PhD Defense: Nils Valentin Hauff

Topological Waveguides and Symmetrical Droplet Quantum Dots for Scalable Photonic Quantum Technology

Deterministic quantum emitters able to generate multiphoton entangled states are essential building blocks for a plethora of quantum information applications. Solid-state spin-photon interfaces integrated into nanophotonic structures show great potential as they can form basic elements of scalable photonic circuits that enable on-chip quantum operations. Particularly, InGaAs quantum dots (QDs) integrated into planar nanophotonic circuits have made significant technological achievements. An essential component is the interface of a QD and a photonic crystal waveguide (PhCW), as it enables engineering the interaction between the QD and guided light. This thesis explores topologically non-trivial PhCW and a novel type of QDs for advancing scalable photonic quantum technology.

Advances such as the design of broken-symmetry PhCWs have led to chiral light-matter interfaces and non-reciprocal devices, which are essential for proposed quantum gates. However, chiral interfaces have yet to achieve as high interaction enhancements as conventional PhCWs and are limited in their performance by backscattering losses. The first part of this thesis explores the potential of a topological PhCW for chiral interfaces with QDs.

Specifically, planar nanophotonic circuits with embedded topological PhCWs with InGaAs QDs are designed and analyzed. Although the devices show adequate transmission in the spectral region of InGaAs QDs, the capabilities of the utilized nanofabrication method limit their practical application as chiral interfaces. The potential of topological PhCWs to support and enhance chiral interactions while suppressing backscattering losses is examined by finite-element calculations and backscattering theory. It is shown that topological PhCW can outperform dispersion-engineered conventional waveguides due to reduced backscattering on fabrication-induced imperfections for high interaction enhancements. The integration of topological PhCWs into nanophotonic circuits of the state-of-the-art InGaAs QD platform and the numerical performance benchmark promise high-performance designs of efficient, on-chip non-reciprocal devices and scalable circuits based on topological PhCWs and may enable the design of optical isolators, circulators, and quantum gates.

Technological developments of planar nanophotonic circuits are confronted with intrinsic limitations of InGaAs QDs in their coherence properties, fine-structure splitting, and spin-photon entanglement fidelity. The second part of this thesis explores the potential of planar nanophotonic circuits with a novel type of QDs: local droplet-etched GaAs QDs.

More concretely, planar nanophotonic circuits are presented that feature interfaces of PhCWs and GaAs QDs, fabricated from an ultra-thin semiconductor heterojunction. The demonstration of charge-state control and optical transition energy tuning of QDs in planar nanophotonic circuits marks an essential step for realizing scalable spin-photon interfaces with GaAs QDs. Embedded QDs are characterized as a source of single photons for quantum information by resonant excitation and using time-resolved, high-resolution, and phonon-sideband resolved spectroscopy. Hanbury Brown and Twiss interferometry confirms the single-photon nature of the QDs' photoemission and low blinking probability in timescales up to 25µs. The single photon purity, the demonstration of mutual coherence between photons created, and coherent optical driving promise prospects for realizing a scalable deterministic source of indistinguishable single photons with GaAs QDs.

Advancing topological nanostructures for chiral light-matter interaction and developing low-decoherence emitters integrated into planar nanostructures are promising avenues for developing a scalable quantum photonic platform for generating entangled states on-chip.