Free-Space Optical (FSO) communication is a technology that enables ultra-high-rate data exchange and will be crucial in enabling secure Quantum Key Distribution (QKD) over long distances. However, FSO communication systems are comparatively complex technologies, comprising a variety of subsystems that all have to work together in a coordinated way. Hence, assessing the feasibility of establishing a (classical or quantum) link in a given communication scenario is a difficult task in itself. Here we introduce QCalc, a software tool that aims at easing and streamlining this process, allowing the user to keep track of all the relevant parameters in a link budget. This software computes the total transmissivity, and thus, the signal intensity observed at the receiver for any given parameters of the transceivers and the FSO communication channel. The estimated signal intensity can then be used to compute either the link budget in a classical communication scenario or the secure key rate of a QKD protocol.
Free-Space Optical (FSO) communication is a technology that enables ultra-high-rate data exchange and will be crucial in enabling secure Quantum Key Distribution (QKD) over long distances. However, FSO communication systems are comparatively complex technologies, comprising a variety of subsystems that all have to work together in a coordinated way. Hence, assessing the feasibility of establishing a (classical or quantum) link in a given communication scenario is a difficult task in itself. Here we introduce QCalc, a software tool that aims at easing and streamlining this process, allowing the user to keep track of all the relevant parameters in a link budget. This software computes the total transmissivity, and thus, the signal intensity observed at the receiver for any given parameters of the transceivers and the FSO communication channel. The estimated signal intensity can then be used to compute either the link budget in a classical communication scenario or the secure key rate of a QKD protocol.
Nowadays, high security levels are required to transmit critical information for government, private and personal sectors. As a countermeasure, the Quantum Key Distribution systems are the best option in order to protect this information because it provides unconditional security. In addition, increasing the transmission distance is a highlight. Therefore, the Free-Space Optical Quantum Key Distribution systems (FSO-QKD) present an innovative way for sharing secure information between two parties located at ground stations, spacecraft or aircraft. However, these scenarios present several challenges regarding the hardware, protocols and techniques used that must be solved in order to enhance the performance parameters (security level, distance link, final secret key rate, among others) for any QKD system; although, in particular, a high transmission performance is required for both the classical and quantum channels. These issues impose the roadmap and trends in the research, academic and manufacturing sectors around the world.
Due to the recent rapid growth of computing technologies, today's encrypted information based on classical computational complexities will soon no longer remain secure. Quantum cryptography, or quantum key distribution (QKD), relying on the physical laws of quantum mechanics, stands out as a more powerful cryptographic solution that enables the secret key to be shared securely over publicly unsecured communication channels in the presence of eavesdroppers. QKD can be possibly implemented over both optical fiber and free-space optical (FSO) links. Compared to optical fiber, FSO has the immediate advantage of flexibility and cost-effectiveness in terms of infrastructure deployment. More interestingly, recent advancements in satellite communications using FSO links promise the future of a global QKD network for highly secure applications. This paper reviews the development of QKD over FSO for both terrestrial and satellite channels, which highlights the most up-to-date QKD protocols. Owing to how the information is encoded, different QKD protocols are classified into technology-focused categories, where their properties are reviewed and compared. The feasibility of current QKD protocols is then investigated with respect to its specific application scenarios for terrestrial and satellite FSO links, equipped with results from both theoretical and experimental studies. Based on the latest free-space QKD achievements, novel perspectives for future developments and challenges to enhance the secrecy performance and reduce implementation cost are then summarized and comprehensively discussed. The potential combination of QKD and physical layer cryptography for the newly emerging wireless mobile communications networks involved with unmanned aerial vehicles (UAV) is also introduced.
