arXiv:1510.04507 · IEEE International Conference on Space Optical Systems and Applications (ICSOS)
Satellite Quantum Communication via the Alphasat Laser Communication Terminal - Quantum Signals from 36 thousand kilometers above Earth
Dominique Elser, Kevin Gunthner, Imran Khan, Birgit Stiller, Christoph Marquardt, Gerd Leuchs, Karen Saucke, Daniel Trondle, Frank Heine, Stefan Seel, Peter Greulich, Herwig Zech, Bjorn Gutlich, Ines Richter, Rolf Meyer
By harnessing quantum effects, we nowadays can use encryption that is in principle proven to withstand any conceivable attack. These fascinating quantum features have been implemented in metropolitan quantum networks around the world. In order to interconnect such networks over long distances, optical satellite communication is the method of choice. Standard telecommunication components allow one to efficiently implement quantum communication by measuring field quadratures (continuous variables). This opens the possibility to adapt our Laser Communication Terminals (LCTs) to quantum key distribution (QKD). First satellite measurement campaigns are currently validating our approach.
By harnessing quantum effects, we nowadays can use encryption that is in principle proven to withstand any conceivable attack. These fascinating quantum features have been implemented in metropolitan quantum networks around the world. In order to interconnect such networks over long distances, optical satellite communication is the method of choice. Standard telecommunication components allow one to efficiently implement quantum communication by measuring field quadratures (continuous variables). This opens the possibility to adapt our Laser Communication Terminals (LCTs) to quantum key distribution (QKD). First satellite measurement campaigns are currently validating our approach.
Quantum optics [1] can be harnessed to implement cryptographic protocols that are verifiably immune against any conceivable attack [2]. Even quantum computers, that will break most current public keys [3, 4], cannot harm quantum encryption. Based on these intriguing quantum features, metropolitan quantum networks have been implemented around the world [5-15]. However, the long-haul link between metropolitan networks is currently missing [16]. Existing fiber infrastructure is not suitable for this purpose since classical telecom repeaters cannot relay quantum states [2]. Therefore, optical satellite-to-ground communication [17-22] lends itself to bridge intercontinental distances for quantum communication [23-40].
Summary form only given. Quantum key distribution (QKD) has raised increased attention over the past years as one of the most attractive quantum technologies for practical implementation. QKD has already been implemented in intra-city networks all around the world. But up to now, bridging global distances with quantum communication remains an outstanding challenge. A promising candidate to provide this link is via optical satellite communication. As space-to-ground communication is already well developed for classical applications, one can make use of the already existing technology for QKD, i.e. modern Laser Communication Terminals (LCTs) may be adapted for quantum communication. An important first step to achieve this goal is a precise characterization of the system and the channel with regard to their quantum noise behaviour. We present quantum-limited measurements of signals from an LCT located on the satellite Alphasat in geostationary Earth orbit. The LCT generates binary phase shift keyed (BPSK) coherent states originally used for classical data communication. Our receiver is a Transportable Adaptive Optical Ground Station (T-AOGS) located at the Teide observatory on Tenerife. It corrects for phase front distortions in order to interfere the incoming signal with a local oscillator. This allows for heterodyne detection or, by phase-locking the local oscillator, for homodyne detection. The detector itself is quantum noise limited such that it can resolve the quantum uncertainty of the detected states (see Fig. 1). Evaluating the signal variance for homodyne detection, we are able to show that the detected states are quantum limited coherent states. Thus, we conclude that the coherence of the quantum states is preserved after propagation over a distance of 38600 km including the passage through turbulent atmosphere and we give an upper bound for any excess noise that the states might have acquired [1]. Moreover, we extent our analysis to heterodyne detection, characterizing arbitrary field quadratures. Our results indicate that satellite quantum communication is feasible in principle in such a scenario.
Quantum communication offers long-term security especially, but not only, relevant to government and industrial users. It is worth noting that, for the first time in the history of cryptographic encoding, we are currently in the situation that secure communication can be based on the fundamental laws of physics (information theoretical security) rather than on algorithmic security relying on the complexity of algorithms, which is periodically endangered as standard computer technology advances. On a fundamental level, the security of quantum key distribution (QKD) relies on the non-orthogonality of the quantum states used. So even coherent states are well suited for this task, the quantum states that largely describe the light generated by laser systems. Depending on whether one uses detectors resolving single or multiple photon states or detectors measuring the field quadratures, one speaks of, respectively, a discrete- or a continuous-variable description. Continuous-variable QKD with coherent states uses a technology that is very similar to the one employed in classical coherent communication systems, the backbone of today's Internet connections. Here, we review recent developments in this field in two connected regimes: (i) improving QKD equipment by implementing front-end telecom devices and (ii) research into satellite QKD for bridging long distances by building upon existing optical satellite links.This article is part of the themed issue 'Quantum technology for the 21st century'.
Quantum key distribution (QKD) is a cryptographic method enabling two parties to establish a private encryption key. The range of communication of ground-based QKD is limited to an order of 100km, due to in-fibre attenuations and atmospheric losses, and the development of quantum repeaters remains technologically challenging. While trusted-node links make communication over large distances possible, satellite-QKD is required for communication over global distances. By using satellites equipped with high-quality optical links, satellite-QKD can achieve ultra-long-distance quantum communication in the 1000-km range. The significant potential of satellite-QKD for the creation of global quantum networks thus makes it a particularly interesting field of research. In this analysis, we begin with an overview of the technical parameters of performing satellite-QKD, including infrastructure and protocols. We continue with a high-level summary of advancements in satellite-QKD by analysing past, present and proposed satellite-QKD missions and initiatives around the world. We conclude by discussing the technical challenges currently faced in satellite-QKD, which can be tackled through future research in this area.
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.
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.