# _photonics_
## 1 Introduction  
The advent of quantum computing may affect classical key cryptography in the next decades, setting a large part of cryptosystems insecure. Quantum computers are constantly gaining computational power with the manipulation of more and more qubits [1], while at the same time [PERSON]'s algorithm ensures that today's classical public key algorithms will become obsolete [2]. While data content is constantly becoming more vital and its security is getting compromised more frequently, the need for resilient schemes in cryptography is of high importance. Quantum cryptography promises unconditional security based on the laws of nature rather than computational security, providing a combat to the threat of quantum computers [3, 4, 5]. Quantum key distribution (QKD) protocols can establish a private key encryption between two remote parties, ensuring the generation of a shared key, which can be used in symmetric cryptographic algorithms, such as the Advanced Encryption Standard (AES). Since the emergence of the first QKD protocol in 1984 [6], rapid progress has been made in the last decade, resulting in increases of transmission distances and key rates [3]. Fiber links have been realized and scaled up to the network size, as in SECOQC [7]. Despite the great progress, this technology has to overcome a major limitation. The lack of efficient quantum repeaters results in unamplified signals [8], thus leaving weak single-photon pulses extremely exposed to the induced attenuation due to the propagation. Therefore, serious limitations occur regarding the distance reach of such links, especially in fiber media. Currently, the maximum ground-based communication range achieved is 509 km infiber [9] and this limitation for repeaterless links sets a major obstacle in the way towards a quantum-secured global mesh network. Even though QKD deployments over terrestrial free-space optics (FSO) links have been studied as a potential candidate for short-reach point-to-point QKD link connections [10], the maximum ground-based QKD link range that has been achieved is 144 km in free space [11]. Recently, satellite QKD has proved to be able to overcome the above range limits, thereby enabling secure communication globally [12]. Moreover, satellite QKD can enable secure network communication between multiple nodes on Earth, based on an efficient scheduling of communication with a set of ground stations [13]. Satellite QKD has been demonstrated for both prepare and measure and entangled-based protocols. Entangled-based QKD does not require the satellite to act as a trusted node and therefore can offer direct inter-optical ground station (OGS) connection. However, the satellite needs to be able to communicate with the two OGSs at the same time, and since the necessary quantum correlations are reduced from loss in the two satellite-to-ground channels, the resultant key rates are relatively low [14; 15]. On the other hand, even though they require the use of a trusted satellite as a relay station, prepare and measure protocol implementations do not face these obstacles and at the same time they can be more practical at the moment. In addition, the technical complexity for the generation of single photons can be significantly reduced by employing attenuated coherent laser sources. In this respect, laser-light QKD schemes are more controllable, require less complex sources and can be used more efficiently to extend the distance of secure QKD to the global scale. Optical satellite communication links are strongly dependent on free-space channel conditions [16]. Among the atmospheric phenomena that degrade optical satellite links, cloud coverage is the dominant one, which causes the blockage of the link [17]. For the mitigation of cloud coverage, the OGS diversity technique is employed [18; 19; 20], taking advantage of the spatial inhomogeneity of clouds.