The recent progress in the manufacturing of semiconductor quantum dots (QD) systems has made possible the coherent control of quantum states in QDs using optical or electrical techniques. Laser pulses have been recently used to coherently coponto quânticontrol the exciton population in QDs. The coherent manipulation of quantum states is a high priority task to the development of quantum information and quantum computation. One particular signature of coherency in quantum systems is the Rabi oscillations, which were recently observed in a few experimental works. Here we theoretically study a system composed of a semiconductor QD, tunnel coupled to electron reservoirs. In the presence of a laser field an electron-hole pair is created in the QD. An external source-drain (bias) voltage allows electrons and holes to tunnel to the reservoirs. The study was developed via the non-equilibrium Green s function technique. We solve numerically a set of coupled differential equations to the retarded and lesser Green functions. This gives the occupation probabilities of the two levels of the QD and the laserinduced photocurrent as a function of time. We focus our attention on the effects of temperature on the Rabi oscillations. Our main findings encompass a thermal activated Pauli blockade of the Rabi oscillations that can be controlled via the reservoirs temperature. We also discussed the effects of this thermal activation of Pauli blockade on the photocurrent. These results suggest that ability to measure temperatures via quantum coherent signals, thus suggesting the possibility of a new quantum-dot based thermometer.
The recent progress in the manufacturing of semiconductor quantum dots (QD) systems has made possible the coherent control of quantum states in QDs using optical or electrical techniques. Laser pulses have been recently used to coherently coponto quânticontrol the exciton population in QDs. The coherent manipulation of quantum states is a high priority task to the development of quantum information and quantum computation. One particular signature of coherency in quantum systems is the Rabi oscillations, which were recently observed in a few experimental works. Here we theoretically study a system composed of a semiconductor QD, tunnel coupled to electron reservoirs. In the presence of a laser field an electron-hole pair is created in the QD. An external source-drain (bias) voltage allows electrons and holes to tunnel to the reservoirs. The study was developed via the non-equilibrium Green s function technique. We solve numerically a set of coupled differential equations to the retarded and lesser Green functions. This gives the occupation probabilities of the two levels of the QD and the laserinduced photocurrent as a function of time. We focus our attention on the effects of temperature on the Rabi oscillations. Our main findings encompass a thermal activated Pauli blockade of the Rabi oscillations that can be controlled via the reservoirs temperature. We also discussed the effects of this thermal activation of Pauli blockade on the photocurrent. These results suggest that ability to measure temperatures via quantum coherent signals, thus suggesting the possibility of a new quantum-dot based thermometer.
Coherent control is a key experimental technique for quantum optics and quantum information processing. We demonstrate a new degree of freedom in coherent control of semiconductor quantum dot (QD) ensembles operating at room temperature using the tunneling injection (TI) processes in which charge carriers tunnel directly from a quantum well reservoir to QD confined states. The TI scheme was originally proposed and implemented to improve QD lasers and optical amplifiers, by providing a direct injection path of cold carriers thereby eliminating the hot carrier injection problem which enhances gain nonlinearity. The impact of the TI processes on the coherent time of the QDs was never considered, however. We show here that since the cold carriers that tunnel to the oscillating QD state are incoherent, the rate of injection determines the coherent time of the QDs thereby controlling coherent light-matter interactions. Coherent interactions by means of Rabi oscillations were demonstrated in absorption and for weak excitation pulses in the gain regime. However, Rabi oscillations are totally diminished under strong excitation pulses which increase the rate of stimulated emission, causing the tunneling processes to dominate what shortens the coherence time significantly. Since the tunneling rate, and hence, the coherence time, were controlled by the optical excitation and electrical bias, our finding paves the way for TI-based coherence switching on a sub-picosecond time scale in room-temperature semiconductor nanometric structures.
In the present work we investigated the physical properties of quantum systems of two [1], three [2] and four [3] levels formed by electrons, excitons and trions using the formalism of the density matrix. The unitary evolution of the systems is based on the Liouville-von Neumann [4] master equation and the decoherence e ects are included according to the Lindblad [5] formalism. We investigated the optical properties [6] of these systems, looking for favorable conditions to the regime of electromagnetically induced transparency [7]. We calculate the geometric phases associated with the two and three level systems for pure and mixed states according to the formalism de ned by Pancharatnan [8]. In the four-level system we manipulate Gaussian pulses of test and control, provoking temporal delays [9], chirps [10] and creating ultra-short pulse trains [11] to induce coherent population accumulation and investigate the soliton propagation of light in the regime of transparency associated with the quantum memory phenomenon. We investigated the e ciency of a photocell of quantum dots coupled by tunnels in the presence of an external source of radiation within the Markovian [12] approximation for the interaction between system and reservoir.
