Screening Nuclear Field Fluctuations in Quantum Dots for Indistinguishable Photon Generation.

A semiconductor quantum dot can generate highly coherent and indistinguishable single photons. However, intrinsic semiconductor dephasing mechanisms can reduce the visibility of two-photon interference. For an electron in a quantum dot, a fundamental dephasing process is the hyperfine interaction with the nuclear spin bath.

High resolution coherent population trapping on a single hole spin in a semiconductor quantum dot.

We report high resolution coherent population trapping on a single hole spin in a semiconductor quantum dot. The absorption dip signifying the formation of a dark state exhibits an atomic physicslike dip width of just 10 MHz. We observe fluctuations in the absolute frequency of the absorption dip, evidence of very slow spin dephasing.

Fine tuning of micropillar cavity modes through repetitive oxidations.

Repetitive wet thermal oxidations of a tapered oxide aperture in a micropillar structure are demonstrated. After each oxidation step the confined optical modes are analyzed at room temperature. Three regimes are identified.

Optical modes in oxide-apertured micropillar cavities.

We present a detailed experimental characterization of the spectral and spatial structure of the confined optical modes for oxide-apertured micropillar cavities, showing good-quality Hermite-Gaussian profiles, easily mode-matched to external fields. We further derive a relation between the frequency splitting of the transverse modes and the expected Purcell factor. Finally, we describe a technique to retrieve the profile of the confining refractive index distribution from the spatial profiles of the modes.

Surface acoustic wave mediated carrier injection into individual quantum post nano emitters.

Acousto-electric charge conveyance induced by a surface acoustic wave (SAW) is employed to dissociate photogenerated excitons. Over macroscopic distances, both electrons and holes are injected sequentially into a remotely positioned, isolated and high quality quantum emitter, a self-assembled quantum post. This process is found to be highly efficient and to exhibit improved stability at high acoustic powers when compared to direct optical pumping at the position of the quantum post.

Electro-elastic tuning of single particles in individual self-assembled quantum dots.

We investigate the effect of uniaxial stress on InGaAs quantum dots in a charge tunable device. Using Coulomb blockade and photoluminescence, we observe that significant tuning of single particle energies (≈-0.22 meV/MPa) leads to variable tuning of exciton energies (+18 to -0.

Probing single-charge fluctuations at a GaAs/AlAs interface using laser spectroscopy on a nearby InGaAs quantum dot.

We probe local charge fluctuations in a semiconductor via laser spectroscopy on a nearby self-assembled quantum dot. We demonstrate that the quantum dot is sensitive to changes in the local environment at the single-charge level. By controlling the charge state of localized defects, we are able to infer the distance of the defects from the quantum dot with ±5  nm resolution.

Semiconductor self-assembled quantum dots: present status and future trends.

Progress in controlling the size, shape, and composition of quantum dots (QDs) as well as their positioning will be crucial to further advances in the fields of quantum information and device applications. The growth of QDs into lattices using controlled positioning of the QD nucleation centers is a possible method. QD positioning is also much needed for further development of QD microcavities and photonic-crystal based devices that are used for quantum information applications.

Terahertz ionization of highly charged quantum posts in a perforated electron gas.

"Quantum posts" are roughly cylindrical semiconductor nanostructures that are embedded in an energetically shallower "matrix" quantum well of comparable thickness. We report measurements of voltage-controlled charging and terahertz absorption of 30 nm thick InGaAs quantum wells and posts. Under flat-band (zero-electric field) conditions, the quantum posts each contain approximately six electrons, and an additional ~2.

Controlling the interaction of electron and nuclear spins in a tunnel-coupled quantum dot.

We present a technique for manipulating the nuclear spins and the emission polarization from a single optically active quantum dot. When the quantum dot is tunnel coupled to a Fermi sea, we have discovered a natural cycle in which an electron spin is repeatedly created with resonant optical excitation. The spontaneous emission polarization and the nuclear spin polarization exhibit a bistability.

Coherent optical writing and reading of the exciton spin state in single quantum dots.

We demonstrate a one-to-one correspondence between the polarization state of a light pulse tuned to neutral exciton resonances of single semiconductor quantum dots and the spin state of the exciton that it photogenerates. This is accomplished using two variably polarized and independently tuned picosecond laser pulses. The first "writes" the spin state of the resonantly excited exciton.

Enhanced sequential carrier capture into individual quantum dots and quantum posts controlled by surface acoustic waves.

Individual self-assembled quantum dots and quantum posts are studied under the influence of a surface acoustic wave. In optical experiments we observe an acoustically induced switching of the occupancy of the nanostructures along with an overall increase of the emission intensity. For quantum posts, switching occurs continuously from predominantly charged excitons (dissimilar number of electrons and holes) to neutral excitons (same number of electrons and holes) and is independent of whether the surface acoustic wave amplitude is increased or decreased.

A coherent single-hole spin in a semiconductor.

Semiconductors have uniquely attractive properties for electronics and photonics. However, it has been difficult to find a highly coherent quantum state in a semiconductor for applications in quantum sensing and quantum information processing. We report coherent population trapping, an optical quantum interference effect, on a single hole.

Externally mode-matched cavity quantum electrodynamics with charge-tunable quantum dots.

