In recent years, fiber-based quantum key distribution systems have emerged in a growing number of sectors. However, fiber optic links limit the distance that can be covered in point-to-point connections, so developments in free-space quantum communications are important. These can be used to establish free-space QKD links between buildings or to connect remote ground stations using satellite systems. In our research, we are developing an entanglement-based free-space QKD system based and we are simulating quantum communication satellite systems based on entanglement.
In recent years, fiber-based quantum key distribution systems have emerged in a growing number of sectors. However, fiber optic links limit the distance that can be covered in point-to-point connections, so developments in free-space quantum communications are important. These can be used to establish free-space QKD links between buildings or to connect remote ground stations using satellite systems. In our research, we are developing an entanglement-based free-space QKD system based and we are simulating quantum communication satellite systems based on entanglement.
The optical quantum network plays an important role in quantum information technology, enabling important applications including quantum communication, distributed quantum computing, quantum remote sensing, etc. Without the fiber connection limitation, free-space quantum network is free from fiber scattering loss, and enables more flexible quantum network connections. In recent years, rapid progress has been achieved in free-space quantum networks using fixed ground stations, satellites, and mobile platforms. Fixed ground stations are the primary testbeds for free-space experiments, and link distances of up to 144 km have been demonstrated between selected locations. Satellites, free from atmospheric turbulence and absorption, further boost the distance to over a thousand kilometers, enabling intercontinental connections. Besides fixed ground stations and satellites, mobile platforms offer highly flexible connections to end users, as fully presented using drones, after preliminary works using balloons, aircraft, helicopters, etc. Here we review the free-space experiments based on these three platforms, and analyse and compare their characteristics, advantages, and limitations in different scenarios, respectively. In the prospectives, we discuss their unique and complementary roles, and the key challenges and enabling technologies toward the ultimate goal of a practical quantum network, in conjunction with the fiber-based quantum network.
Quantum key distribution is now a mature quantum communication protocol which allows the verifiably secure sharing of encryption keys between two communicating parties. It seeks to address potential vulnerabilities of data transmission and storage, offering a realistic possibility to share encryption keys which are robust to eavesdropping attacks and future-proof against hacking. Fibre-optic implementations of quantum key distribution currently have a limited practical transmission distance, of less than 400 km, making commercial applications limited. Quantum-specific amplifier/repeater technology is not yet mature enough to increase the transmission distance to achieve global capabilities. Optical fiber is also impractical and expensive for applications where a remote area or moving platform are involved. In recent years, long-distance free-space quantum communications using low-Earth orbit satellites has seen an increase in interest from the academic community as well as from industrial organisations and national research institutes. The source of this new interest was a series of proof-of-principle demonstrations of satellite-based quantum key distribution in 2017. The use of free-space channels implementing airborne or satellite platforms also opens a range of new applications for quantum communications, as they allow coverage of remote areas, moving platforms, and also avoid the requirement of spooling fibre through volatile regions. This presentation will give a general introduction to satellite-based quantum communications, an overview of the field, and discuss future endeavours. The talk will also include an overview of our research into novel photonic technology, such as the use of detector array technology, for the optical ground station receiver.
Various optical quantum key distribution protocols have been proposed and examined to implement a global quantum communication system both in fiber-based network and in free-space systems. Employing a satellite as a quantum relay is a possible solution to increase the secure covered distance, which is available for sharing secure keys between two remote ground stations. In this paper, combining with the idea of measurement device independent quantum key distribution, a modified free-space quantum communication protocol is proposed to build a QKD network with untrusted nodes, where the parties on the ground send quantum signals to the untrusted satellite relay that perform a joint measurement on the incoming signals. Our results demonstrate that though the modified protocol suffers higher link attentuation than the entanglement-based quantum communication protocol owing to higher effects of atmospheric turbulence for uplink, our modified protocol provides secure quantum communications with untrusted relays. Furthermore, the complex and non-space-qualified sources remain on the ground in our scheme , which makes our scheme more practical for bridging a seamless connection form ground to space.
On the brink of the so-called quantum age, researchers from around the globe go to great lengths to better understand the quantum nature of light. Browne et al. (2017, p.1) state: 'Quantum light remains at the heart of many of the most promising and potentially transformative quantum technologies'. With the huge progress in quantum computing over the last years, quantum key distribution (QKD) is equally growing in importance. By exchanging secure keys using an optical link, QKD is able to solve security vulnerabilities arising through the insufficient protection of traditional encryption techniques against brute force quantum computer attacks threatening future communications. Pushing the limits of optical free space links for many years, the German Aerospace Center (DLR) has also gathered a lot of experience in QKD applications and is now aiming for a safe and powerful future optical communication network. As QKD networks become reality, optical freespace links ensure ground-satellite and satellite-satellite communication lines. Essential for ground-satellite links are accurate telescopes, which reduce satellite optics requirements due to their ability to gather light and focus collimated beams.
Quantum key distribution (QKD) enables tap-proof exchange of cryptographic keys guaranteed by the very laws of physics. One of the last remaining roadblocks on the way towards widespread deployment of QKD is the high loss experienced during terrestrial distribution of photons, which limits the distance between the communicating parties. A viable solution to this problem is to avoid the terrestrial distribution of photons via optical fibers altogether and instead transmit them via satellite links, where the loss is dominated by diffraction instead of absorption and scattering. First dedicated satellite missions have demonstrated the feasibility of this approach, albeit with relatively low secure key rates. In order for QKD to become commercially viable, the design of future satellite missions must be focused on achieving higher key rates at lower system costs. Current satellite missions are already operating at almost optimal system parameters, which leaves little room for enhancing the key rates with currently deployed technology. Instead, fundamentally new techniques are required to drastically reduce the costs per secret bit shared between two distant parties. Entanglement-based protocols provide the highest level of security and offer several pathways for increasing the key rate by exploiting the underlying quantum correlations. In this contribution, we review the most relevant advances in entanglement-based QKD which are implementable over free-space links and thus enable distribution of secure keys from orbit. The development of satellite missions is notoriously lengthy. Possible candidates for a new generation of quantum payloads should therefore be scrutinized as early as possible in order to advance the development of quantum technologies for space applications.
Long-distance quantum key distribution (QKD) is a critical component of a quantum network. When network users are within a city, they often use existing telecommunication fiber-optic networks. This method is problematic for QKD on a large scale between cities and countries. Direct connecting of distant consumers with quantum single-photon lines is not possible, because optical fiber involves attenuation. This problem is addressed by constructing a large number of trusted intermediate nodes, but there are concerns with the reliability of each node. To increase the quantum network's dependability, a method of building a satellite quantum network is actively developing. The only one trusted satellite node is required for satellite QKD between any two points on the Earth's surface. Furthermore, a satellite quantum link can also be utilized to add another communication channel in the existing fiber-optic networks that are thousands of kilometers apart. We describe the creation of ground infrastructure elements for receiving single photons in various polarization quantum states from satellites. Ground-based receiving nodes are based on two telescopes with an aperture of 1.2 m and 0.6 m located in remote cities.