Publications

DOI:

10.1117/12.3044303External link

University Bibliography Jena:

fsu_mods_00024480External link

Quantum communication, particularly quantum key distribution (QKD), leverages the principles of quantum mechanics to provide secure information exchange. A key challenge in airborne quantum communication setups is the preservation of polarisation states during photon transmission between a moving aircraft and a ground station. In this work, I propose a polarisation compensation model designed to mitigate the distortions introduced by dynamic optical elements, including the aircraft’s coarse pointing assembly (CPA), the periscope system of the ground station (QuBus), and other optical components. These distortions can degrade the performance of both QKD and any interfacing experiments requiring coupling of these transmitted photons. By characterising the polarisation transformations introduced by these elements, I develop a polarisation correction mechanism based on a three-waveplate configuration consisting of two quarter-wave plates (QWPs) and one half-wave plate (HWP). This dynamic compensation system adjusts in real-time, based on positional data from the QuBus periscope. Laboratory tests have demonstrated the feasibility of this correction mechanism in terms of speed, achieving adjustments within 2.54 seconds, thus allowing for compensation on aircraft with a traverse angle rate of change of up to 0.413°/sec. The effectiveness of this system will be further evaluated in the upcoming 935 nm interfacing experiment, scheduled for March 2025, providing critical insights into its real-world applicability and its potential to enhance the reliability of free-space quantum communication systems.

DOI:

10.22032/dbt.67430External link

University Bibliography Jena:

fsu_mods_00027455External link

Optical clock networks connected by phase-coherent links offer significant potential for advancing fundamental research and diverse scientific applications. Free-space optical frequency transfer extends fiber-based connectivity to remote areas and holds the potential for global coverage via satellite links. Here we present a compact and robust portable, rack-integrated two-way free-space link characterization system. Equipped with plug-and-play capabilities, the system enables straightforward interfacing with various optical systems and facilitates quick deployment for field experiments. In this work, we achieve a fractional frequency instability of 2.0 × 10−¹⁹ for an averaging time of 10 s over a 3.4 km horizontal fully folded intra-city free-space link. Moreover, the system maintains an uptime of 94% over 15 hours, illustrating its reliability and effectiveness for high-precision optical frequency comparisons over free-space.

DOI:

10.1364/OE.557708External link

University Bibliography Jena:

fsu_mods_00027552External link

DOI:

10.1109/CLEO/EUROPE-EQEC65582.2025.11110183External link

University Bibliography Jena:

fsu_mods_00027651External link

Quantum information is transforming modern society with concepts like entanglement, crucial for communication, computing and sensing. To mainstream these technologies, improved efficiency in encoding, transmission and information processing is required, especially in quantum key distribution (QKD). This method aims to securely distribute encryption keys while improving key rates and overcoming propagation loss and noise. Photons serve as ideal carriers due to their various properties for encoding information and minimal environmental interaction, making them vital for the global internet. High-dimensional and hyper-entanglement offer advantages over traditional two-mode encoding methods, including increase noise tolerance and information capacity. However, practical implementation is often limited by measurement device complexities and errors. Frequencies, or photon colours, present a promising approach to boost quantum communication thanks to their compatibility with existing telecommunications infrastructure and naturally occurs in photon pair generation. This thesis explores the frequency degree of freedom for applications in QKD, utilizing efficient all-fiber photon-pair sources to generate hyper-entangled states in polarization and frequency. The incorporation of fast modulators allows for high-speed operation while maintaining high entanglement fidelity. Reconfigurable spectral filters enable tailored spectro-temporal properties to fulfil requirements in multi-user networks and protocols beyond QKD, such as quantum secret sharing. To maximize quantum information capacity, methods for efficiently characterizing frequency entanglement are developed, achieving a certification of 35 modes, a record for the time and frequency degrees of freedom. Finally, estimates of secure key rates and a discussion of noise effects is shown. This foundation supports broadband and flexible quantum networks that effectively utilize frequency encoding in the future quantum internet.

DOI:

10.22032/dbt.69494External link

University Bibliography Jena:

fsu_mods_00034587External link

DOI:

10.1109/PN66844.2025.11097157External link

University Bibliography Jena:

fsu_mods_00029072External link

DOI:

10.1364/quantum.2024.qth2b.3External link

University Bibliography Jena:

fsu_mods_00018763External link

Photonic quantum technology requires precise, time-resolved identification of photodetection events. In distributed quantum networks with spatially separated and drifting time references, achieving high precision is particularly challenging. Here we build on recent advances of using single-photons for time transfer and employ and quantify a fast postprocessing scheme designed to pulsed single-photon sources. We achieve an average root mean square synchronization jitter of 3.0 ps. The stability is comparable to systems with Rb vapor cell clocks with 19 ps at 1 s integration time, in terms of Allan time deviation. Remarkably, our stability is even better than classical high-precision time transfer, like the White Rabbit protocol, although we use significantly less signal (single-photon level). Our algorithms allow local processing of the data and do not affect the secure key rate. It compensates substantial clock imperfections from crystal oscillators and we foresee great potential for low signal scenarios. The findings are naturally suited to quantum communication networks and provide simultaneous time transfer without adding hardware or modifying the single-photon sources.

DOI:

10.1088/2058-9565/ad0ce0External link

University Bibliography Jena:

fsu_mods_00009581External link

DOI:

10.1364/quantum.2024.qw2a.7External link

University Bibliography Jena:

fsu_mods_00018757External link

DOI:

10.1364/quantum.2024.qth3a.14External link

University Bibliography Jena:

fsu_mods_00018764External link

DOI:

10.1117/12.3022982External link

University Bibliography Jena:

fsu_mods_00016181External link

DOI:

10.1364/quantum.2024.qth3a.13External link

University Bibliography Jena:

fsu_mods_00018760External link