Asymmetric Coupled Quantum Dots Research Articles

Phonon-induced decoherence of a spin based qubit made with asymmetric coupled quantum dots

A semiconductor heterostructure made with two asymmetric coupled self-assembled quantum dots (ACSAQDs) has been studied. Such a structure can serve as a spin based quantum dot qubit which is an important manufacture for quantum communication and quantum information processing . The shape of the confinement potential, the used semiconductor materials and the size of the investigated quantum dots have been chosen to be comparable to the fabricated and the experimentally studied quantum dots. The spin-flip decoherence and more specifically the effects associated with acoustic phonons have been researched. A rather sophisticated theoretical model (8-band strain dependent k ⋅ p theory) has been employed to evaluate the carrier eigenfunctions and eigenvalues within the quantum dots. Using the Fermis golden rule, the spin-flip relaxation time ( T 1 ) due to phonon coupling to electrons has been evaluated. The volume of the quantum dots is a parameter of crucial importance for controlling the spin-flip relaxation mechanism. Moreover, other parameters like the space between the coupled quantum dots, the lattice temperature and the presence of an external applied magnetic field (along the quantum dots growth direction) have considerable effects on the spin-flip relaxation time. • In this paper, a heterostructure made of two asymmetric coupledquantum dots and which can be implemented as quantum logic gate has been studied. • The qubit is manipulated by controlling the two-level electron spin systems. • The electron-spin eigenfunctions and eigenvalues have been calculated by using the 8-band strain dependent k . p theory. • The dependence of the electron energy on the quantum structure geometry and the existence of an applied magnetic field was highlighted. • The evaluation of spin-flip time T 1 via the emission of acoustic phonons showed that T 1 strongly depends on the size and on the relative volume of the dots under certain circumstances (geometry and small magnetic field).

Quantum repeaters using orbitals in quantum dot molecules

We propose quantum repeaters using quantum dot molecules, in which matter-photon entanglement is generated by Raman scatterings in lambda systems composed of various coherent exciton levels formed in the ensembles of asymmetric coupled quantum dots. In our scheme, the wavelength of Stokes and anti-Stokes photons can be chosen to fulfill the requirements of optical fiber communication. Further, the relative superposition phase in the entangled states can be stabilized by the active feedback to the gate voltage in quantum dot system. These characteristics are favorable for implementing our scheme in practice.

电场调控下不对称耦合量子点中激子的发光特性

We theoretically analyzed the bonding and anti-bonding energy states in the GaAs/Al0.35Ga0.65As asymmetric coupled quantum dots under a series of electric fields.The carrier tunneling effect strongly influences the exciton energy and oscillator strength.We find that energy splitting of anti-crossing occurs under an critical electric field for hole bounding and anti-bonding states,and both the energy gap and the critical field decrease with increasing the barrier distance.Meanwhile,the exciton luminescence intensity changes from bright(dark) to dark(bright) under external fields.

Donor impurity states in zinc-blende InGaN/GaN asymmetric coupled quantum dots: Hydrostatic pressure effect

Abstract Based on the effective-mass approximation, the ground-state donor binding energy in a cylindrical zinc-blende (ZB) InGaN/GaN asymmetric coupled quantum dots (QDs) is investigated variationally, considering the hydrostatic pressure effect. Numerical results show that the donor binding energy increases on increasing the hydrostatic pressure for any impurity position. The hydrostatic pressure has an obvious influence on the impurity localized inside the wide dot of the asymmetric coupled QDs. In the case of any hydrostatic pressure, our results show that the donor binding energy is distributed asymmetrically with respect to the center of the asymmetric coupled QDs. When the impurity is localized inside the middle barrier layer, the donor binding energy has a maximum value with varying the dot height of the asymmetric coupled QDs. Moreover, for the impurity localized inside the narrow dot, the donor binding energy is decreased monotonically on increasing the middle barrier width. In particular, for the impurity located inside the wide dot, the donor binding energy is insensitive to the middle barrier width if the middle barrier width is large in the ZB InGaN/GaN asymmetric coupled QDs.

Shallow-donor impurity in zinc-blende InGaN/GaN asymmetric coupled quantum dots: Effect of electric field

We have performed a theoretical calculation of the shallow-donor impurity states in cylindrical zinc-blende (ZB) InGaN/GaN asymmetric coupled quantum dots (QDs), considering the effect of the electric field applied to the left (opposite to the growth direction). Numerical results show that the donor binding energy in ZB InGaN/GaN asymmetric coupled QDs is highly dependent on the impurity positions, asymmetric coupled QD structure parameters, and the electric field. In the presence of the electric field, if the left dot height is increased from zero, the donor binding energy of the impurity localized inside the middle barrier layer has a maximum value; if the right dot is increased from zero, the donor binding energy of impurity localized inside the right dot has a maximum value. It is also found that for the impurity located at the center of the right dot, the donor binding energy is insensible to the electric field when the electric field is large; however, the critical electric field is smaller if the right dot is wide. In particular, numerical results show that if the right dot is wide, the asymmetric coupled QDs are much easier to be decoupled under the influence of the electric field.

Hydrogenic impurity states in zinc-blende InGaN/GaN asymmetric coupled quantum dots

Based on the effective-mass approximation, the ground-state donor binding energy in cylindrical zinc-blende (ZB) InGaN/GaN asymmetric coupled quantum dots (QDs) is investigated variationally. Numerical results show that the ground-state donor binding energy is highly dependent on the impurity positions and asymmetric coupled QDs structure parameters. The donor binding energy is distributed asymmetrically with respect to the center of the asymmetric coupled QDs. The position of the maximum value is located inside the wide QD. When the impurity is localized inside the middle barrier layer, the donor binding energy has a maximum value with the variation of any dot height in ZB InGaN/GaN asymmetric coupled QDs. In particular, for the impurity localized inside the wide dot, the donor binding energy is insensitive to large middle barrier width ( L m b ≥ 3 nm ) in the ZB In 0.1Ga 0.9 N/GaN asymmetric coupled QDs.

Carrier tunneling in asymmetric coupled quantum dots

Exciton spin relaxation at low temperatures in InAlAs–InGaAs asymmetric double quantum dots embedded in AlGaAs layers has been investigated as a function of the barrier thickness by the time-resolved photoluminescence measurements. With decreasing the thickness of the AlGaAs layer between the dots, the spin relaxation time change from 3 ns to less than 500 ps . The reduction in the spin relaxation time was considered to originate from the spin-flip tunneling between the ground state in InAlAs dot and the excited states in InGaAs dot, and the resultant tunneling leads to the spin depolarization of the ground state in InGaAs dot.

Dynamical control of quantum tunneling due to ac Stark shift in an asymmetric coupled quantum dot

Dynamical suppression and enhancement of the quantum tunneling in an asymmetric coupled-quantum-dot structure is predicted to occur when a laser field drives a subband transition in one of the dots. The laser field induces the splitting and shift of the quasiresonant energy levels, i.e., ac Stark (or light) shift. As a result, the tunneling between the energy levels perturbed by the laser field and another level in the neighboring dot is strongly modified as the amplitude of the laser field is increased. Furthermore, it is shown that the amount of the ac Stark shift has an oscillatory behavior for large amplitudes of the laser field, which is not predicted from the conventional rotating-wave approximation for the matter-light interaction.