Quantum-limited measurements of optical signals from a geostationary satellite
Kevin Günthner, Imran Khan, Dominique Elser, Birgit Stiller, Ömer Bayraktar, Christian R. Müller, Karen Saucke, Daniel Tröndle, Frank F. Heine, S. Seel, Peter Greulich, Herwig Zech, Björn Gütlich, S. Philipp-May, Christoph Marquardt, Gerd Leuchs
The measurement of quantum signals that traveled through long distances is of fundamental and technological interest. We present quantum-limited coherent measurements of optical signals, sent from a satellite in geostationary Earth orbit to an optical ground station. We bound the excess noise that the quantum states could have acquired after having propagated 38600 km through Earth's gravitational potential as well as its turbulent atmosphere. Our results indicate that quantum communication is feasible in principle in such a scenario, highlighting the possibility of a global quantum key distribution network for secure communication.
The measurement of quantum signals that traveled through long distances is of fundamental and technological interest. We present quantum-limited coherent measurements of optical signals, sent from a satellite in geostationary Earth orbit to an optical ground station. We bound the excess noise that the quantum states could have acquired after having propagated 38600 km through Earth's gravitational potential as well as its turbulent atmosphere. Our results indicate that quantum communication is feasible in principle in such a scenario, highlighting the possibility of a global quantum key distribution network for secure communication.
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.
This study aims at extending the concept of QKD to the optical free-space quantum communications domain. Moreover, the feasibility of a global satellite based quantum key distribution, in the low earth orbit (LEO) is investigated. Inline with this, three different link scenarios, including ground-to-ground, ground-to-satellite and satellite-to-satellite are considered. Moreover, system performance is analysed to show the sensitivity of the link to effect such as atmospheric attenuation, turbulence and detector noise. Generally, these factors are assumed to impair system performance by causing an overall increase in the systems quantum bit error rate (QBER), let alone limiting the communication bit rate and transmission distance of the link. The obtained results reveal the feasibility of satellite-based quantum key distribution in the LEO platform, a concept which could be potentially extended to geostationary earth orbit (GEO).
Optical quantum communication utilizing satellite platforms has the potential to extend the reach of quantum key distribution (QKD) from terrestrial limits of ∼200 km to global scales. We have developed a thorough numerical simulation using realistic simulated orbits and incorporating the effects of pointing error, diffraction, atmosphere and telescope design, to obtain estimates of the loss and background noise which a satellite-based system would experience. Combining with quantum optics simulations of sources and detection, we determine the length of secure key for QKD, as well as entanglement visibility and achievable distances for fundamental experiments. We analyse the performance of a low Earth orbit satellite for downlink and uplink scenarios of the quantum optical signals. We argue that the advantages of locating the quantum source on the ground justify a greater scientific interest in an uplink as compared to a downlink. An uplink with a ground transmitter of at least 25 cm diameter and a 30 cm receiver telescope on the satellite could be used to successfully perform QKD multiple times per week with either an entangled photon source or with a weak coherent pulse source, as well as perform long-distance Bell tests and quantum teleportation. Our model helps to resolve important design considerations such as operating wavelength, type and specifications of sources and detectors, telescope designs, specific orbits and ground station locations, in view of anticipated overall system performance.
The establishment of quantum communication links over a global scale is enabled by satellite nodes. We examine the influence of the Earth's atmosphere on the performance of quantum optical communication channels with emphasis on the downlink scenario. We derive the geometrical path length between a moving low Earth orbit satellite and an optical ground station as a function of the ground observer's zenith angle, his geographical latitude, and the meridian inclination angle of the satellite. We show that the signal distortions due to regular atmospheric refraction, atmospheric absorption, and turbulence have a strong dependence on the zenith angle. The observed saturation of transmittance fluctuations for large zenith angles is explained. The probability distribution of the transmittance for slant propagation paths is derived, which enables us to perform the security analysis of decoy-state protocols implemented via satellite-mediated links.
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.
Satellite quantum communications are emerging within the panorama of quantum technologies as a more effective strategy to distribute completely secure keys at very long distances, therefore playing an important role in the architecture of a large-scale quantum network.In this work, we apply and extend recent results in free-space quantum communications to determine the ultimate limits at which secret (and entanglement) bits can be distributed via satellites.Our study is comprehensive of the various practical scenarios, encompassing both downlink and uplink configurations, with satellites at different altitudes and zenith angles.It includes effects of diffraction, extinction, background noise, and fading, due to pointing errors and atmospheric turbulence (appropriately developed for slant distances).Besides identifying upper bounds, we also discuss lower bounds, i.e., achievable rates for key generation and entanglement distribution.In particular, we study the composable finite-size secret key rates that are achievable by protocols of continuous variable quantum key distribution, for both downlink and uplink, showing the feasibility of this approach for all configurations.Finally, we present a study with a sun-synchronous satellite, showing that its key distribution rate is able to outperform a ground chain of ideal quantum repeaters.