Quantum operations on charge qubits with the electrostatic control in semiconductor cavities

We consider one- and two-qubit operations on charge qubits that represent double quantum dots with one electron in each of them. The dots are formed inside a high-Q semiconductor cavity (disk, toroidal, or spherical) in the antinodes of one of its optical eigenmodes; the frequencies of the transitions between the ground (logic) and excited (auxiliary) states of the discrete electron spectrum in quantum dots are close to the frequency of this mode. The precise tuning of the transition frequency is performed by applying an electric potential on the qubit control gate. Within the model of qubits coherently interacting with a cavity quantum field, a few methods for controlling their states are developed. In particular, we propose different variants for implementing two-qubit CNOT and CZ gates and generating qubit entangled states. By the example of a micro-disk cavity, we calculate the operating characteristics that ensure a high rate of quantum gate implementation.     

Excitonic dynamics of a quantum dot coupled to a laser-driven semiconductor microcavity

Within the density matrix formalism, we report on the quantum control of the excitonic coherences in quantum dots coupled to a single mode field resonant semiconductor cavity. We use an external classical laser field to drive the dynamical response of the excitonic states. Dissipation mechanisms associated with the cavity field and the excitonic states are explicitly included in the model. Our numerical simulations of the excitonic dynamics are in good agreement with recent experimental reports. Furthermore, we compute and show how to tailor such a dynamics in the presence of the laser field by means of controlling the detuning between the laser and the cavity field frequencies. The results are analyzed with a view to implementing quantum control of local qubit operations.     

腔耦合量子点系统的非常规几何量子计算

提出了在量子点与腔相互作用系统中，利用双拉曼过程来实现非常规几何量子逻辑门方案。在本方案中，所有的演化本身与腔场态无关，因而对热场是不敏感的。在总位相中既包含有几何位相,又包含有动力学位相,但它的确仅依赖于量子态演化的整体几何特征。通过调节耦合常数和失谐量来选择我们所要的量子控制门，在实际的操作中不需要考虑消除动力学相位问题因而易于操作，且避免因消除动力学相位引入的误差。     

Implementation of a Two-Qubit C-NOT Quantum Gate in a System of Two Double Quantum Dots,a Microcavity,and a Laser

We study the possibility of implementing a two-qubit controlled-NOT gate operation in a structure consisting of two semiconductor double quantum dots placed in a high-Q optical microcavity and governed by a resonant laser field. The effect of relaxation processes on the dynamics of the two-electron system is discussed. The dissipation rates allowing quantum error correction algorithms to be used are determined. The existing additional excitation channel (laser pulse) is shown to weaken the effect of the nonideality of the cavity on the evolution of the states. Optimal electron coupling coefficients in the quantum dots with control fields are fitted, in which the controlled qubit is switched with the highest probability and the gate implementation takes several hundreds of picoseconds.     

Quantum-Dot Sources for Single Photons and Entangled Photon Pairs

Quantum dots show excellent promise as triggered sources of both single and polarization entangled photons for quantum information applications. Our recent progress developing nonclassical light sources with single quantum dots is presented in this paper. Following radiative emission of an exciton confined in a quantum dot, there is a finite delay before re-excitation can occur; this results in an anti-bunching of the photons emitted providing a source of single photons. Excitation of a quantum dot with two electrons and two holes leads to the emission of a pair of photons; we show here that, provided the spin splitting of the intermediate exciton state in the decay is erased, the photon pair is emitted in an entangled polarization state. The fidelity of this entangled state is shown to exceed 70%. Using quantum dots to generate quantum light allows contacts for electrical injection to be integrated into a compact and robust device. A cavity may also be integrated into the semiconductor structure to enhance the photon collection efficiency and control the recombination dynamics. We detail a process to form a submicrometer current aperture within an electrical device, allowing individual quantum dots to be addressed electrically in devices.     

