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Title
Slow sngle photons and entangled photon-pairs from strain-tunable quantum dot devices / submitted by Johannes S. Wildmann
Additional Titles
Langsame Einzelphotonen und Photonenpaare von Quantenpunktstrukturen mit durchstimmbarer Verspannung
AuthorWildmann, Johannes Siegfried
CensorRastelli, Armando ; Nika, Akopian
PublishedLinz, 2017
Descriptionxiii, 190 Seiten : Illustrationen
Institutional NoteUniversität Linz, Dissertation, 2017
Annotation
Zusammenfassung in deutscher Sprache
Abweichender Titel laut Übersetzung der Verfasserin/des Verfassers
LanguageEnglish
Bibl. ReferenceOeBB
Document typeDissertation (PhD)
Keywords (DE)Halbleiterphysik / Festkörperpysik / Nanostrukturen / Quantenpunkte / Verschränkung
Keywords (EN)Semiconductor physics / solid state physics / nanostructures / quantum dots / entanglement
Keywords (GND)Quantenpunkt / Einzelphotonenemission / Paarerzeugung / Nanostruktur / Verschränkter Zustand
URNurn:nbn:at:at-ubl:1-13571 Persistent Identifier (URN)
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 The work is publicly available
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Slow sngle photons and entangled photon-pairs from strain-tunable quantum dot devices [139.29 mb]
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Abstract (English)

Semiconductor quantum dots (QDs) are nanostructures made of several thousand atoms that can self-assemble during heteroepitaxial growth. When their size is smaller than the De Broglie wavelength of charge carriers trapped in these structures, quantum confinement effects become apparent and, similar to real atoms, QDs feature discrete energy levels. This remarkable property has opened up the unprecedented possibility to use QDs as quantum bits (qubits) for quantum information processing. This can be achieved by exploiting, for example, the spin/charge properties of single electrons (holes), or one degree of freedom of photons emitted during the radiative recombination of electron-hole complexes (i.e. excitons).Very recently, QDs are emerging as one of the most promising sources of flying qubits (photons), since they are ompatible with current photonic integration technologies and they are capable to deliver single [83, 107] and entangled photons [3, 15] with high efficiency [32, 130] and on demand [91]. However, in order to implement these nanostructures in solid-state based quantum information processing, several challenges have to be overcome. Two major challenges are: (i) the fabrication of technologically robust sources of highly entangled photons [105, 139] and (ii) the development of efficient interfaces between QD photons and other quantum systems [29, 75, 94]. In this work, these two challenges are tackled. First, semiconductor piezoelectric devices are presented that can be used for the generation of polarization entangled photon pairs featuring the highest degree of entanglement reported to date for QD photon sources. These sources can be driven electrically at repetition rates up to 0.4 GHz. Second, it is reported how the same device can be used to interface single photons from a QD with a cloud of Cesium (Cs) atoms, which is used as a slow light medium. The first part of the results section thus starts with the basic layout of the semiconductor-piezoelectric device, which consists of a diode- like nanomembrane integrated onto a piezoelectric actuator. This allows the electronic structure of QDs to be reshaped by the simultaneous application of stress and electric fields. It is demonstrated that a balanced combination of these fields allows to compensate for the structural asymmetries of QDs, which are detrimental for the generation of highly entangled photons. Alternatively, the electric field across the the diode-like nanomembrane can be used to inject carriers electrically into the QD , thus leading to the generation of en- tangled photon pairs. This allows for the demonstration of the first entangled light emitting diode (ELED), driven at a repetition rate of 0.4GHz, which is an important step towards high data rate quantum information processing. The second part of the work reports on interfacing photons from QDs with warm Cs vapors. More specifically, the same device discussed above was used to precisely tune the energy of the quantum dot (QD) photons to the hyperfine split D 1 levels of Cs. Under this conditions, photons propagate in a strongly dispersive medium and are slowed down. A differential delay of 2.4ns over the length of the Cs cell (7.5 cm) is demonstrated in this work. Moreover, the Cs vapor is utilized as a spectrally selective delay line, which is capable of introducing a significant temporal delay between photons that are separated in frequency by only a few GHz. The temporal and spectral distribution of photons before and after transmission through Cs vapor is measured and quantitatively explained by a theoretical model. This model uses the temperature dependent group velocity of Cs and takes into account the transmission of the inhomogeneously broadened QD emission to calculate the temporal distribution of the photons escaping through the Cs cell. In combination with a QD device that allows to suppress the biexciton binding energy, this spectrally selective delay opens up the possibility to test the theoretically proposed, but not yet experimentally demonstrated, entanglement generation through temporal reordering [8]. Finally, the last part of this work discusses the first experimental steps towards this reordering scheme. In particular, the complete cancellation of the biexciton binding energy and the simultaneous tuning of the emission energy to the absorption of Cs D1 are demonstrated. Thus, time reordering operations on photons from the biexciton-exciton radiative cascade are demonstrated. Further refinements of the devices that are needed to demonstrate the feasibility of this scheme for entangled photon generation are discussed.

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