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Journal of Applied Physics

Double hydrogenic impurities in double quantum dots

Xuefeng Wang, Yonghui Liu

Published Sep 15, 2007• 3 citations
Publisher

Abstract

The ground states and degree of entanglement of double hydrogenic impurities in a pair of vertically stacked InGaAs/GaAs quantum dots are studied with a proposed diagonalization technique. It is found that at short barrier widths, the entanglement is small due to the coupling between the intra- and interdot orbitals. At large barrier widths, large entanglement occurs.

DOI
10.1063/1.2781323

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The ground states and degree of entanglement of double hydrogenic impurities in a pair of vertically stacked InGaAs/GaAs quantum dots are studied with a proposed diagonalization technique. It is found that at short barrier widths, the entanglement is small due to the coupling between the intra- and interdot orbitals. At large barrier widths, large entanglement occurs.
Exciton states and optical spectra in a pair of vertically stacked InGaAs∕GaAs\mathrm{InGaAs}∕\mathrm{GaAs}InGaAs∕GaAs quantum dots are studied with a proposed variation-diagonalization technique. The spectra obtained are in good agreement with experiments. The tunnel coupling and the exciton entanglement are also investigated. It is found that at short barrier widths, the entanglement is small due to the coupling between the intradot and interdot orbitals. At large barrier widths, large entanglement appears in coupled identical quantum dots; however, such an entanglement can be easily destroyed by the broken symmetry because of the weak tunneling of the hole.
We theoretically consider the effects of having unintentional charged impurities in laterally coupled two-dimensional double (GaAs) quantum-dot systems, where each dot contains one or two electrons and a single charged impurity. Using molecular orbital and configuration interaction methods, we calculate the effect of the impurity on the two-electron energy spectrum of each individual dot as well as on the spectrum of the coupled-double-dot two-electron system. We find that the singlet-triplet exchange splitting between the two lowest-energy states, both for the individual dots and the coupled-dot system, depends sensitively on the location of the impurity and its coupling strength (i.e. the effective charge). A strong electron-impurity coupling breaks down the equality of the two doubly occupied singlets in the left and the right dots, leading to a mixing between different spin singlets. As a result, the maximally entangled qubit states are no longer fully obtained in the zero-magnetic-field case. Moreover, a repulsive impurity results in a triplet-singlet transition as the impurity effective charge increases or the impurity position changes. We comment on the impurity effect in spin qubit operations in the double-dot system based on our numerical results.
Incompressible (magic) states that result from many-body effects in vertically coupled quantum dots submitted to strong magnetic fields such that only the lowest Landau level is relevant are studied within an exact diagonalization calculation for N=3,N=3,N=3, 5, and 6, electrons. We find that the sequences of total angular momentum M for which these incompressible states exist depend on the interplay between the interdot hopping parameter ${\ensuremath{\Delta}}_{t}$ and the interdot distance d. For d of the order of the magnetic length and for all values of ${\ensuremath{\Delta}}_{t},$ we conclude that, in contrast to previous claims, these incompressible states appear at magic values of M which do not differ from those obtained for a single dot: namely, $M=N(N\ensuremath{-}1)/2+jN,$ where j is a positive integer number. For large interdot distance and simultaneously small interdot hopping parameter, new sequences of magic values of M are observed. These new sequences can be easily understood in terms of a transition regime towards a system of two decoupled single dots. However, important differences in the nature of the incompressible ground states are found with respect to those of a single dot.
PACS. 73.23.-b – Electronic transport in mesoscopic systems. Abstract. – We study entanglement of charge qubits in a vertical tunnel-coupled double quantum dot containing two interacting electrons. Exact diagonalization is used to compute the negativity characterizing entanglement. We find that entanglement can be efficiently generated and controlled by sidegate voltages, and describe how it can be detected. For large enough tunnel coupling, the negativity shows a pronounced maximum at an intermediate interaction strength within the Wigner molecule regime. Semiconductor few-electron quantum dots (QDs) continue to attract a lot of interest, as it has now become possible to experimentally control both the electronic spin and charge in a condensed-phase environment in an unprecedented manner. In particular, double quantum dots (DQDs) can be fabricated in a well-controlled fashion in high-quality semiconductor devices, and are currently under intense study [1–9]. Vertical or lateral tunnel-coupled DQDs are among the most promising candidates for realizing spin or charge qubits in a quantum information processor [10–12]. Their main advantages are scalability, good control of physical properties via tunable external (sidegate-) voltages or magnetic fields, and spatial separation
Quantum dots comprise a type of quantum impurity system. The entanglement and coherence of quantum states are significantly influenced by the strong electron-electron interactions among impurities and their dissipative coupling with the surrounding environment. Competition between many-body effects and transfer couplings plays an important role in determining the entanglement among localized impurity spins. In this work, we employ the hierarchical-equations-of-motion approach to explore the entanglement of a strongly correlated double quantum dots system. The relation between the total system entropy and those of subsystems is also investigated.