Electrically injected photonic crystal edge-emitting quantum-dot light source

An electrically injected photonic crystal (PC) edge-emitting light source with horizontally coupled waveguide, based on a GaAs heterostructure with self-organized InGaAs quantum dots emitting near 1 /spl mu/m, is demonstrated. The emission spectra and near-field images indicate that the spontaneous emission from the low-Q resonant PC cavity is coupled to an adjacent PC missing defect waveguide and guided over the 70-/spl mu/m waveguide length.     

扫描近场光学显微镜测量量子点团簇超辐射

用扫描近场光学显微镜的针尖照明模式对ZnSe量子点团簇进行精确定位测量,研究了量子点团簇的超辐射效应。在理论上根据Wannier激子超辐射模型阐述了量子点系统的超辐射发光机制;实验上用荧光光谱表征ZnSe量子点溶液的荧光性质,用扫描近场光学显微镜(SNOM)表征单个量子点团簇的超辐射光谱。结果表明,在Wannier激子超辐射模型中,量子点团簇辐射衰变率受到量子点团簇的大小和辐射光谱的共同影响,在实验上得出团簇的辐射衰变率随团簇尺寸的增加而增大,同时,不同尺寸的量子点团簇产生的辐射光谱也会对其产生影响,理论和实验的结合验证了激子超辐射的适用性。此研究结果可广泛用于生物传感器和光子器件等领域。     

Power conversion efficiency of quantum dot laser diodes

The power conversion efficiency of laser diodes with an array of quantum dots in the active region is analyzed. A model is proposed which allows analytical determination of the optimal cavity length corresponding to the highest conversion efficiency for a given output power. A comparison is made with experimental data for high-power lasers based on submonolayer quantum dots emitting at 0.94 μm. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 34, No. 5, 2000, pp. 628–632. Original Russian Text Copyright ? 2000 by Zhukov, Kovsh, Mikhrin, Maleev, Odnoblyudov, Ustinov, Shernyakov, Kondrat’eva, Livshits, Tarasov, Ledentsov, Kop’ev, Alferov, Bimberg.     

渐逝波耦合半导体量子点光纤放大器

基于半导体量子点的特性,结合光纤渐逝波耦合器,提出了一种新型的光纤放大器件,它将以溶液形式的硫化铅(PbS)半导体量子点材料沉积于耦合器熔锥区,信号光和抽运光通过渐逝波共同与半导体量子点材料相互作用,实现光的放大作用。PbS量子点材料是采用工艺容易控制的反胶束法制备的,通过透射电镜(TEM)测量得到其粒子尺寸小于10 nm。利用工作波长为980 nm,功率为30 mW的半导体激光器抽运光源对该光纤放大器抽运,在1310 nm波段得到了大于4 dB的增益,这是半导体量子点尺寸效应引起的光谱蓝移现象的体现。因此,这种有源区短、器件结构紧凑的光纤放大器在高速、宽带光纤接入等领域具有重要的实际意义和应用价值。     

Role of thermal ejection of carriers in the burning of spatial holes in quantum dot lasers

The role of thermal carrier ejection from quantum dots and free carrier diffusion in the burning of spatial holes in semiconductor quantum dot lasers is analyzed. The balance of the spatially inhomogeneous population inversion in the longitudinal direction of the cavity is shown to be controlled by thermal ejection from quantum dots. Because of this circumstance, hole burning in quantum dot lasers can show up more strongly and the threshold for multimode lasing can be lower than in semiconductor lasers with three-dimensional active regions or quantum-well lasers. The threshold for multimode lasing is determined as a function of the dispersion in the quantum dot size, cavity length, and temperature for structures that have been optimized to minimize the threshold current density of the fundamental mode. Fiz. Tekh. Poluprovodn. 33, 1076–1079 (September 1999)     

半导体量子点自组织生长过程的控制

对控制半导体量子点的自组织生长进行了研究. 在对平均场模型和非平均场模型计算机模拟结果分析的基础上，提出了利用隔离单元抑制量子点的熟化生长的方法来控制量子点的大小，并对生长过程进行了计算机模拟. 结果表明通过将衬底表面的吸附原子海分割成一个个独立的隔离单元可以抑制量子点熟化生长，调节量子点的生长速度，达到制备大小均匀、排列有序的半导体量子点阵列的目的.     

