Semiconductor Quantum Dot Molecules Research Articles

Tunneling induced transparency and slow light in an asymmetric double quantum dot molecule—Metal nanoparticle hybrid

We investigate the optical properties appearing in a nanostructure that is composed of an asymmetric double semiconductor quantum dot (SQD) molecule and a metal nanoparticle (MNP). The profile of the total linear absorption spectrum is proportional to the SQD contribution, while the MNP contribution is important. The profile of the doublet of resonances detected on the total linear absorption spectrum creates a transparency window. The doublet is asymmetric for small SQD-MNP distances and has a narrow peak and a wide peak. The width of the transparency window is increased, either with the enhancement of the rate at which the electron tunneling effect takes place within the double SQD molecule or with the decrease of the distance that separates the SQD molecule from the center of the MNP. The steep slope detected on the linear dispersion spectrum for frequencies laying within the transparency window owes its presence to the tunneling induced transparency and leads to slow light production. The corresponding value of the slow down factor is maximized for low values of the electron tunneling rate as well as for low center-to-center distances between the components of the hybrid nanostructure.

Study of optical bistability in a double-cavity hybrid optomechanical system consisting of an optical cavity coupled with a mechanical resonator filled with semiconductor quantum dot molecules

Study of optical bistability in a double-cavity hybrid optomechanical system consisting of an optical cavity coupled with a mechanical resonator filled with semiconductor quantum dot molecules

Controlled Coherent Coupling in a Quantum Dot Molecule Revealed by Ultrafast Four-Wave Mixing Spectroscopy.

Semiconductor quantum dot molecules are considered promising candidates for quantum technological applications due to their wide tunability of optical properties and coverage of different energy scales associated with charge and spin physics. While previous works have studied the tunnel-coupling of the different excitonic charge complexes shared by the two quantum dots by conventional optical spectroscopy, we here report on the first demonstration of a coherently controlled interdot tunnel-coupling focusing on the quantum coherence of the optically active trion transitions. We employ ultrafast four-wave mixing spectroscopy to resonantly generate a quantum coherence in one trion complex, transfer it to and probe it in another trion configuration. With the help of theoretical modeling on different levels of complexity, we give an instructive explanation of the underlying coupling mechanism and dynamical processes.

Coherent effects in energy absorption in double quantum dot molecule – Metal nanoparticle hybrids

We theoretically study the optical properties of a hybrid structure consisting of a metal nanoparticle (MNP) and an asymmetric double semiconductor quantum dot (SQD) molecule, which are coupled together, via long-range Coulomb interaction. We derive and solve numerically the relevant density-matrix equations and use electromagnetic calculations to evaluate the energy absorption rate of the MNP and the asymmetric double SQD molecule, separately, as well as, the total absorption rate for the entire hybrid structure, as a function of the energy of an applied electromagnetic field , for two different applied field polarization directions. We also investigate the impact of the applied field intensity and the electron tunnelling coupling rate on the spectral profiles, for several values of the interparticle distance. The Autler-Townes splitting and double nonlinear Fano effects are obtained in the absorption spectra of the asymmetric double SQD molecule, the MNP and the total system.

Controlling the Pump-Probe Optical Response in Asymmetric Tunneling-Controlled Double Quantum Dot Molecule—Metal Nanoparticle Hybrids

In the present work, we investigate the modified nonlinear pump-probe optical properties due to the excitonic–plasmonic interaction of a double semiconductor quantum dot (SQD) molecule coupled to a metal nanoparticle (MNP). More specifically, we study the absorption and the dispersion spectra of a weak electromagnetic field in a hybrid structure with two counterparts, a molecule of two coupled SQDs, and a spherical MNP driven by a field of high intensity. We solve the relevant density matrix equations, calculate the first-order optical susceptibility of the probe field in the strong pumping regime, and investigate the way in which the distance between the two counterparts modifies the optical response, for a variety of values of the physical constants of the system, including the pump-field detuning, the tunnelling rate, and the energy separation gap associated with the excited states of the coupled SQDs.

