We successfully tested the suitability of probabilistically shaped constellations in a German nationwide fiber ring of the R&D field test network of Deutsche Telekom. In this ring, eight German cities can be reached by add/drop nodes implemented by passive optical add/drop multiplexers, offering 12 bands of 400 GHz widths in the extended C-band. Multiple cascaded add/drop multiplexers are reducing the available bandwidth. We transmitted a 1 Tb/s four-carrier super-channel with an extended family of probabilistically shaped constellations using 16QAM, 36QAM, and 64QAM and variable bandwidth. We verified in the field environment by two examples that probabilistic shaping is outperforming standard 64QAM and 16QAM formats in reach. We experimentally tested two typical 1 Tb/s working paths with 419 and 951 km, demonstrating spectral efficiencies of 6.75 and 5 bit/s/Hz, respectively. The maximum reach of about 1500 km was extrapolated for a 1 Tb/s probabilistically shaped super-channel transmitted in a 200 GHz bandwidth. For protection paths with longer distances than 1500 km, the spectral efficiency has to be reduced with the net-bitrate in steps of 100 Gb/s. For the longest bidirectional path of the test network, i.e., a protection path of 2159 km, the maximum achieved net-bitrate was 800 Gb/s.

We report an industry leading optical dense wavelength division multiplexing (DWDM) field trial with line rates per channel.1.66 Tb/s using 130 GBaud dualpolarization probabilistic constellation shaping 256-ary quadrature amplitude modulation (DP-PCS256QAM) in a high capacity data center interconnect (DCI) scenario. This research trial was performed on 96.5 km of field-deployed standard single mode G.652 fiber infrastructure of Deutsche Telekom in Germany employing Erbium-doped fiber amplifier (EDFA)-only amplification. A total of 34 channels were transmitted with 150 GHz spacing for a total fiber capacity of 56.51 Tb/s and a spectral efficiency higher than 11bit/s/Hz. In the single-channel transmission scenario 1.71 Tb/s was achieved over the same link. In addition, we successfully demonstrate record net bitrates of 1.88 Tb/s in back-to-back (B2B) using 130 GBaud DP-PCS400QAM.

Quantum-key-distribution (QKD) networks are gaining importance and it has become necessary to analyze the most appropriate methods for their long-distance interconnection. In this paper, four different methods of interconnecting remote QKD networks are proposed. The methods are used to link three different QKD testbeds in Europe, located in Berlin, Madrid, and Poznan. Although long-distance QKD links are only emulated, the methods used can serve as a blueprint for the secure interconnection of distant QKD networks in the future. Specifically, the presented approaches combine, in a transparent way, different fiber and satellite physical media, as well as common standards of key delivery interfaces. The testbed interconnections are designed to increase the security by utilizing multipath techniques and multiple hybridizations of QKD and post-quantum cryptography (PQC) algorithms.

Quantum Key Distribution technology is evaluated for carrier-grade provider networks in the Berlin OpenQKD testbed, which is operated by Deutsche Telekom. It includes the investigation of modular architectures, standardized interfaces and layered network structures containing quantum, key management, and application layers as well as hybrid technologies, consisting of Quantum Key Distribution and Post-Quantum Cryptography. The testbed incorporates field-installed wireline fiber links together with wireless radio connections, enabling carrier-grade use cases and proof of concepts under real telecommunication operating environments for various scenarios of layer interworking, vendor agnostic networking, and quantum security for network as well as end-to-end user applications.

Quantum Key Distribution (QKD) systems have the potential to provide secure communication by offering unconditionally secure key exchange between distant parties. However, the integration of QKD into existing communication networks presents challenges, particularly when it comes to the deployment of trusted nodes and the establishment of secure connections over metropolitan distances. In this study, we present a novel approach to address these issues by introducing a trusted-node-free design on a metropolitan scale for the seamless integration of a QKD system into the Berlin OpenQKD optical fiber testbed. Our innovative design leverages conventional telecom modulators and a standard Wavelength-Division Multiplexing - Arrayed Waveguide Grating (WDM-AWG), offering a practical solution for flexible multi-partner communication and enabling full connectivity between non-adjacent nodes. In this study, we not only observed relatively low QBER ($< $ 5.5%) but also a rather stable key rate between nodes in the Qline system over approximately 130 hours ($\approx$ 5.4 days) for a lab-based back-to-back setup. Later on, a more complex experimental scheme is integrated into Berlin OpenQKD optical fiber testbed encompassing multiple C-band frequencies (191.75–195.75 THz) and extending over metropolitan distances (up to 39.8 km), providing a foundation for secure quantum communication networks with enhanced scalability and versatility.