Tommaso Pregnalato

A thesis for the degree of Doctor of Philosophy defended October 2019.

The PhD School of Science, Faculty of Science, Quantum Optics, Niels Bohr Institute, University of Copenhagen

Principal Supervisors:
Professor Peter Lodahl
Assistant Professor Nir Rotenberg

Deterministic quantum photonic devices based on self-assembled quantum dots

In this thesis we present our recent progress towards the creation of complex quantum photonic circuits. These devices would integrate on the same chip efficient single-photons sources and photonic components that guide light and interface it with quantum emitters. Such a quantum circuit would constitute the ideal solid-state platform to process quantum states and would play a key role in large-scale quantum networks.Self-assembled InAs quantum dots (QDs) embedded in GaAs photonic nanostructures are particularly promising candidates for these applications since they have been shown to have good coherence properties and the ability to emit single photons at very high rates. However, major challenges still hinder the implementation of advanced quantum photonic experiments on this platform. For example, structural imperfections on the fabricated devices introduce optical losses that reduce the overall performances. Furthermore, QDs grow at random positions and emit photons at different frequencies due to their large range of sizes. The resulting lack of spatial and spectral knowledge makes the coupling between emitters and photonic devices less efficient and precludes the production of large-scale quantum photonic circuits.

In this thesis, we first describe the protocols we developed to produce complex nanostructures such as electrically contacted nanobeam and photonic crystal waveguides. These techniques allowed for the fabrication of devices in which appealing quantum effects such as single-photon nonlinearities and spin-photon interactions could be observed. In the second part we overcome the problem of randomly distributed QDs, presenting a method to pre-locate QDs and subsequently fabricate photonic nanostructures about their positions. This photoluminescence-based protocol allowed us to fabricate photonic nanostructures aligned to the pre-selected QDs with final accuracy better than 50 nm.

The newly acquired knowledge over the QD locations lets us explore for the first time the effects of nanofabrication on the spectral properties of InAs QDs in suspended nanostructures. We registered average spectral shifts of up to  1nm, which we were able to correct for by applying an electric field across the sample. Furthermore, we also characterised the effects of fabrication on different excitonic complexes. Finally, to demonstrate for the first time the potential of combining all these capabilities, we deterministically interface QDs with photonic crystal waveguides, which we carefully position both in space and spectrally close to the optical bandgap. Time-resolved measurements allowed us to probe the emission properties of these QDs, revealing a dramatic improvement to their quantum efficiencies. In total, these results constitute an important step towards the fully deterministic spatial and spectral interfacing of quantum emitters with photonic nanostructures, a key requisite to the development of complex quantum circuits.