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Semiconductor Quantum Optics at Telecom Wavelengths

Time: Fri 2020-12-04 09.00

Location:, (English)

Subject area: Physics, Optics and Photonics

Doctoral student: Katharina Zeuner , Kvant- och biofotonik, Quantum Nano Photonics

Opponent: Professor Stephan Reitzenstein, TU Berlin

Supervisor: Val Zwiller, Kvant- och biofotonik; Klaus D. Jöns, Kvant- och biofotonik

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Quantum technologies are an expanding field in physics and engineering concerning the development of protocols and devices that enable augmented or novel applications based on quantum mechanics. This includes amongst others quantum computation and quantum communication. Quantum computers promise a computational speed–up based on superposition relevant for optimization and simulation problems, as well as for factorizing of large numbers, which poses a threat to our classical encryption schemes. Quantum communication offers a solution to this issue by providing an unconditionally secure communication channel based on the laws of quantum mechanics. Moreover, quantum communication would allow the exchange of quantum information between remote quantum computers, enabling distributed quantum computing. An infrastructure that links quantum computers or processors is referred to as a quantum network. Stationary quantum bits at the network nodes are used for performing information processing or storing operations, while flying quantum bits connect the nodes and enable the transfer of quantum information. Photons are excellent flying quantum bits, as they travel at the speed of light and have a small interaction cross–section. Consequently, quantum networks require sources of quantum states of light to provide flying quantum bits. These quantum states of light need to be entangled, indistinguishable and wavelength–matched such that they either experience low transmission losses in networks or can be interfaced with other quantum technologies like atom–based quantum memories. In this thesis the emission of single, indistinguishable or entangled photons from single self–assembled optically active semiconductor quantum dots, our quantum emitter of choice, has been studied. The investigated quantum dots emit either in the telecom range or close to the D1–transition in Rubidium. The main aspects of the experiments performed in this thesis were to research the integrability of the emitters into future quantum networks by making them wavelength–tunable, integrating them into photonic structures and employing resonant excitation schemes in order to generate photons with unprecedented purity, indistinguishability or entanglement concurrence. In the telecom range, we study InAsP nanowire quantum dots whose emission is shifted from the near–infrared range up to the telecom O–band and C–band. Single photon emission is demonstrated with decay times of the quantum dots similar to their near–infrared counterparts. Furthermore, InAs/GaAs quantum dots emitting in the telecom C–band are integrated onto piezo–electric substrates, and their emission is modulated into sidebands by using commercial telecommunication equipment. We generate on–demand single photons using a two–photon resonant excitation scheme and on–demand entangled photons via a phonon–assisted resonant scheme. Droplet–etched GaAs quantum dots with emission in the vicinity of the D1–Rubidium transition have been excited via two–photon resonant excitation to generate single photons with unparalleled purity and highly entangled photon pairs to perform entanglement swapping. Under resonance fluorescence, single and highly indistinguishable photons are extracted. Both resonant excitation schemes are theoretically compared to reveal the limitations of the two techniques. Moreover, these quantum dots are integrated into piezo–tunable broad–band micro parabolic cavities for an enhanced extraction efficiency.