## 2 State of the Art  
Although QKD is still considered by some to be a fringe issue belonging only to the arenas of intelligence communities and large enterprises, a growing number of new vendors are emerging, whether among universities or startups. Thus, QKD is currently being developed in many distinct directions, and the various systems are based on different physical principles and are at various stages of maturity. Although the vast majority of QKD transmissions are over optical fibers, many \"wireless\" versions are now also being actively developed. These are often referred to as free-space protocols. Among the most interesting are satellite systems such as the Chinese Micius project [3; 4], which serves, among other things, as a link between Beijing and Vienna, and the European Space Agency's Eagle-1 project, where a satellite launch is planned for 2024 [5].  
As in other areas of electrical engineering, there is a trend to minimize the individual components of a QKD system, which has led to multiple attempts to develop chip-based QKD and quantum random number generation (QRNG). An example may be Toshiba, which in 2021 introduced their first chip-based QRNG and QKD prototype [6]. Other organizations and startups, such as KETS, are developing their own solutions [7; 8; 9]. An alternative to most classical discrete-variable QKD (DV-QKD) protocols is the so-called continuous-variable QKD (CV-QKD), which stores information in multiphoton pulses by means of modulations. An example is the system implementation of the Technical University of Denmark (DTU) [10]. Although the coexistence of CV-QKD protocols with classical channels is not as well studied as DV-QKD, they are generally considered to be more robust to noise and thus more suitable for deployment in multiplexed networks [11; 12]. Their second advantage is generally a higher key rate; however, their current disadvantage is usually a shorter range [13].  
One of the largest obstacles of the recent QKD systems is the short maximum range of the quantum channel, which can, however, be extended in several ways. One is a well-known concept of a trusted repeater, which is a device that operates above the quantum layer itself and forward encrypted keys. In this way, it is possible to not only extend the link, but also build more complex topologies. However, a trusted node contains keys in nonquantum form; for this reason, it must itself be protected from access by an eavesdropper [14; 15].  
Various successes have also been achieved with different Measurement-device Independent QKD (MDI-QKD) protocols. Here, the most well-known representative is the twin-field QKD (TF-QKD). With this protocol, it is already possible to communicate over distances greater than 830 km under experimental conditions [16; 17]. Recently, however, there have been reports of new protocols based on a similar principle that can outperform TF-QKD in terms of key rate [18; 19].  
In the near future, the so-called quantum repeaters might revolutionize the whole field of the quantum key distribution. Such repeaters are basically a new QKD protocol scheme based on the phenomenon of entanglement swap [20; 21]. However, building them requires functional quantum memory that is not yet available. Therefore, quantum repeaters have not yet appeared. For this reason, quantum memories are currently one of the main areas of interest of many research labs, and there is gradual progress in this field [22; 23].