Academic literature on the topic 'Single-photon sources characterization'

AbstractIn our laboratory, we employ two biphoton sources for quantum key distribution. The first is based on cw parametric down-conversion of photons at 404 nm in PPKTP waveguide chips, while the second is based on the pulsed parametric down-conversion of 775 nm photons in PPLN waveguides. The spectral characterization is important for the determination of certain side-channel attacks. A Hong-Ou-Mandel experiment employing the first photon source revealed a complex structure of the common Hong-Ou-Mandel dip. By measuring the spectra of the single photons at 808 nm, we were able to associate these structures to the superposition of different transverse modes of the pump photons in our waveguide chips. The pulsed source was characterized by means of single-photon spectra measured by a sensitive spectrum analyzer as well as dispersion-based measurements. Finally, we also describe Hong-Ou-Mandel experiments using the photons from the second source.

Quantum Key Distribution allows two users to exchange secret keys and it is based on the transmission of single photons or attenuated laser pulses. Recently, sources based on multiple single-photon emitters were demonstrated to be suitable for QKD. Here, we present a CMOS compatible multiple single-photon emitters source realized on a SOI wafer by a standard silicon diode doped with erbium ions. Particular emphasis is placed on the fabrication of such a device enhancing the erbium electroluminescence signal by adopting a proper oxygen co-doping. Finally, electroluminescence characterization at room temperature of the device is presented.

ABSTRACTWe report the formation and optical properties of site-controlled InAs/GaAs quantum dots (QDs) embedded in GaAs nanowires (NWs) by selective metalorganic chemical vapor deposition for application to single photon sources. InAs/GaAs QD-in-NWs with various InAs thicknesses are realized on patterned GaAs(111)B substrates in the form of InAs/GaAs heterostructures and identified by structural analyses using scanning transmission electron microscopy and photoluminescence characterization. Sharp excitonic emission peaks at 10 K from single QD-in-NWs with the narrowest exciton linewidth of 87 μeV are observed. Light emission from the single QD-in-NW shows photon antibunching which evidences single photon emission from high-quality QD-in-NWs.

AbstractStriving for nanometer-sized solid-state single-photon sources, we investigate atom-like quantum emitters based on single germanium-vacancy (GeV) centers isolated in crystalline nanodiamonds (NDs). Cryogenic characterization indicated symmetry-protected and bright (>106 counts/s with off-resonance excitation) zero-phonon optical transitions with up to 6-fold enhancement in energy splitting of their ground states as compared to that found for GeV centers in bulk diamonds (i.e. up to 870 GHz in highly strained NDs vs. 150 GHz in bulk). Utilizing lithographic alignment techniques, we demonstrate an integrated nanophotonic platform for deterministic interfacing plasmonic waveguides with isolated GeV centers in NDs, which enables 10-fold enhancement of single-photon decay rates along with the emission direction control by judiciously designing and positioning a Bragg reflector. This approach allows one to realize the unidirectional emission from single-photon dipolar sources, thereby opening new perspectives for the realization of quantum optical integrated circuits.

We have used optical tweezers with shaped apertures in metal films to hold onto individual nanoparticles including erbium containing nanoparticles and perovskite quantum dots. In this talk, I will talk about our ability to isolate nanocrystals with just one erbium emitter, paving the way for single photon sources at low loss fiber wavelengths. I will also talk about our new work on in-situ characterization and assembly of perovskite quantum dots, allowing to relate size to emission wavelength and look at quantum hybridization.

Photoluminescent carbon nanotubes are expected to become versatile room-temperature single-photon sources that have applications in quantum information processing. Quantum emission from carbon nanotubes is often induced by localization of excitons or exciton-exciton annihilation. Here, we modify the local energy landscape of excitons by decorating nanoscale pentacene particles onto air-suspended single-walled carbon nanotubes [1]. Directional exciton diffusion from the undecorated region to the decorated site is demonstrated, suggesting exciton trapping induced by local dielectric screening from pentacene particles. Photoluminescence and photon correlation measurements on a representative carbon nanotube reveal enhanced exciton-exciton annihilation and single-photon emission at room temperature. Pentacene particles are shown to promote strong photon antibunching at the decorated site, indicating that noncovalent functionalization using molecules can be an effective approach for energy landscape modification and quantum emission in carbon nanotubes. Work supported in part by MIC (SCOPE 191503001), JSPS (KAKENHI JP20H02558, JP19J00894, JP20J00817, JP20K15121, JP20K15137, JP20K15112 and JP20K15199), and MEXT (Nanotechnology Platform JPMXP09F19UT0079). K.O. and D.Y. are supported by the JSPS Research Fellowship. D.K. is supported by the RIKEN Special Postdoctoral Researcher Program. We acknowledge RIKEN Materials Characterization Team for access to the transmission electron microscope, and RIKEN Advanced Manufacturing Support Team for technical assistance. [1] Z. Li, K. Otsuka, D. Yamashita, D. Kozawa, Y. K. Kato, ACS Photonics 8, 2367 (2021).

Single-photon photoemission of electrons for incident photon energies below the surface work function is reported for a single-crystal Cu(111) photocathode. Spectral characterization of the quantum efficiency and mean transverse energy of the emitted electrons is shown to be consistent with emission from a thermalized hot electron distribution photoexcited on sub-picosecond time scales into an upper conduction band at the L-point of the Brillouin zone of copper. To our knowledge, this is the first time that such excited-state thermionic emission has been observed from a planar metal photocathode, and then from a commonly used photocathode material. The results, therefore, indicate the potential importance of such band structure dependent sub-threshold photoemission mechanisms on the performance of photocathodes employed for the generation of short electron pulses using sub-picosecond laser pulses. Consequently, the combined experimental and theoretical work presented in this paper contributes to solid-state photocathode-based research aimed at our understanding and selection (or discovery) of high brightness photo-electron sources required for many electron-based diffraction, imaging, and accelerator applications.

A promising approach for direct detection of quantum entanglement in solid state materials is probing with entangled particles. X-rays are ideal for characterization of many materials but have not been generated as entangled pairs in useful numbers, even at the most powerful synchrotron sources. This proposal describes how x-rays in fully entangled N00N states (with N = 2) can be produced by applying Hong-Ou-Mandel interferometry at an x-ray synchrotron source. The technical requirements for indistinguishable photons, x-ray beamsplitters, and interferometer design are met with existing resources. The entangled x-ray biphotons can be filtered from the unavoidable single photon background by simple diffraction from an analyzer crystal, resulting in intense beams suitable for characterizing quantum materials.

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

We report on BB84 quantum key distribution tests employing a benchtop plug&play quantum-dot based single-photon source operating at O-band wavelengths. We perform a detailed characterization and exploit optimized temporal filters to maximize the tolerable losses.