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.

A two-layer switch exploiting orbital angular momentum (OAM) and wavelength of the light as switching domains is presented, aiming to increase the scalability with respect to the single-layer switches. The switch is able to accept 160 optical Gaussian data inputs on a 16-channel wavelength division multiplexing (WDM) grid and direct each input signals to different output ports exploiting 10 OAMs. The optical switch is based on an integrated OAM multiplexer followed by a compact OAM demultiplexer consisting of two refractive elements. Its experimental characterization confirmed a total enabled throughput of 19.2 Tb/s, thanks to the 30 GHz bandwidth available for each port. The switching time can be lower than 1 s. The OAM switch power consumption, solely due to the thermal tuning of the OAM emitters, since the OAM demux is passive, is 1.35 mW/Gb/s. In the proposed switching architecture the number of active components, i.e. the power consumption, scales linear with the number of ports. This is favorable in comparison with single-layer switches that cascade e.g. 2x2 elementary blocks to obtain large port counts, which scale with the square of the number of ports. The switch accepts input and output signals with Gaussian phase profile that propagate through optical fibers and waveguides, thus making it compatible with standard telecom devices. The suitability of the switch to support real data-traffic is proved by successfully testing it with 10G Ethernet and Fiber Channel over Ethernet (FCoE) data traffic and video traffic. A possible application scenario is represented by a data-center network where the switch can be used to create a low-power consumption network parallel to the network based on standard electronic routers, to manage large traffic flows.

We investigate buffer-aware demand-routing of key-consumption demands in QKD networks and implement a measurement-based framework for path selection. The proposed ML-based algorithm outperforms state-of-the-art heuristics by 22 % and 92 % for networks under test.

Current technological advancements of quantum computers highlight the need for application-driven, practical and well-defined methods of benchmarking their performance. As the existing NISQ device's quality of two-qubit gate errors rate is even around one percent and the number of qubits is still limited to a few or several dozen, naturally, we need to propose rather small algorithms instances taken from key promising application areas, such as quantum chemistry, combinatorial optimisation or machine learning. While many techniques for assessing the performance of logical components, such as gate fidelity and qubit coherence exist, it is still challenging to extrapolate those values onto the performance of different quantum algorithms and subroutines. This work aims to introduce a series of initial quantum application benchmarks together with a methodology of execution for measuring the performance and fidelity of the results. The proposed suite refers to several variational algorithms widely used on available NISQ devices but also includes examples of quantum circuits designed for a fault-tolerant quantum computer.