Exploiting phonon and coulomb interactions in semiconductor quantum dots

Semiconductor quantum dots have been investigated in many different aspects, from fundamental semiconductor physics to advanced quantum technologies. After many years of growth improvements, the quantum dot emits single photons of high purity and high indistinguishability. The noise in the semiconductor is reduced to a level where it starts to be negligible (compared to other measurement errors). Recently advances also have been made in controlling a single electron(hole)-spin confined by the quantum dot. This makes the quantum dot a perfect candidate for any application involving single photons and also a spin, for example, boson sampling and cluster state generation, respectively.
The quantum dot can also be used to study its coupling to additional degrees of freedom, such as for example the surrounding nuclear spins or coupling to an optical cavity. In this thesis, two individual studies are presented. First, the interaction of the quantum dot with the mechanical surrounding, i.e. phonons, and second, radiative Auger processes due to Coulomb interactions.
The quantum dot naturally couples to its mechanical environment by deformation-potential coupling. The interaction is exploited by engineering the mechanical environment (density of states) by patterning a mechanical resonator (mechanical cavity). The main focus lies on reaching gigahertz mechanical frequencies, the so-called resolved-sideband regime. However, this is not trivial for two reasons. First, fabricating a mechanical resonator at such high frequencies is challenging due the small size. Second, measuring such a fast modulation of the quantum dot requires a special measurement parameter set. Nonetheless, the coupling to mechanical resonators from a few megahertz to more than a gigahertz mechanical frequency is shown. Furthermore, an in-depth study of the exciton-phonon interaction is presented which includes a semi-classical master-equation description of the coupled system as well as the observation of acoustic sideband emission. The current limit, for applications such as optomechanical cooling, is the coupling rate between the two systems. However, such experiments are within reach with a five- to ten-fold increase in the coupling rate.
The quantum dot itself presents a coupled system if charged with an additional electron (or hole). Then, in the excited state, three carriers (one hole and two electrons) are tightly confined inside the dot which are coupled to each other via Coulomb interactions. The effect of this coupling is studied which gives rise to the so-called radiative Auger process. During the decay of the trion, one of the carriers (the Auger electron) is promoted to a higher energy state inside the quantum dot (p- and d-shell) and the emitted photon is correspondingly red-shifted. In more detail, it is found that the wavefunction of the trion is composed of admixtures of higher shells and the emitted photon projects the state of the remaining electron to the corresponding shell. Furthermore, the radiative Auger process gives rise to an optical transition which can be addressed with a laser. In a two-laser experiment (Λ-configuration), the radiative Auger transition is optically driven. This leads to a coherent superposition of the auger carrier being in two different quantum dot shells. These measurements pose a first step toward coherent control of the orbital state of the Auger carrier.