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 38 600 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.Quantum mechanics has successfully undergone a number of fundamental experimental tests since its development [1][2][3]. Still some aspects pose both a theoretical and an experimental challenge, such as the relation of quantum mechanics and gravity [4][5][6]. Quantum-limited measurements of quantum states traveling through long distances in outer space provide both an offer to test quantum mechanics under such extreme conditions and a prerequisite for its use in quantum technology [7]. To this end satellite quantum communication [8][9][10][11][12][13][14][15] promises to provide the currently missing links for global quantum key distribution (QKD). Important experiments in satellite quantum communication have been reported or are currently being devised and set up [16][17][18][19][20][21][22].This work presents and discusses quantum-limited measurements on optical signals sent from a GEOstationary satellite. We report on the first bound of the possible influence of physical effects on the quantum states traveling through Earth's gravitational potential and evaluating its impact on quantum communication.Optical [27]). In parallel, free space quantum communication has made its steps out of laboratories into real-world scenarios [28][29][30][31]. It has turned out that detecting field quadratures (continuous variables) is well suited to combat disturbances from atmospheric turbulence and stray light [32][33][34]. Using these methods, the first implementation of an intra-urban free space quantum link using quantum coherent detection has been reported [35,36]. The advantage of stray light immunity applies as well to classical coherent satellite communication [37]. The similarity between these classical and quantum technologies allows us to make use of the platform of a technologically mature Laser Communication Terminal (LCT) [38][39][40] for future quantum communication (see Fig 1).An important step on this way is to precisely characterize system and channel with respect to their quantum noise behavior. Coherent quantum communication employs encoding of quantum states in phase space and works at the limit of the Heisenberg uncertainty relation [41], but is susceptible to additional technical noise. Our task here is to characterize whether quantum coherence properties are preserved after propagation of quantum states over 38 600 km, through a large part of graviarXiv:1608.03511v2 [quant-...

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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. 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. Standard telecommunication components allow one to efficiently implement quantum communication by measuring field

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