arXiv:cond-mat/0405526 · Physical Review B

Coherent state monitoring in quantum dots

Ameenah N. Al‐Ahmadi, Sergio E. Ulloa

Published Nov 3, 2004• 9 citations

Abstract

We study the dynamics of excitonic states in dimer and trimer arrangements of colloidal quantum dots using a density-matrix approach. The dots are coupled via a dipole-dipole interaction akin to the F\"orster mechanism. Coherent oscillations of tuned donor dots are shown to appear as plateaus in the acceptor dot, and therefore in its optical response. This behavior provides one with an interesting and unique handle to monitor the quantum state of the dimer, an ``eavesdroping arrangement.'' A trimer cluster in a symmetrical loop shows steady states in a shorter characteristic time than the typical radiative lifetime of the dots. Breaking the symmetry of the loop results again in damping oscillatory states in the donor dots and plateaus in the eavesdropping∕acceptor dot. The use of realistic parameters allows direct comparison with recent experiments and indicates that coherent-state monitoring is possible in real experiments.

DOI: 10.1103/physrevb.70.201302

Related papers

EXCITATION ENERGY TRANSFER IN QUANTUM-DOT SOLIDS

Ameenah N. Al‐Ahmadi

We study the dynamics of excitonic states in one dimensional, dimer and trimer, and two dimensional arrangements of colloidal quantum dots using a density-matrix approach.The dots are coupled via a dipole-dipole interaction akin to the Förster mechanism.Coherent oscillations of tuned donor dots are shown to appear as plateaus in the acceptor dots, and therefore in their optical response.This behavior provides one with an interesting and unique handle to monitor the quantum state of clusters in an eavesdroping arrangement.A trimer cluster in a symmetrical loop shows steady states in a shorter characteristic time than the typical radiative lifetime of the dots.Breaking the symmetry of the loop results again in damping oscillatory states in the donor dots and plateaus in the eavesdropping/acceptor dot.The use of realistic parameters allows direct comparison with recent experiments and indicates that coherent state monitoring is possible in real experiments.Estimates of the Förster coupling V F are obtained from a microscopic description of the exciton levels in the quantum dot.We present results for different materials CdS, CdSe, GaAs, and InP as function of dot features.V F shows a nonmonotonic variation with dot sizes and a maximum value that depends on the specific dot sizes, material parameters, and dot separation.We also study the effect of the orientation factor on coherent energy transfer from a donor quantum dot to an acceptor quantum dot by calculating the polarization of the acceptor dot.We study how the measurement of the ac-ceptor polarization can be used to determine the relative orientation between the donor-acceptor pair.We also find that Förster coupling between dots results in a polarization memory transfer.

Jan 1, 20060 citations

Coherent state excitons in anisotropic quantum dots: classical and quantum correlations

Alagu Thilagam

We investigate the entanglement dynamics of excitons confined in two adjacent quantum dots, which we describe through their algebraic properties using in the z -direction. We use two explicit forms of coherent states: the Perelomov and Barut–Girardello states to represent the electronic component of the excitonic state in the z -direction. Our results show that in a coherent state basis, the concurrence of an excitonic-qubit pair shows subtle variations which are dependent on the algebraic parameters of the group. These variations also appear in the classical and quantum correlations, present in the qubit–qubit density matrix that is associated with the purely Förster-coupled excitonic-qubit pair. A brief discussion of a plausible experimental technique of detecting excitonic coherent states is provided. This article is part of a special issue of Journal of Physics A: Mathematical and Theoretical devoted to 'Coherent states: mathematical and physical aspects'.

Journal of Physics A Mathematical and TheoreticalMay 30, 20120 citations

Coherent nonlinear optical spectroscopy of coupled quantum dots.

