Quantum communication relies on the availability of light pulses with strong quantum correlations among photons. An example of such an optical source is a single-photon pulse with a vanishing probability for detecting two or more photons. Using pulsed laser excitation of a single quantum dot, a single-photon turnstile device that generates a train of single-photon pulses was demonstrated. For a spectrally isolated quantum dot, nearly 100% of the excitation pulses lead to emission of a single photon, yielding an ideal single-photon source.

Quantum dots or rings are artificial nanometre-sized clusters that confine electrons in all three directions. They can be fabricated in a semiconductor system by embedding an island of low-bandgap material in a sea of material with a higher bandgap. Quantum dots are often referred to as artificial atoms because, when filled sequentially with electrons, the charging energies are pronounced for particular electron numbers; this is analogous to Hund's rules in atomic physics. But semiconductors also have a valence band with strong optical transitions to the conduction band. These transitions are the basis for the application of quantum dots as laser emitters, storage devices and fluorescence markers. Here we report how the optical emission (photoluminescence) of a single quantum ring changes as electrons are added one-by-one. We find that the emission energy changes abruptly whenever an electron is added to the artificial atom, and that the sizes of the jumps reveal a shell structure.

Thin TiO2 films are demonstrated to be an excellent electron-selective contact for crystalline silicon solar cells. An efficiency of 21.6% is achieved for crystalline silicon solar cells featuring a full-area TiO2 -based electron-selective contact.

Storage and retrieval of excitons were demonstrated with semiconductor self-assembled quantum dots (QDs). The optically generated excitons were dissociated and stored as separated electron-hole pairs in coupled QD pairs. A bias voltage restored the excitons, which recombined radiatively to provide a readout optical signal. The localization of the spatially separated electron-hole pair in QDs was responsible for the ultralong storage times, which were on the order of several seconds. The present limits of this optical storage medium are discussed.

We investigate the intensity correlation properties of single photons emitted from an optically excited single semiconductor quantum dot. The second order temporal coherence function of the photons emitted at various wavelengths is measured as a function of the excitation power. We show experimentally and theoretically, for the first time, that a quantum dot is not only a source of correlated non-classical monochromatic photons but is also a source of correlated non-classical multicolor photons with tunable correlation properties. We found that the emitted photon statistics can be varied by the excitation rate from a sub-Poissonian one, where the photons are temporally antibunched, to super-Poissonian, where they are temporally bunched.PACS numbers: 78.67. Hc, 42.50.Dv, 78.55.Cr, 85.30.Vw Semiconductor quantum dots have been extensively investigated recently as a potential, technology-compatible quantum light emitters.1-3 Such emitters are important for possible future quantum computing 4 and cryptography. 5It has been recently demonstrated that under continuous wave (cw) excitation, a single quantum dot emits antibunched photons obeying a sub-Poissonian statistics 6 , while under optical pulse excitation, they emit single photon per each excitation pulse.2,3,7 . Similar effects were previously observed also in optical studies of the fluorescence from single atoms and molecules. 8,9In this work we report on measurements of temporal correlations among multi-color photons emitted from cw optically excited single semiconductor quantum dots (SCQD). We show for the first time, that there is a tunable intensity correlation among photons emitted at the same and at different wavelengths due to the recombination of excitons from different collaborative quantum states. We show that the temporal correlations among the photons emitted by the SCQD change dramatically with the excitation power. While strong characteristic antibunching correlations are observed for low power excitations, these correlations disappear with the increase in the excitation power and gradually transform into bunching correlations for yet higher excitation power. These observations demonstrate that a multiply populated quantum light source may emit bunched photons, obeying super-Poisson statistics.We quantitatively account for the experimentally measured distribution of the time interval, τ , between consecutively emitted photons. Specifically, we explain the changes in the distribution under variable excitation powers, both for photons originating from the same spectral line, as well as for photons from two different spectral lines. We do that by analytically solving a set of coupled rate equations 7,10 describing the conditional probability that a photon is emitted from a collective state of j confined electron-hole (e-h) pairs (i.e. the j th multiexciton) following a photon emission event from a collective state of i > j e-h pairs.The SCQD sample was grown by molecular beam epitaxy of a strained epitaxial layer of InAs on (100) oriented GaAs subst...