PhD Defense by Freja T. Pedersen

Title: ’Deterministic Single and Multi-Photon Sources with Quantum dots in Planar Nanostructure’

Abstract: Photonic qubits are key ingredients in the implementation of quantum-information processing and are ideal for interconnecting distributed nodes in a quantum network. Solid-state quantum dots have proven to be powerful and versatile sources of photonic qubits, both for single-photon emission and entanglement generation. Strong confinement in the quantum dot leads to discretized energy levels, which can be optically addressed to generate single-photons. Furthermore, the solid state environment enables engineering of the local photonic environment around the quantum dot, using a nanostructure, which is crucial in extracting and guiding the emitted photons with near-unity efficiency. The approach taken in this thesis is one among many and is employing a photonic crystal waveguide. Among the diverse array of nanostructures that were investigated for efficient single-photon generation, photonic crystal waveguides are promising owing to their broadband operation, near-unity efficiency, possibility for Purcell enhancement of the radiative decay rate, and a direct coupling to a single propagating mode. However, the introduction of nanophotonic structures and the solid-state environment typically introduce new sources of noise that should be suppressed to achieve emission of high quality photons. Moreover, to enable deterministic operation, resonant excitation schemes must be employed, where laser background suppression can be experimentally challenging. 

Nearly ideal operation of self-assembled InAs quantum dots in photonic crystal waveguides as single-photon sources is presented in this thesis. We achieve close to perfect noise suppression by embedding the quantum dots in a p-i-n diode heterostructure for charge control. Robust resonant excitation is achieved in a thoroughly optimized experimental setup using carefully characterized quantum dots. We observe high single-photon emission 10 MHz, single-photon purity g(2)(0) < 1 % and indistinguishability of the emitted photons of > 98 %. 

We also investigate the generation of polarization-entangled photon pairs from these high-quality quantum dots. Typically, polarization information of the photons is lost when the emission is coupled to a single-mode waveguide. However, photonic crystal waveguides support special locations called chiral points, which enable directional coupling of polarized emission. In this way, polarization entanglement can be converted to a spatial basis enabling on-chip entanglement generation.

Finally, we present the first results towards the integration of droplet etched GaAs quantum dots into photonic crystal waveguides, which exhibit ideal properties for high-fidelity entanglement generation. 

This thesis presents a quantum dot based source of photonic qubits which accommodates both scalable single-photon emission and the possibility for deterministic on-chip entanglement generation. Our source is therefore a strong resource in a future quantum based network.