E. T. Batteh

Quantum dots have been the focus of both fundamental and applied research due to their atomic-like properties. The delta-function-like density of states along with strong nonlinear optical properties have made quantum dots the focus of novel opto-electronic devices and of quantum computing proposals. The key issue for implementing quantum dots in devices such as quantum computers is the ability to coherently entangle separately localized quantum dot states. Using continuous wave coherent nonlinear optical spectroscopy techniques, this thesis focuses on creating coherent super-position states built from separately localized quantum dot states. The narrow bandwidths and mutual coherence of the applied laser fields allow for the creation of radiative and nonradiative coherences which can be understood from the coherent nonlinear optical response. The quantum dots under study are formed due to the width fluctuations of a thin quantum well. By applying nondegenerate mutually coherent fields on the sample in the traditional pump and probe geometry and homodyne detecting the nonlinear signal field, nonradiative coherences between two laterally coupled quantum dots are created and the coupling between their states examined. The coherent nonlinear optical response is compared with theoretical predictions based on the density matrix formalism for the interacting two exciton model. From this comparison, binding energies of the two-exciton bound state created with separately localized excitons is found to be on the order of 20 mueV for like-polarized excitons and on the order of 100 mueV for unlike-polarized excitons. A second order coherence between the ground and two-exciton state is also noted in the experimental data after comparison with theory. Further work attempting to study a vertically coupled quantum dot system was unsuccessful. The spectrum of a quantum dot is further understood from the comparison of the linear and nonlinear excited state spectra. Excited states which appear in both spectra imply a Coulomb interaction between them and their respective ground states. Coherent nonlinear optical signatures of excited states correlated with their linear excitation spectra are presented, and the importance of such states for a quantum computer read-out is described.

Jan 1, 20030 citations

Optical Control of Excitons in a Pair of Quantum Dots Coupled by the Dipole-Dipole Interaction

Thomas Unold, Kerstin Mueller, Christoph Lienau +2 more

We demonstrate coherent nonlinear-optical control of excitons in a pair of quantum dots (QDs) coupled via dipolar interaction. The single-exciton population in the first QD is controlled by resonant picosecond excitation, giving rise to Rabi oscillations. As a result, the exciton transition in the second QD is spectrally shifted and concomitant Rabi oscillations are observed. We identify coupling between permanent excitonic dipole moments as the dominant interaction mechanism, whereas quasiresonant (Förster) energy transfer is weak. Such control schemes based on dipolar interaction are a prerequisite for realizing scalable quantum logic gates.

Physical Review LettersApr 8, 2005205 citations

Coherent Light-Matter Interaction in InGaAs Quantum Dots: Dephasing Time and Optical Rabi Oscillations

Paola Borri, W. Langbein, Stephan Schneider +4 more

We present measurements of the dephasing time and of optical Rabi oscillations in the excitonic ground-state transition of an InGaAs quantum dot ensemble at low temperature, using an heterodyne pump–probe technique. Dephasing times of several hundred picoseconds are measured, enabling coherent light–matter interaction with picosecond optical pulses. Optical Rabi oscillations which are damped versus pulse area and change period when changing pulse duration are observed. Comparison with calculations indicates that the observed damping is not intrinsic to a single dot. Dephasing processes and the biexciton resonance change the amplitude and the period of the oscillations, respectively, while the damping versus pulse area is due to a distribution of transition dipole moments in the ensemble.

physica status solidi (b)Oct 1, 20026 citations

Coherent spectroscopy of InGaAs quantum dots

Stefan Maximilian Strauß

Excitons in quantum dots represent a remarkably good two-level system with a large optical dipole transition moment. Although most research currently focuses on enhancing their already outstanding properties as single photon emitters they become also increasingly interesting for experimental fundamental research due to their large dipole transition moment. This thesis presents studies which explore fundamental phenomena in quantum dots using coherent spectrocopy methods. First, we will first examine the optical properties of site-controlled quantum dots which were grown using a buried stressor via resonance fluorescence. Subsequently, we will show results that were obtained using photocurrent spectroscopy to study single quantum dots that are embedded in micropillar resonators. This is followed by a study of the Wigner time delay induced by a single quantum dot and which shows that the maximum delay is larger than its dephasing time T2 which is the theoretical limit for coherently scattered fraction. The next section demonstrates that the response of a resonantly driven two-level system which in our case is represented by an exciton in a seminconductor quantum dot depends significantly on the photon statistics of the exciting light field. Most strikingly, the Mollow triplet which we observe under coherent excitation vanishes under thermal excitation. The second part presents results concerned with the resonantly driven biexciton cascade via two- photon excitation. The coherent interaction between light field and excitons gives rise to a complex five-level system. Here, we show for the first time two-photon Rabi oscillations in the temporal domain. Furthermore, we examine the correlations between the different transitions. The measurements show that we can manipulate the time-ordering of the emitted photons by changing for example the laser power. All these studies share the resonant excitation scheme which allows for a coherent manipulation of the states of the quantum dot and grants access to phenomena typical for resonance fluorescence such as the Mollow triplet and Rabi oscillations. In particular the experiments on photon excitation spectroscopy and the studies on the dressed biexciton cascade show that fundamental effects of quantum optics can be explored in a scalable and application oriented setting using quantum dots.

Jan 1, 20180 citations