9 research outputs found
Radiative cascades in charged quantum dots
We measured, for the first time, two photon radiative cascades due to
sequential recombination of quantum dot confined electron hole pairs in the
presence of an additional spectator charge carrier. We identified direct, all
optical cascades involving spin blockaded intermediate states, and indirect
cascades, in which non radiative relaxation precedes the second recombination.
Our measurements provide also spin dephasing rates of confined carriers.Comment: 4 pages, 3 figure
Radiative cascade from quantum dot metastable spin-blockaded biexciton
We detect a novel radiative cascade from a neutral semiconductor quantum dot.
The cascade initiates from a metastable biexciton state in which the holes form
a spin-triplet configuration, Pauli-blockaded from relaxation to the
spin-singlet ground state. The triplet biexciton has two photon-phonon-photon
decay paths. Unlike in the singlet-ground state biexciton radiative cascade, in
which the two photons are co-linearly polarized, in the triplet biexciton
cascade they are crosslinearly polarized. We measured the two-photon
polarization density matrix and show that the phonon emitted when the
intermediate exciton relaxes from excited to ground state, preserves the
exciton's spin. The phonon, thus, does not carry with it any which-path
information other than its energy. Nevertheless, entanglement distillation by
spectral filtering was found to be rather ineffective for this cascade. This
deficiency results from the opposite sign of the anisotropic electron-hole
exchange interaction in the excited exciton relative to that in the ground
exciton.Comment: 6 pages, 4 figure
Accessing the dark exciton with light
The fundamental optical excitation in semiconductors is an electron–hole pair with antiparallel spins: the ‘bright’ exciton. Bright excitons in optically active, direct-bandgap semiconductors and their nanostructures have been thoroughly studied. In quantum dots, bright excitons provide an essential interface between light and the spins of interacting confined charge carriers. Recently, complete control of the spin state of single electrons and holes in these nanostructures has been demonstrated, a necessary step towards quantum information processing with these two-level systems. In principle, the bright exciton’s spin could also be used directly as a two-level system. However, because of its short radiative lifetime, its usefulness is limited. An electron–hole pair with parallel spins forms a long-lived, optically inactive ‘dark exciton’, and has received less attention as it is mostly regarded as an inaccessible excitation. In this work we demonstrate that the dark exciton forms a coherent two-level system that can fairly easily be accessed by external light. We demonstrate: optical preparation of its spin state as a coherent superposition of two eigenstates, coherent precession of its spin state at a frequency defined by the energy difference between its eigenstates, and readout of the spin by charge addition and subsequent polarized photon detection