量子点-聚合物纳米复合材料的光电器件研究进展

量子点因具有优异的光电特性,近年来备受关注。但量子点的规模化应用因受到其加工工艺及稳定性等因素限制而尚待开发。量子点-聚合物纳米复合材料的出现有效弥补了这一问题,将量子点分散到有机聚合物中形成纳米复合材料,集合量子点与聚合物的各自优势于一体,是解决量子点当前应用问题的一种有效方法,具有显著的发展潜力。文中介绍了量子点的主要制备技术,并在此基础上对量子点-聚合物复合材料的制备方法及其在激光器、发光二极管、光电探测器、量子点电视等光电子器件中的应用进展进行了概述,最后对其在光电器件领域的应用进行了展望。     

Photoluminescence of InAs quantum dots grown on disoriented GaAs substrates

The photoluminescence spectra in an external magnetic field of an ensemble of InAs quantum dots grown by molecular beam epitaxy on a (001) GaAs substrate with a disorientation in the [010] direction are studied. A redistribution of the photoexcited carriers among different groups of dots under the influence of the magnetic field is observed. The concentration of quantum dots is determined by analyzing the data. Fiz. Tekh. Poluprovodn. 33, 1084–1087 (September 1999)     

1040 nm vertical external cavity surface emitting laser based on inGaAs quantum dots grown in Stranski-Krastanow regime

Quantum dots grown by metal organic vapour phase epitaxy in the Stranski-Krastanow regime are succesfully implemented for the first time as active media in optically pumped vertical external cavity surface emitting lasers. To optimise the gain the quantum dots are engineered to match the excited state luminescence to the cavity resonance. Room temperature continuous-wave operation at 1040 nm wavelength is demonstrated. An output power of 280 mW is achieved, limited only by the onset of thermal rollover. The differential conversion efficiency is 6.7%.     

Characteristics of pH sensors fabricated by using protein-mediated CdSe/ZnS quantum dots

The pH sensors using protein-mediated CdSe/ZnS quantum dots in an electrolyte-insulator-semiconductor (EIS) structure have been investigated. The hydrophobic cylindrical cavity of the chaperonin (GroEL) protein template was used to trap CdSe/ZnS quantum dots on hydrophobically treated SiO₂ surface. The CdSe/ZnS quantum dot with a small diameter of 3.98 nm is observed by atomic force microscope. A fair pH response with a sensitivity of 39 mV/pH and a linearity of 99.48% are obtained by using CdSe/ZnS quantum dot based EIS sensor, while those values are found to be 53 mV/pH and 99.95% for bare SiO₂ based EIS pH sensors. The pH response and linearity of CdSe/ZnS based quantum dot sensors are inferior (slightly) as compared to the bare SiO₂ sensors owing to the initial negative charges of CdSe quantum dots membrane, which has been explained by energy band diagrams. It is expected that this kind of quantum dot membrane can be useful in future bio-molecule detections.     

Stark effect in vertically coupled quantum dots in InAs-GaAs heterostructures

The results of studies of hole energy states in vertically coupled quantum dots in InAs-GaAs p-n heterostructures by deep-level transient spectroscopy are reported. Spectra were recorded at different reverse-bias voltages. Levels related to bonding and antibonding s and p states of vertically coupled quantum dots were revealed. The energies of these states significantly depend on an external electric field applied to a heterostructure. This dependence was attributed to the quantum-dimensional Stark effect for the hole states of vertically coupled quantum dots. In addition to this, it was found that the energy of thermal activation of carriers from vertically coupled quantum dots depends on the conditions of isochronous annealing that was carried out both with the reverse bias switched-on and switched-off and both in the presence and absence of illumination. These changes, as in the case of isolated quantum dots, are typical of a bistable electrostatic dipole formed by carriers, localized in a coupled quantum dot, and ionized lattice point defects. The built-in electric field of this dipole reduces the energy barrier for the carriers in the coupled quantum dot. The investigated structures with vertically coupled quantum dots were grown using molecular-beam epitaxy taking account of self-assembling effects.     