We present coherent reflection spectroscopy on a charge and dc Stark tunable quantum dot embedded in a high-quality and externally mode-matched microcavity. The addition of an exciton to a single-electron-charged quantum dot forms a trion that interacts with the microcavity just below the strong-coupling regime of cavity quantum electrodynamics. Such an integrated, monolithic system is a crucial step towards the implementation of scalable hybrid quantum-information schemes that are based on an efficient interaction between a single photon and a confined electron spin.

Comparative magneto-photoluminescence study of ensembles and of individual InAs quantum dots.

We report on magneto-photoluminescence studies of InAs/GaAs quantum dots (QDs) of considerably different densities, from dense ensembles down to individual dots. It is found that a magnetic field applied in Faraday geometry decreases the photoluminescence (PL) intensity of QD ensembles, which is not accompanied by the corresponding increase of PL signal of the wetting layer on which QDs are grown. The model suggested to explain these data assumes considerably different strengths of suppression of electron and hole fluxes by a magnetic field.

Optically induced hybridization of a quantum dot state with a filled continuum.

We present an optical signature of a hybridization between a localized quantum dot state and a filled continuum. Radiative recombination of the negatively charged trion in a single quantum dot leaves behind a single electron. We show that in two regions of vertical electric field, the electron hybridizes with a continuum through a tunneling interaction.

Optical detection of single-electron spin resonance in a quantum dot.

We demonstrate optically detected spin resonance of a single electron confined to a self-assembled quantum dot. The dot is rendered dark by resonant optical pumping of the spin with a laser. Contrast is restored by applying a radio frequency (rf) magnetic field at the spin resonance.

A semiconductor exciton memory cell based on a single quantum nanostructure.

We demonstrate storage of excitons in a single nanostructure, a self-assembled quantum post. After generation, electrons and holes forming the excitons are separated by an electric field toward opposite ends of the quantum post inhibiting their radiative recombination. After a defined time, the spatially indirect excitons are reconverted to optically active direct excitons by switching the electric field.

Optical pumping of a single hole spin in a quantum dot.

The spin of an electron is a natural two-level system for realizing a quantum bit in the solid state. For an electron trapped in a semiconductor quantum dot, strong quantum confinement highly suppresses the detrimental effect of phonon-related spin relaxation. However, this advantage is offset by the hyperfine interaction between the electron spin and the 10(4) to 10(6) spins of the host nuclei in the quantum dot.

The nonlinear Fano effect.

The Fano effect is ubiquitous in the spectroscopy of, for instance, atoms, bulk solids and semiconductor heterostructures. It arises when quantum interference takes place between two competing optical pathways, one connecting the energy ground state and an excited discrete state, the other connecting the ground state with a continuum of energy states. The nature of the interference changes rapidly as a function of energy, giving rise to characteristically asymmetric lineshapes.

Fine structure of negatively and positively charged excitons in semiconductor quantum dots: electron-hole asymmetry.

We present new understanding of excitonic fine structure in close-to-symmetric InAs/GaAs and InGaAs/GaAs quantum dots. We demonstrate excellent agreement between spectroscopy and many-body pseudopotential theory in the energy splittings, selection rules and polarizations of the optical emissions from doubly charged excitons. We discover a marked difference between the fine structure of the doubly negatively and doubly positively charged excitons.

Growth, structural, and optical properties of self-assembled (In,Ga)as quantum posts on GaAs.

Self-assembled quantum dots embedded in semiconductor heterostructures have proved to be a rich system for exploring the physics of three dimensionally confined charges and excitons. We present here a novel structure, which allows adjusting the level of confinement between 3D and 2D for electrons and holes, respectively. The quantum post consists of a quantum dot connected to a short quantum wire.

Enhancement of the luminescence intensity of InAs/GaAs quantum dots induced by an external electric field.

InAs/GaAs quantum dots have been subjected to a lateral external electric field in low-temperature microphotoluminescence measurements. It is demonstrated that the dot PL signal could be increased several times depending on the magnitude of the external field and the strength of the internal (built-in) electric field, which could be altered by an additional infrared illumination of the sample. The observed effects are explained by a model that accounts for the essentially faster lateral transport of the photoexcited carriers achieved in an electric field.

Photonic crystal-assisted light extraction from a colloidal quantum dot/GaN hybrid structure.

We present a study of the light extraction from CdSe/ZnS core/shell colloidal quantum dot thin films deposited on quantum well InGaN/GaN photonic crystal structures. The two-dimensional photonic crystal defined by nanoimprint lithography is used to efficiently extract the guided light modes originating from both the quantum dot thin films and the InGaN quantum wells. Far-field photoluminescence spectra are used to measure the extraction enhancement factor of the quantum dot emission (x1.

Entangled photon pairs from semiconductor quantum dots.

Tomographic analysis demonstrates that the polarization state of pairs of photons emitted from a biexciton decay cascade becomes entangled when spectral filtering is applied. The measured density matrix of the photon pair satisfies the Peres criterion for entanglement by more than 3 standard deviations of the experimental uncertainty and violates Bell's inequality. We show that the spectral filtering erases the "which path" information contained in the photons' color and that the remanent information in the quantum dot degrees of freedom is negligible.