Nonlinear Faraday rotation in electromagnetically induced transparency medium of semiconductor three quantum dots

In the past few years, many interesting optical phenomena, such as electromagnetically induced transparency, coherent optical control of a biexciton, slow light and optical solitons, have been investigated in single quantum dot (QD). However, in an actual semiconductor device there exist many quantum dots (QDs). Recently, QD molecule, which is comprised of double semiconductor QDs coupled by tunneling coupling, has been proposed. In this new semiconductor structure, many complex but interesting phenomena have been discovered. In fact, three QD molecules may also be composed of three QDs, which can be coupled by interdot tunneling coupling. For the three semiconductor QDs molecules, the influence of the interdot tunneling coupling strength must be considered. So, in this paper, with considering that a weak, -linear-polarized probe field can form left- and right-polarized components under the control of the parallel magnetic field, and when they are combined with the tunneling coupling among the QDs, an electromagnetically induced transparency medium of a five-level M configuration semiconductor three QDs is proposed. Subsequently, the nonlinear Faraday rotation in the semiconductor three QDs is analytically studied. For the linear case, the linear dispersion relation is driven by a method of multiple scales. Then, by studying the linear optical properties, it is found that the system exhibits a single tunneling induced transparency window due to the quantum destructive interference effect driven by the interdot tunneling coupling under appropriate conditions, and the width of the tunneling induced transparency window can be effectively controlled by the strength of the interdot tunneling coupling. Meanwhile, the switch regulatory effect, which changes from the anomalous dispersion regime to the normal dispersion regime, is likely to be achieved by changing the strength of the interdot tunneling coupling. For the nonlinear case, two coupled nonlinear Schrdinger equations, which govern the evolutions of left- and right-polarized components of the weak, -linear-polarized probe field under the applied longitudinal magnetic field, are derived. By studying the nonlinear properties, it is shown that a large nonlinear Faraday rotation angle can be obtained due to the quantum interference effect which is induced by the interdot tunneling coupling with a very low absorption of the weak, -linear-polarized probe field. In addition, it is also found that the nonlinear Faraday rotation direction is opposite to line Faraday rotation for the same magnetic field. What is more, the nonlinear Faraday rotation angle grows bigger than the linear Faraday rotation. These results mean that the Faraday rotation of the three semiconductor QDs with the electromagnetically induced transparency can be more effectively controlled by the nonlinear effect.

Pump-probe optical response of an asymmetric double quantum dot molecule

We study the interaction of an asymmetric double semiconductor quantum dot molecule with a weak probe field and a strong pump field. We show that the optical properties of the system are controlled by a gate voltage and the pump field. For example, we find that the application of the pump field leads to controlled probe absorption, optical transparency, and gain for weak tunneling rates, while for stronger tunneling rates optical gain disappears and absorption spectra with double peaks are formed.

Voltage-controlled multipartite entanglement with distant quantum dot molecules via adiabatic-varying tunnel coupling

An alternative voltage-controlled scheme is proposed for the generation of an N-qubit W state with distant self-assembled semiconductor quantum dot molecules (QDMs) via adiabatic-varying tunnel coupling. The N semiconductor QDMs are trapped in N spatially separated cavities coupled with N−1 fibers. The present scheme takes full advantage of adiabatic passage and the exceptional features of semiconductor QDMs. The decoherence caused by the excited state spontaneous emission and the fiber loss was efficiently suppressed. In addition, our calculations show that the present scheme is robust against the deviation of the parameters.

Control of indirect exciton population in an asymmetric quantum dot molecule

Abstract We analyze the problem of coherent population transfer to the indirect exciton state in an asymmetric double semiconductor quantum dot molecule that interacts with an external electromagnetic field. Using the controlled rotation method, we obtain analytical solutions of the time-dependent Schrodinger equation and determine closed-form conditions for the parameters of the applied field and the quantum system that lead to complete population transfer to the indirect exciton state, in the absence of decay effects. Then, by numerical solution of the relevant density matrix equations we study the influence of decay mechanisms to the efficiency of population transfer.

