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I.INTRODUCTIONQuantum 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]. A.Laser Communication Terminal for Quantum Key DistributionA space borne Laser Communication Terminal (LCT) is capable to relay quantum key distribution (QKD) between a large number of hubs on ground. In contrast to other satellite-to-ground communication scenarios, QKD does not require real-time availability. Keys can be produced at any convenient time in advance and stored until used. B.Continuous Variable Quantum CommunicationStandard telecommunication components allow for an efficient implementation of quantum communication using continuous variables (CV) of light [41-45]. MPL Erlangen implements free space CV quantum communication (see Fig. 1) based on binary phase-shift keying of coherent states and homodyne detection of the optical field quadratures [46-50]. This operating principle is equal to the one of the TESAT/DLR LCTs [51-53]. Therefore, we have a unique shortcut to implement space borne quantum communication at lower cost than other ambitious programs around the world. III.QKD FEASIBILITY VERFICATIONWe have tested the feasibility of this proposal by quantum-limited measurements of signals from the Alphasat TDP1 LCT in geostationary Earth orbit (GEO). The Transportable Adaptive Optical Ground Station (TAOGS) [54, 21], currently located at the Teide observatory on Tenerife, is capable of establishing bi-directional communication links with the Alphasat TDP1 LCT (see Fig. 2). We utilize the downlink by extending the TAOGS with equipment for quantum-limited measurements. Our measurement results [55] show that coherence is well preserved after propagation over more than 38 000 km and through Earth’s atmosphere. By using a low-noise phase-locking mechanism based on homodyne detection, phase-encoded quantum communication protocols become feasible. Fig. 2.Space-to-ground link setup: A Laser Communication Terminal (LCT) is embarked as Technology Demonstration Payload (TDP) on Alphasat in geostationary Earth orbit (GEO). The LCT links to the Transportable Adaptive Optical Ground Station (TAOGS), currently located at the Teide Observatory on Tenerife, Spain. By using homodyne detection for quantum signal acquisition, daylight operation is possible without any constraints. (Picture of Alphasat: ESA)  III.CONCLUSIONOur results underpin the feasibility of satellite quantum communication using existing hardware. In order to optimize the system for quantum communication, our next step is the adaptation of the sender and receiver for applied QKD. Furthermore, on the fundamental research side, the large gravitational potential difference between GEO and ground allows to investigate gravitational effects on quantum states [56-60]. AcknowledgementsThe Laser Communication Terminal (LCT) and the Transportable Adaptive Optical Ground Station (TAOGS) are supported by the German Aerospace Center (DLR) Space Administration, with funds from the Federal Ministry for Economic Affairs and Energy according to a decision of the German Federal Parliament. We thank our colleagues at the FAU computer science building for hosting our receiver setup and Zoran Sodnik for hosting the TAOGS next to the ESA Optical Ground Station (OGS) on Tenerife. REFERENCESC. Gerry and P. Knight, Introductory Quantum Optics, Cambridge University Press,2005). Google Scholar
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