PhD Defense by Tommaso Pregnolato

Deterministic quantum photonic devices based on self-assembled quantum dots

In this talk I will 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 (e.g. efficient fabrication of nanodevices and the random growth of QDs) still hinder the implementation of advanced quantum photonic experiments on this platform.

In this talk, I will 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 I will discuss how we overcome the problem of randomly distributed QDs, presenting a method to pre-locate QDs and subsequently fabricate photonic nanostructures about their positions. The final accuracy we achieved was 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 approx. 1nm, which we were able to correct for by applying an electric field across the sample.

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.