The use of satellites to distribute entangled photon pairs (and single photons) provides a unique solution for long-distance quantum communication networks. This overcomes the principle limitations of Earth-bound technology, i.e. the narrow range of some 100 km provided by optical fiber and terrestrial free-space links.

EPN 40/3 27of quantum communication on a global scale, a task impossible on ground with current optical fiber and photon-detector technology. Currently, quantum communication on ground is limited to the order of 200 kilometers [4]. Bringing quantum communication into space is the only way to overcome this limit with state-of-the-art technology. Another area of applications is metrology, where quantum clock synchronization and quantum positioning [5] are studied. Furthermore, sources of quantum states in space may have applications in the new field of quantum astronomy. The proposed experimentsWe propose to ESA to perform these experiments in space by placing a quantum transceiver on the external pallet of the European Columbus module at the ISS (see Fig. 1). The entire terminal must not exceed the specifications given for pallet payloads as provided by ESA. The requirements are: size 1.39×1.17×0.86 m 3 , mass < 100 kg, and a peak power consumption of < 250 W, respectively. A preliminary design of a satellite-based quantum transceiver (including an entangled photon source, a weak pulse laser source, single photon detection modules together with two transceiver telescopes) based on state-of-the-art optical communication terminals and adapted to the needs of quantum communication has already been published in [6] (see Fig. 2). e entangled photons are transmitted to two distant ground stations via simultaneous down-links [7], allowing a test on entanglement and the generation of an unconditional secure quantum cryptographic key between stations separated by more than 1000 km.t is an open issue whether quantum laws, originally established to describe nature at the microscopic level of atoms, are also valid in the macroscopic domain such as long distances. Various proposals predict that quantum entanglement is limited to certain mass and length scales or is altered under specific gravitational circumstances. Testing the quantum correlations over distances achievable with systems placed in the Earth orbit or even beyond would allow verifying both the validity of quantum physics and the preservation of entanglement over distances impossible to achieve on ground. Using the large relative velocity of two orbiting satellites, one can perform experiments on entanglement where -due to special relativity -both observers can claim that they have performed the measurement on their system prior to the measurement of the other observer. In such an experiment it is no longer possible to think of any local realistic mechanisms that potentially influence one measurement outcome according to the other one. Moreover, quantum mechanics is also the basis for emerging technologies of quantum information science, presently one of the most active research fields in physics. Today's most prominent application is quantum key distribution (QKD) [3], i.e. the generation of a provably unconditionally secure key at distance, which is not possible with classical cryptography. The use of satellites allows for demonstrations SPAcE-quESt ExPERimE...

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