Satellite based quantum key distribution (QKD) enables the delivery of keys for quantum secure communications over long distances. Maturity of the technology as well as industrial interest keep increasing. So does the technology readiness of satellite free-space optical communications. A satellite QKD system comprises a quantum communication subsystem and a classical communication subsystem (public channel). Both are implemented with free-space optics. Thus, in satellite QKD system design, there are strong synergies that should be exploited as much as possible and lead to an all-optical satellite QKD system. In this paper, we present a system like this locating all optical channels in ITU DWDM C-band. We focus on the overall conceptual design and the setup of the optical channels for quantum and classical signal transmission. The system description addresses the breadboards of a transmitter laser terminal (Alice terminal), a receiver laser terminal, (Bob terminal), the public channel implementation, the interfaced QKD system and the deployed encryption system. The design basis for the Alice terminal is the laser terminal development OSIRISv3. The design basis for the Bob terminal is the ground station development THRUST. The later contains an adaptive optics correction to enable single mode fiber coupling. This enables the interfacing to almost arbitrary quantum receivers such as the Bob modules used in the described experiment. The public channel is composed of a bi-directional 1 Gbps IM/DD system and a MODEM that implements a proprietary waveform optimized for free-space channels. The system was experimentally analyzed in a field test in the framework of the German initiative QuNET which addresses the use case of quantum secure communication for authorities. The results of the experiment are used to model a feasible LEO satellite-ground link. Performance indicators such as quantum bit error rate and secure key rate of a potential mission are estimated analytically.
In order to asses free space optical links, using a Cassegrain telescope, for both classical (i.e. coherent and direct detection) and quantum (coherent, Dolinar-Kennedy system) several scenarios were analyzed. This analysis was conducted through a software tool that uses some specific parameters imposed by the environmental condition and hardware used, such as: pointing and tracking errors, turbulence, modulations schemes, and visibility, among others; in order to obtain the general performance parameters of the simulated link, such as: bit error rate and the photons number. The software developed allows the user to produce useful way to assess the possible success of the implementation of the free space optical link, or a way to identify phenomena that might suggest hardware adjustments to improve the overall performance of the system. Results allow the revision of the expected link performance for a quantum key distribution system; although it is possible to use it for general optical quantum communications systems.
In response to the increasing demands for reliable, fast, and secure communications beyond 5G scenarios, the high-capacity networks have become a focal point. Quantum communication is at the forefront of this research, offering unmatched throughput and security. A free space optics (FSO) communication system integrated with fiber-end is designed and investigated using the Bennett–Brassard 1984 quantum key distribution (BB84-QKD) protocol. Simulation results show that reliable transmission can be achieved over a 10–15 km fiber length with a signal power of −19.54 dBm and high optical-to-signal noise of 72.28–95.30 dB over a 550 m FSO range under clear air, haze, fog, and rain conditions at a data rate of 1 Gbps. Also, the system using rectilinearly and circularly polarized signals exhibits a Stokes parameter intensity of −4.69 to −35.65 dBm and −7.7 to −35.66 dBm Stokes parameter intensity, respectively, over 100–700 m FSO range under diverse weather conditions. Likewise, for the same scenario, an FSO range of 100 m incorporating 2.5–4 mrad beam divergence provides the Stokes power intensity of −6.03 to −11.1 dBm and −9.04 to −14.12 dBm for rectilinearly and circularly polarized signals, respectively. Moreover, compared to existing works, this work allows faithful and secure signal transmission in free space, considering FSO–fiber link losses.
Due to the advancements in the field of quantum computing and possible compromise of existing key-exchange algorithms, it is essential to explore performance and efficiency of new key-exchange techniques, such as Quantum Key Distribution (QKD). QKD is possible over fiber cable and Free Space Optical (FSO) links. Fiber based QKD may be used if direct line of sight link is not possible, but it brings along many problems related to material properties of fiber cable and detector limitations. It is therefore more convenient to use free-space based QKD whenever physically possible. Furthermore, FSO links can be satellite-supported in order to generate worldwide distribution of quantum keys. The FSO however greatly depends on weather conditions and link length, thus it is very important to evaluate these effects on the size of quantum key. In this paper, simulation-based evaluation of FSO QKD is done given tropical weather conditions specific for Malaysia. The paper discusses how different factors, such as link distance, weather, beam intensity and transceivers setup, influence key size when BB84 QKD protocol is used. For each weather condition specific for Malaysia key size was obtained, and the percentage of the received key compared to the total generated key is calculated. The results show the size of the final encryption key after being transmitted over FSO channel influenced by various weather conditions, which implies that it is possible to use FSO based QKD in Malaysia successfully.