In this paper, by applying generalized density-matrix equations, we explore the ultrafast dynamics of photo-generated electron–hole pairs in the system composed of electromagnetically coupled GaAs/AlGaAs quantum dots and reveal the physics mechanism behind laser-controlled nonlinear-optical response of electron–hole pairs. For the single-dot system, we present numerical results to show the transient dependence of carrier occupations and induced quantum coherence of electron–hole pairs in the presence of either a single or double laser pulses, as well as for a step turning-on of the laser field. For a dipole-radiation coupled double-dot system, we study numerically the time dependence of both intradot and interdot self-consistent depolarization fields within and between two dots, respectively. Meanwhile, we further demonstrate the significance of phase matching between induced depolarization fields and incident-laser pulse in order to maximize the stimulated transitions of electron–hole pairs and ensure non-quenched Rabi oscillations in each quantum dot. The employed theoretical model and the associated numerical analysis method are expected to facilitate designing and developing artificial quantum molecules composed of arbitrarily dielectric-embedded quantum dots with non-local quantum interactions and entanglement, as well as optically manipulating electron states of artificial quantum molecules.
We provide a theoretical study for the dynamical behavior of two-photon Rabi oscillations in two coupled quantum dots. Each dot is a three-level quantum system (ground, one-exciton, and biexciton states). We describe the optical dynamics within the density matrix formalism framework. The two-photon Rabi oscillations are found to be significantly affected by the strength of the interdot dipolar interaction. Using a detuned laser pulse with an exciton state of the driven quantum dot in our numerical simulations, a direct coherent excitation between the biexciton states can be established, but whether Rabi oscillations can be observed depend mainly on the strength of the dipolar interaction and bandwidth of the excitation laser. When the strength of the dipolar coupling is less than the excitation laser bandwidth, the time-dependent populations of the various two biexciton states of the system exhibit coherent oscillations with several frequencies. However, as we increase the dipolar coupling strength, the population transfer between the quantum dots levels amazingly becomes forbidden, and the excitation gets trapped at the initial state. The reduction of the influence of the dipolar interaction leads to a manipulation of two-photon Rabi oscillation in an ultrashort timescale (∼ps), opening possibilities to achieve ultrafast quantum state control and quantum logic gate generation.
The influence of electron-hole dipole-dipole interactions (local fields) on the excitonic Rabi oscillations in an isolated quantum dot (QD) has been theoretically investigated. An analysis for the QD interaction with monochromatic field and ultrashort Gaussian pulse has been performed. On the basis of optical Bloch equations the Rabi oscillation dynamics has been investigated. As a result, the bifurcation and essentially anharmonic regimes in the Rabi oscillations in a QD exposed to the monochromatic field have been predicted. The strong dependence of the period of Rabi oscillations on the QD depolarization has been revealed. For the Gaussian pulse it has been shown that the final state of inversion as a function of the pulse peak strength demonstrates step-like transitions.
The focus of the thesis is nonlinear transient optical properties and coherent control of single interface fluctuation quantum dots (QD's). When single exciton and biexciton resonances were driven by strong resonant laser pulses, Rabi oscillations of these transitions were observed in the differential transmission (DT) geometry. These results extend the concept of coherent control to the limit of single quantum confined systems in solids and have important implications for the feasibility of building quantum information processing devices using semiconductor QD's. Exciton Rabi oscillations are equivalent to arbitrary single-bit operations. The pulsed control of the exciton-biexciton transition is utilized as the optical operation for building a two-bit controlled rotational gates (CROT) in a single QD. A fidelity of 0.79 was obtained from numerical evaluation of the gate operation itself assuming typical decay parameters measured from these dots. Next, the nonradiative quantum coherence between two orthogonally polarized excitons confined in a single QD was studied. By using pulses with large enough bandwidth and the proper polarization, two exciton states are simultaneously excited. The quantum beats between the two exciton states were temporally resolved as the relative phase between the two dipole oscillations changed. It was found that the nonradiative coherence time is limited by the lifetimes of the excitons involved even in the strong field regime. In the basis of the pseudo-spin Bloch vectors of exciton transitions, the total wavefunction following the pulsed excitation is an entangled state, i.e. the wavefunction is non-factorizable. Finally, the quantum coherence between the biexciton, exciton, and the crystal ground states was studied in a novel quantum beat experiment, where the oscillations only existed when the pump and probe pulses overlapped in time. Laser pulses generated after a passive pulse shaper with just enough bandwidth to cover both the exciton and biexciton transitions were used in this experiment. The appearance of the beats is indicative of a reasonably long coherence time between the biexciton and other states even in the presence of strong pulsed excitations.