The most advanced quantum application is quantum cryptography, or more correctly known as quantum key distribution (QKD)\ [Scarani et al., Rev.Mod.Phys, 2009] Thereby, the transmission of individual photons between two distant users, called Alice and Bob, allows them to create an highly secure key. The security stems from the simple fact that the state of one individual photon (such as its polarization) can not be fully determined due to Heisenberg's uncertainty relation. Therefore, Alice and Bob can verify the integrity of the transmitted photons simply be testing their error rate.
Currently, such quantum cryptography systems can reach distances on the order of 200 kilometres [Waks et al, Phys.Rev.A,2002], mainly limited by current optical fiber and single-photon-detectors. One clear solution for extending the range is to bring these quantum systems into space onto satellites. Since satellites typically travel over a large portion of Earth's surface, it can achieve quantum communication on a global scale.
As is the case for QKD, also the tests of quantum entanglement, such as Bell-inequality [Bell, Physics 1,1964], can only reach distances of about 200 km on the ground\cite{Ursin2007}. However, it is important to test quantum mechanical predictions such as the strength of entanglement correlations over distances well beyond these limits. By performing such experiments with satellite systems in Earth's orbit, and possibly even beyond that, testing the validity of quantum physics and entanglement on such length scales will give insights into the interplay of quantum physics and relativity.
Experimental demonstrations of these concepts can be realized with existing technology. One viable approach is to design a small-scale payload, such as a micro-sat. This mission can be undertaken on one the relatively short time frame within two to three years. I will outline the envisioned quantum experiments and the requirements for the technology.
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