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AIP conference proceedings

Spin mixing of two electrons in a quantum dot

Ştefan C. Bǎdescu, T. L. Reinecke

Published Jan 1, 2007• 0 citations
Publisher

Abstract

The optical initialization and readout of an electron spin in a quantum dot involves often the lowest excited states of two electrons in a negative trion. The dynamics involves mixing of electron triplets and excited singlets. Typically, the mixing mechanism invoked has been electron‐hole exchange in the presence of dot lateral asymmetries, but this has not been always consistent with the observed dephasing rates. We describe here a more general and stronger singlet‐triplet mixing mechanism that does not require dot asymmetry.

DOI
10.1063/1.2730274

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The optical initialization and readout of an electron spin in a quantum dot involves often the lowest excited states of two electrons in a negative trion. The dynamics involves mixing of electron triplets and excited singlets. Typically, the mixing mechanism invoked has been electron‐hole exchange in the presence of dot lateral asymmetries, but this has not been always consistent with the observed dephasing rates. We describe here a more general and stronger singlet‐triplet mixing mechanism that does not require dot asymmetry.
Two electron spins form the spin singlet or triplet state if these spins have the same g -factor value, or a mix state consisting of both the singlet and triplet states if these spins have different g -factor values. It is considered that two energy levels of electrons are formed in a double quantum dot device for use in quantum information processing; however, the g -factors of the electrons have not been treated as an important factor because they cannot be changed easily in experiments. We introduce a numerical calculation method for the singlet–triplet mix state due to the g -factor mismatch in double quantum dots. It is found that this mismatch changes the energy levels of the original singlet and triplet states; thus, it affects the intersecting points of the energy levels of the singlet and triplet states where a dynamic nuclear polarization appears via hyperfine interaction.
Two electron spins form the spin singlet or triplet state if these spins have the same g-factor value, or a mix state consisting of both the singlet and triplet states if these spins have different g-factor values. It is considered that two energy levels of electrons are formed in a double quantum dot device for use in quantum information processing; however, the g-factors of the electrons have not been treated as an important factor because they cannot be changed easily in experiments. We introduce a numerical calculation method for the singlet–triplet mix state due to the g-factor mismatch in double quantum dots. It is found that this mismatch changes the energy levels of the original singlet and triplet states; thus, it affects the intersecting points of the energy levels of the singlet and triplet states where a dynamic nuclear polarization appears via hyperfine interaction.
We show that spin-orbit coupling in a quantum dot molecule allows for coherent manipulation of two-electron spin states using Raman transitions. Such two-electron spin states defined by the singlet and triplet states of two exchange-coupled quantum dots can have favorable coherence properties. In addition, two of the four metastable ground states in this system can be used as auxiliary states that could facilitate implementation of tasks such as mapping of spin states to that of a single propagating photon. We find that even weak spin-orbit effects---manifesting themselves as slightly different ggg factors for the electron and the hole---would allow for the coherent Raman coupling of the singlet-triplet states. We also discuss the possibilities for implementing quantum optical techniques for spin preparation and manipulation.