Exciton-Spin Memory with a Semiconductor Quantum Dot Molecule

We report on a single photon and spin storage device based on a semiconductor quantum dot molecule. Optically excited single electron-hole pairs are trapped within the molecule, and their recombination rate is electrically controlled over 3 orders of magnitude. Single photons are stored up to 1 μs and read out on a subnanosecond time scale. By using resonant excitation, the circular polarization of individual photons is transferred into the spin state of electron-hole pairs with a fidelity above 80%, which does not degrade for storage times up to the 12.5 ns repetition period of the experiment.

Voltage-controlled entanglement and quantum-information transfer between spatially separated quantum-dot molecules

We propose two schemes for generating entanglement and quantum-state transfer (QST) between two spatially separated semiconductor quantum dot molecules (QDMs) based on voltage-controlled tunneling effects. In the present schemes, two QDMs are trapped in two spatially separated cavities connected by a fiber. By numerically simulating the evolution of the system, we show that the generation of entanglement and QST can be controlled by an external gate voltage in our schemes. Moreover, proposed schemes are robust against the noise of system parameters.

Voltage-controlled coherent population transfer in an asymmetric semiconductor quantum dot molecule

We present a scheme for the coherent population transfer of a three-level system in a quantum dot molecule by means of the interdot tunnel-coupling which can be controlled by a bias voltage. Under proper combination of the pump laser pulse and the tunnel-coupling pulse, an almost complete population transfer from the state without excitations to the indirect exciton state can be achieved. Additionally, by tuning the tunnel-coupling pulse, an arbitrary coherent superposition of the two states can also be prepared.

Computer simulation of the non-uniform and anisotropic diamagnetic shift of electronic energy levels in double quantum dot molecules

In this study we calculated the lowest energy states of electrons confined in an asymmetrical InAs/ GaAs double lens-shaped quantum dot molecule in external magnetic field. Based on the effective three-dimensional one electronic-band Hamiltonian approximation (with the energy and position dependent electronic effective mass) the electronic energy states of the system were computed by non-linear iterative method using Comsol MultiPhysics package. Our description allows us to simulate the semiconductor quantum dot molecule in arbitrary directed magnetic field. Simulation results clearly show that the diamagnetic shifts of the electronic energy levels are anisotropic and non-uniform. Therefore we demonstrate an opportunity to dynamically manipulate electronic states not only by varying the magnitude but also changing the direction of the magnetic field.

Self-Assembled Quantum Dot Molecules.

Semiconductor quantum dot molecules (QDMs) are systems composed of two or more closely spaced and interacting QDs. QDMs are receiving much attention both as playground for studying coupling and energy transfer processes between "artificial atoms" and as new systems, which substantially extend the range of possible applications of QDs. QDMs can be conveniently fabricated by self-assembly either through chemical synthesis or epitaxial growth. Although QDMs relying on the random occurrence of nearby QDs can be used for fundamental studies, special fabrication protocols must be used to create QDMs with well-defined properties. In this article, we focus on self-assembled QDMs obtained by epitaxial growth and embedded in a semiconductor matrix, which are appealing for the possible realization of quantum gates based on two-level systems defined in QDs. We provide a comprehensive overview of the development and current stage of the research on QDMs composed of vertically (in the growth direction) or laterally (in the growth plane) aligned QDs. The review highlights some recent milestone works and points out the challenges and future directions in the field.