Optical properties of wurtzite GaN/AlN quantum dots grown on non-polar planes: The effect of stacking faults in the reduction of the internal electric field

The optical emission of non-polar GaN/AlN quantum dots has been investigated. The presence of stacking faults inside these quantum dots is evidenced in the dependence of the photoluminescence with temperature and excitation power. A theoretical model for the electronic structure and optical properties of non-polar quantum dots, taking into account their realistic shapes, is presented which predicts a substantial reduction of the internal electric field but a persisting quantum confined Stark effect, comparable to that of polar GaN/AlN quantum dots. Modeling the effect of a 3 monolayer stacking fault inside the quantum dot, which acts as zinc-blende inclusion into the wurtzite matrix, results in an additional 30% reduction of the internal electric field and gives a better account of the observed optical features.     

Reprint of: Optical properties of wurtzite GaN/AlN quantum dots grown on non-polar planes: The effect of stacking faults in the reduction of the internal electric field

The optical emission of non-polar GaN/AlN quantum dots has been investigated. The presence of stacking faults inside these quantum dots is evidenced in the dependence of the photoluminescence with temperature and excitation power. A theoretical model for the electronic structure and optical properties of non-polar quantum dots, taking into account their realistic shapes, is presented which predicts a substantial reduction of the internal electric field but a persisting quantum confined Stark effect, comparable to that of polar GaN/AlN quantum dots. Modeling the effect of a 3 monolayer stacking fault inside the quantum dot, which acts as zinc-blende inclusion into the wurtzite matrix, results in an additional 30% reduction of the internal electric field and gives a better account of the observed optical features.     

On the physics of semiconductor quantum dots for applications in lasers and quantum optics

The progression of carrier confinement from quantum wells to quantum dots has received considerable interests because of the potential to improve the semiconductor laser performance at the underlying physics level and to explore quantum optical phenomena in semiconductors. Associated with the transition from quantum wells to quantum dots is a switch from a solid-state-like quasi-continuous density of states to an atom-like system with discrete states. As discussed in this paper, the transition changes the role of the carrier interaction processes that directly influence optical properties. Our goals in this review are two-fold. One is to identify and describe the physics that allows new applications and determines intrinsic limitations for applications in light emitters. We will analyze the use of quantum dots in conventional laser devices and in microcavity emitters, where cavity quantum electrodynamics can alter spontaneous emission and generate nonclassical light for applications in quantum information technologies. A second goal is to promote a new connection between physics and technology. This paper demonstrates how a first-principles theory may be applied to guide important technological decisions by predicting the performances of various active materials under a broad set of experimental conditions.     

不同形状InN/GaN量子点应变场的有限元分析

量子点中的应变场分布对量子点的力学稳定性、压电性能以及光电性能有着重要的影响。基于有限元方法,并考虑了InN/GaN材料的六方纤锌矿结构特性,分别对透镜形、平顶六角金字塔形和六角金字塔形量子点的应变分布进行了比较,结果表明应变主要集中在浸润层和量子点内,在讨论量子点中电子能级时必须考虑浸润层的影响。量子点内的应变分布及静水应变和双轴应变受几何形状的影响明显。此外还计算了三种形状量子点的总能量,六角金字塔形量子点总能量最小,而透镜形量子点总能量最大,因此六角金字塔形是最稳定的结构,而透镜形是最不稳定的结构。