron filling rule in the coupled multiple nanocrystal quantum-dot systems, i.e. quantum chain and quantum pattern, by the unrestricted Hartree–Fock–Roothaan method. Assuming each quantum dot of quantum pattern to be confined in a three-dimensional spherical potential well of finite depth, we have studied the intradot and interdot electron Coulomb and exchange interactions. By varying the center distance d between the coupled quantum dots, the transition from the strong- to weak-coupling situation is realized. For the systems in question, our results show that, with the filling of excess electrons into the quantum pattern, the corresponding chemical potentials form quasi-band structure, which is similar to the energy-band structure of crystal material. In each chemical-potential band of quantum pattern, the number of chemical-potential curves is equal to the number of quantum dots, and the distributions of them depend strongly on the quantum-dot arrangement structure of quantum pattern.     

Continuous weak measurement and nonlinear dynamics in a cold spin ensemble

A weak continuous quantum measurement of an atomic spin ensemble can be implemented via Faraday rotation of an off-resonance probe beam, and may be used to create and probe nonclassical spin states and dynamics. We show that the probe light shift leads to nonlinearity in the spin dynamics and limits the useful Faraday measurement window. Removing the nonlinearity allows a nonperturbing measurement on the much longer time scale set by decoherence. The nonlinear spin Hamiltonian is of interest for studies of quantum chaos and real-time quantum state estimation.     

载流子复合对时间分辨法拉第旋转光谱的影响

利用二能级体系速率方程,推导了半导体中探测光探测到的法拉第旋转光谱的理论模型,发现电子-空穴对的复合对法拉第旋转信号随时间的衰减有重要影响,并利用该模型对GaAs量子阱中实验测得的法拉第旋转光谱进行拟合,得到GaAs量子阱材料中的电子自旋弛豫时间为73.5 ps,而直接利用单指数进行拟合得到的电子自旋弛豫时间仅为51.3 ps. 因此,直接利用单指数对法拉第旋转光谱进行拟合得到电子自旋弛豫时间的传统做法是不准确的. 关键词： 自旋弛豫时间 时间分辨法拉第旋转光谱 GaAs量子阱     

Pressure-induced modulation of the confinement in self-organized quantum dots produced and detected by a near-field optical probe

Near-field optical probing, or nanoprobing, achieves spatial resolution that surpasses the diffraction limit of light and makes possible the luminescence imaging and spectroscopy of single quantum dots in dense arrays of dots. We use optical nanoprobing to study self-organized InGaAs quantum dots grown on (3 1 1)B oriented GaAs substrates. Here, we emphasize a new feature of nanoprobing: pressure-induced strain modulation near the surface. Operating in near-field optical excitation–collection mode, the probe makes contact with the surface and exerts direct pressure whose main effect is a compressive uniaxial strain under the probe. By adjusting the applied pressure, we modulate the local strain environment in and around a quantum dot, but still preserve the capability to capture its near-field luminescence. Nanoprobe pressure effects modify the confinement potential and radiative emission of single quantum dots, and the coupling strength between dots. This opens new possibilities for the study and control of the optical and electronic properties of single- and coupled-quantum dots.     

Splitting and storing excitons in strained coupled self-assembled quantum dots

We present a novel self-assembled quantum dot structure designed to spatially separate and store photo-generated electrons and holes in pairs of strain coupled quantum dots. The spatial separation of electron–hole pairs into quantum dots and strain-induced quantum dots has been investigated and verified by photoluminescence experiments. Results from time-resolved PL demonstrates that at low temperatures (3 K) the electron–hole pair can be stored for several seconds.