Engineering coherent photons from semiconductor quantum dots

Self-assembled semiconductor quantum dots (QDs) have great promise as quantum light sources due to their ability to generate single indistinguishable photons and entanglement. In this thesis, confocal microscopy experiments have been carried using non-resonant photoluminescence (PL) and resonant uorescence (RF) on QDs with the goal of characterising and developing them into high-quality quantum light sources. Through the application of uniaxial strain and an electric eld, single particle energies in a QD and their behaviour with strain are determined using a perturbative Coulomb blockade model. The exciton energy tuning magnitude is found to be a result of the near-cancellation of much larger single electron and hole tuning tuning. In addition, the rate of electron con nement energy tuning with strain is found to be correlated with the nominal unstrained con nement energy. An attempt is made at characterising the composition of the QDs through extracting deformation potentials, but the simple model does not capture the full system. Further, strain tuning of the ne structure splitting (FSS) of the neutral exciton X0 from QDs emitting at telecommunications wavelengths is shown. FSS tuning as large as 46 eV was observed, and using a phenomenological model select QDs were identi ed to achieve FSS < 1 ueV. RF is used to examine noise sources in QDs. Two sources of noise are considered: electric charge noise due to a uctuating charge environment, and nuclear spin noise due to the hyper ne interaction of single electron spins with a large number ( ~105) of nuclear spins. While the charge noise contributes to a loss in overall photon emission rates, but does not negatively impact the photon antibunching or indistinguishability at low Rabi frequencies, spin noise allows inelastic Raman scattering which reduces photon indistinguishability. The application of an external magnetic eld in the Faraday geometry screens the electrons from the nuclear spins, recovering a high degree of photon indistinguishability

Frequency-encoded linear cluster states with coherent Raman photons

Entangled multi-qubit states are an essential resource for quantum information and computation. Solid-state emitters can mediate interactions between subsequently emitted photons via their spin, thus offering a route towards generating entangled multi-photon states. However, existing schemes typically rely on the incoherent emission of single photons and suffer from severe practical limitations, for self-assembled quantum dots most notably the limited spin coherence time due to Overhauser magnetic field fluctuations. We here propose an alternative approach of employing spin-flip Raman scattering events of self-assembled quantum dots in Voigt geometry. We argue that weakly driven hole spins constitute a promising platform for the practical generation of frequency-entangled photonic cluster states

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 (~ -0.5 meV/MPa) leads to variable tuning of exciton energies (+18 to -0.9 micro-eV/MPa) under tensile stress. Modest tuning of the permanent dipole, Coulomb interaction and fine-structure splitting energies is also measured. We exploit the variable exciton response to tune multiple quantum dots on the same chip into resonance.Comment: 16 pages, 4 figures, 1 table. Final versio

Exciton fine-structure splitting of telecom-wavelength single quantum dots : Statistics and external strain tuning

In a charge tunable device, we investigate the fine structure splitting of neutral excitons in single long-wavelength (1.1\mu m < \lambda < 1.3 \mu m) InGaAs quantum dots as a function of external uniaxial strain. Nominal fine structure splittings between 16 and 136 \mu eV are measured and manipulated. We observe varied response of the splitting to the external strain, including positive and negative tuning slopes, different tuning ranges, and linear and parabolic dependencies, indicating that these physical parameters depend strongly on the unique microscopic structure of the individual quantum dot. To better understand the experimental results, we apply a phenomenological model describing the exciton polarization and fine-structure splitting under uniaxial strain. The model predicts that, with an increased experimental strain tuning range, the fine-structure can be effectively canceled for select telecom wavelength dots using uniaxial strain. These results are promising for the generation of on-demand entangled photon pairs at telecom wavelengths.Comment: 15 pages, 3 figure

Untersuchungen zur optimalen Quantisierung und zur Stoeranfaelligkeit der zweidimensionalen Hadamard-Koeffizienten bei der Echtzeittransformation von Fernsehbildsignalen Abschlussbericht

TIB: AC 5912. / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Supplement 1: Indistinguishable single photons with flexible electronic triggering

Supplemental-document Originally published in Optica on 20 May 2016 (optica-3-5-493