Sources of single and polarization-entangled photons are an essential component in a variety of potential quantum information applications. Suitable emitters need to generate photons deterministically and at fast repetition rates, with highest degrees of single-photon purity, entanglement and indistinguishability. Semiconductor quantum dots are among the leading candidates for this task, offering entangled-photon pair emission on-demand, challenging current state-of-the-art sources based on the probabilistic spontaneous parametric down-conversion (SPDC). Unfortunately, their susceptibility to perturbations from the solid-state environment significantly affects the photon coherence and entanglement degree. Furthermore, most quantum dot types suffer from poor wavelength control and emitter yield, due to a random growth process.
This thesis investigates the emerging family of GaAs/AlGaAs quantum dots obtained by in-situ Al droplet etching and nanohole infilling. Particular focus is laid on the interplay of growth parameters, quantum dot morphology and optical properties. An unprecedented emission wavelength control with distributions as narrow as ± 1 nm is achieved, using four independent growth parameters: The GaAs infilling amount, the deposition sequence, the migration time and the Al concentration in the barrier material. This enables the generation of large emitter ensembles tailored to match the optical transitions of rubidium, a leading quantum memory candidate. The photon coherence is enhanced by an optimized As flux during the growth process using the GaAs surface reconstruction. With these improvements, we demonstrate for the first time two-photon interference from separate, frequency-stabilized quantum dots using a rubidium-based Faraday filter as frequency reference.
Two-photon resonant excitation of the biexciton state is employed for the coherent and deterministic generation of photon pairs with negligible multi-photon emission probability. The GaAs/AlGaAs quantum dots exhibit a very small average fine structure of (4.8 ±2.4) µeV and short average radiative lifetimes of 200 ps, enabling entanglement fidelities up to F = 0.94, which are among the highest reported for any entangled-photon source to date. Furthermore, almost all fabricated emitters on a single wafer exhibit fidelities beyond the classical limit - without any post-growth tuning. By embedding the quantum dots into a broadband-optical antenna we enhance the photon collection efficiency significantly without impairing the high degrees of entanglement. Thus, for the first time, quantum dots are able to compete with SPDC sources, paving the way towards the realization of a semiconductor-based quantum repeater - among many other key enabling quantum photonic elements.:Contents
List of Figures ix
List of Tables xiii
1 Introduction 1
1.1 Researchmotivation ...................1
1.1.1 Structure of this thesis ................. 3
1.2 Applications based on entangled photons ............. 4
1.2.1 Quantum bits ...................4
1.2.2 Quantum key distribution ................ 5
1.2.3 Qubit teleportation .................. 7
1.2.4 Teleportation of entanglement ..............9
1.2.5 The photonic quantumrepeater .............. 10
1.3 Generation of entangled photons ...............12
1.3.1 The ideal entangled-photon source ............. 12
1.3.2 Non-deterministic photon sources ............. 13
1.3.3 Deterministic photon sources ..............14
2 Fundamentals 17
2.1 Semiconductor quantumdots ................17
2.1.1 Introduction to semiconductor quantum dots .......... 17
2.1.2 Formation of confined excitonic states ............ 19
2.1.3 Energy hierarchy of excitonic states ............. 21
2.2 Entangled photons from semiconductor quantumdots ......... 22
2.2.1 The concept of entanglement ............... 22
2.2.2 Polarization-entangled photon pairs fromthe biexciton radiative decay .. 23
2.2.3 Origin and impact of the exciton fine structure splitting ....... 25
2.2.4 Impact of spin-scattering, dephasing and background photons on the degree
of entanglement ..................29
2.3 Quantum dot entangled-photon sources - State of the art ........32
2.4 Exciton radiative lifetime .................. 34
2.4.1 The concept of radiative lifetime .............. 34
2.4.2 Measurement of the radiative lifetime ............35
2.5 Single-photon purity ...................37
2.5.1 Photon number distributions ............... 37
2.5.2 Second-order correlation function .............38
2.5.3 Measurement of the second-order correlation function ....... 40
2.6 Measurement of entanglement ................42
2.6.1 Quantum state tomography ...............43
2.7 Photon coherence and spectral linewidth .............46
2.7.1 The concept of coherence ................ 46
2.7.2 First-order coherence ................. 46
2.7.3 Relation between coherence and spectral linewidth ........ 49
2.7.4 homogeneous vs. inhomogeneous broadening in single quantumdots ..50
2.8 Photon indistinguishability .................51
2.8.1 Hong-Ou-Mandel interference ..............51
2.8.2 Hong-Ou-Mandel interference between photons fromseparate sources .. 52
2.8.3 The Bell state measurement with linear optics .......... 53
3 Experimentalmethods 55
3.1 The GaAs and AlAs material system ............... 55
3.2 Molecular beam epitaxy ..................56
3.2.1 The Concept of molecular beam epitaxy ...........56
3.2.2 Layout and components of the III-V Omicron MBE ........58
3.2.3 Growth parameters .................. 59
3.2.4 Reflection high-energy electron diffraction (RHEED) ........ 60
3.2.5 Growth rate determination using RHEED oscillations .......61
3.3 Optical setups .....................63
4 Results 67
4.1 Growth of GaAs/AlGaAs quantum dots by in-situ Al droplet etching .....68
4.1.1 Motivation for the study of GaAs / AlGaAs quantum dots ......68
4.1.2 GaAs / AlGaAs quantum dot growth process ..........69
4.1.3 Interplay between growth parameters, quantumdot morphology
and optical properties ................. 71
4.1.4 Nanohole morphology and quantumdot formation ........ 73
4.1.5 Optical characterization ................75
4.1.6 Deterministic wavelength control .............77
4.1.7 Photon coherence and radiative lifetime ...........84
4.1.8 Decoherence processes in semiconductor quantum dots ......86
4.1.9 Chamber conditioning and growth process optimization ......87
4.1.10 Arsenic flux calibration using the GaAs surface reconstruction ..... 88
4.1.11 Enhanced photon coherence after growth process adjustments ....92
4.2 Two-photon interference from frequency-stabilized
GaAs/AlGaAs quantum dots .................94
4.2.1 Frequency tuning of semiconductor quantumdots ........95
4.2.2 Experimental setup .................. 95
4.2.3 Optical characterization of the separate GaAs/AlGaAs quantum dots ... 98
4.2.4 Faraday anomalous dispersion optical filter and frequency feedback ... 99
4.2.5 Two-photon interference between remote, frequency-stabilized quantum dots 100
4.3 Solid-state ensemble of highly entangled photon sources at rubidiumatomic transitions
........................102
4.3.1 Fine-structure splitting ................103
4.3.2 Resonant excitation of the biexciton state ...........105
4.3.3 Single photon purity and radiative lifetime ........... 107
4.3.4 Radiative lifetime of GaAs/AlGaAs quantumdots - comparison to other quantumdot
types ...................108
4.3.5 Degree of entanglement ................109
4.3.6 Highly-efficient extraction of the obtained entangled photons ..... 116
5 Conclusions 119
5.1 Summary ....................... 119
5.2 Discussion and outlook ..................122
Bibliography 127
Publications and scientific presentations 150
Acknowledgments 154
Selbstständigkeitserklärung 157
Curriculum vitae 157
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:72729 |
| Date | 19 November 2020 |
| Creators | Keil, Robert |
| Contributors | Schmidt, Oliver G., Ding, Fei, Technische Universität Chemnitz |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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