Coherent photons and coherent spins in a GaAs quantum dot

This thesis explores droplet-etched GaAs quantum dots as potential candidates for spin-photon interfaces.
The first part of the thesis focuses on the optical coherence of the emitted photons. We confirm the emission of single photons from a single quantum dot with a high single-photon purity characterised by $1-g^{(2)}(0)$ of $99\,\%$. A Hong-Ou-Mandel experiment reveals a two-photon interference visibility of $V = 98\,\%$ for consecutively emitted photons, proving high photon indistinguishability. A more striking experiment is to analyse the indistinguishability of photons emitted further apart in time, ideally infinitely apart. Such an experiment can be realised by interfering photons emitted from separate quantum dot sources with uncorrelated noise environments - an experiment that has previously led to visibilities just above $67\,\%$. Using GaAs quantum dots, we determine a high two-photon visibility of $V = 93\,\%$ for photons emitted from remote sources, indicating that the noise environments in the quantum dot devices are exceptionally low. This high visibility allows us to perform a final experiment, entangling two streams of photons from different sources - a proof-of-principle demonstration of the probabilistic generation of a Bell state.
The second part of the thesis focuses on the spin coherence of an electron spin in a GaAs quantum dot. We implement an all-optical spin control scheme and combine fast optical pulses with flexible microwave spin manipulation. We find short electron-spin coherence times ($T_2^* = 4\,$ns), a consequence of strong interactions between the central electron spin and the host nuclei. This interaction is enhanced when the electron spin is manipulated at Rabi frequencies matching the Larmor frequencies of the nuclei. Exploiting this interaction, we implement two nuclear cooling schemes and increase the coherence time $T_2^*$ to $80\,$ns and $0.608\,$µs, respectively. This constitutes a 155-fold increase in the coherence time and a reduction of nuclear-spin fluctuations comparable to an effective temperature of $100\,$µK (starting from $T =4\,$K). Moreover, we implement refocusing pulses and show that the electron-spin coherence can be extended using dynamical decoupling pulses up to $T_2^\text{DD} = 22\,$µs with the Carr-Purcell-Meiboom-Gill sequence.
The combination of highly coherent single photons and fast control of a long-coherent spin constitutes ideal properties for a spin-photon interface. The enhanced spin coherence time is much larger than the radiative lifetime of the exciton ($\tau = 300\,$ps) and the time required to rotate the spin ($T_\pi<2\,$ns), enabling the emission of hundreds of spin-photon entangled states within the coherence time.