Theoretical study of phononassisted singlet‐singlet relaxation in two‐electron semiconductor quantum dot molecules

AbstractPhonon‐assisted singlet‐singlet relaxation in semiconductor quantum dot molecules is studied theoretically. Laterally coupled quantum dot structures doped with two electrons are considered. We take into account interaction with acoustic phonon modes via deformation potential and piezoelectric coupling. We show that piezoelectric mechanism for the considered system is of great importance and for some ranges of quantum dot molecule parameters is the dominant contribution to relaxation.It is shown that the phonon‐assisted tunneling is much faster (down to ∼ 6 ps even at zero temperature) in comparison with other decoherence processes. The influence of Coulomb interaction is discussed and its consequences are indicated. We calculate the relaxation rates for GaAs quantum dot molecules and study the dependence on quantum dot size, distance and offset between the constituent quantum dots. In addition the temperature dependence of the tunneling rates is analyzed. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Phonon-assisted tunneling between singlet states in two-electron quantum dot molecules

We study phonon-assisted electron tunneling in semiconductor quantum dot molecules. In particular, singletsinglet relaxation in a two-electron-doped structure is considered. The influence of Coulomb interaction is discussed via comparison with a single-electron system. We find that the relaxation rate reaches similar values in the two cases but the Coulomb interaction shifts the maximum rates toward larger separations between the dots. The difference in electron-phonon interaction between deformation potential and piezoelectric coupling is investigated. We show that the phonon-induced tunneling between two-electron singlet states is a fast process, taking place on the time scales of the order of a few tens of picoseconds.

Fine-structure splitting of exciton states in semiconductor quantum dot molecules

Exciton levels and fine-structure splitting in laterally coupled quantum dot molecules arestudied. The electron and hole tunnelling energies as well as the direct Coulomb interactionare essential for the exciton levels. It is found that fine-structure splittings of the two lowestexciton levels are contributed by the intra- and interdot exchange interactions which aregreatly influenced by the symmetry and tunnel-coupling between the two dots. As theinterdot separation is reduced, fine-structure splitting of the exciton ground state is largelyincreased while those of the second and fourth states are decreased. Moreover, thedependence of the fine-structure splitting in quantum dot molecules on the Coulombcorrelation is clearly clarified.

One-step preparation of cluster states in quantum-dot molecules

Cluster states, a special type of highly entangled states, are a universal resource for measurement-based quantum computation. Here, we propose an efficient one-step generation scheme for cluster states in semiconductor quantum dot molecules, where qubits are encoded on singlet and triplet states of two coupled quantum dots. By applying a collective electrical field or simultaneously adjusting interdot bias voltages of all double-dot molecules, we get a switchable Ising-like interaction between any two adjacent quantum molecule qubits. The initialization, the single-qubit measurement, and the experimental parameters are discussed. It is shown that preparation of large cluster states and one-way quantum computation are implementable in semiconductor quantum dots with the present techniques.

Optimal quantum gates for semiconductor qubits

We employ optimal control theory to design optimized quantum gates for solid-state qubits subject to decoherence. At the example of a gate-controlled semiconductor quantum dot molecule we demonstrate that decoherence due to phonon couplings can be strongly suppressed. Our results suggest a much broader class of quantum control strategies in solids.

A model for semiconductor quantum dot molecule based on the current spin density functional theory

Based on the current spin density functional theory, a theoretical model of three vertically aligned semiconductor quantum dots is proposed and numerically studied. This quantum dot molecule (QDM) model is treated with realistic hard-wall confinement potential and external magnetic field in three-dimensional setting. Using the effective-mass approximation with band nonparabolicity, the many-body Hamiltonian results in a cubic eigenvalue problem from a finite difference discretization. A self-consistent algorithm for solving the Schrödinger–Poisson system by using the Jacobi–Davidson method and GMRES is given to illustrate the Kohn–Sham orbitals and energies of six electrons in the molecule with some magnetic fields. It is shown that the six electrons residing in the central dot at zero magnetic field can be changed to such that each dot contains two electrons with some feasible magnetic field. The Förster–Dexter resonant energy transfer may therefore be generated by two individual QDMs. This may motivate a new paradigm of Fermionic qubits for quantum computing in solid-state systems.