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Satellites offer the means to extend quantum communication and quantum key distribution towards global distances. We will outline the proposed QEYSSat mission proposal, which involves a quantum receiver onboard a satellite that measures quantum signals sent up from the ground. We present recent studies on the expected performance for quantum links from ground to space. Further studies include the demonstration of high-loss quantum transmission, and analyzing the effects of a fluctuating optical link on quantum signals and how these fluctuations can actually be exploited to improve the link performance.

We design and built a novel optical terminal specifically designed for free-space communication operating at light levels at the quantum limit, such as in quantum communication. Our system is particularly well suited for this task, as it is based on an all-spheric catadioptric design, which allows for large and un-obstructed apertures. This design offers an easier and cheaper approach to building high-quality optical terminals with large apertures than schemes based on offaxis parabolic mirrors. We utilized an off-axis version of the original Schwarzschild concentric design, and correct the spherical aberration by substituting the original on-axis secondary spherical mirror with an off-axis catadioptric secondary mirror. A prototype of the optical terminal was realized and tested and it meet the expected performance.

Fundamental quantum optics test as well as quantum cryptography and quantum teleportation are based on the distribution of single quantum states and quantum entanglement respectively. We will discuss recent experimental achievements in the field of long-distance quantum communication via optical fiber as well as in free-space over a record breaking distance of 144 km. The European Space Agency (ESA) has supported a range of studies in the field of quantum physics and quantuminformation science in space for several years, and consequently a mission proposal Space-QUEST Quantum Entanglement for Space Experiments was submitted to the European Life and Physical Sciences in Space Program. This proposal envisions to perform space-to-ground quantum communication tests from the International Space Station (ISS) and will presented in this article.

We show how the probabilistic interpretation of quantum mechanics leads to unavoidable quantum noise, even for deterministic evolution of the quantum state. Far from being a nuisance, this consequent quantum randomness is at the heart of new concepts in technology. We discuss explicitly the quantum random number generator based on the partitition noise at the beam splitter. Another application of quantum noise is quantum cryptography, where the randomness of the detection event leads to the generation of a random cryptographic key at two locations without the necessity of transporting that key from A to B. Finally, we will show how quantum noise is an intrinsically important part of quantum teleportation, and we conclude with a brief discussion of the possibilities of free-space quantum communication.

Quantum physics experiments in space using entangled photons and satellites are within reach of current technology. We propose a series of fundamental quantum physics experiments that make advantageous use of the space infrastructure with specific emphasis on the satellite-based distribution of entangled photon pairs. The experiments are feasible already today and will eventually lead to a Bell-experiment over thousands of kilometers, thus demonstrating quantum correlations over distances which cannot be achieved by purely earth-bound experiments., Comment: 15 pages, 10 figures, to appear in: SPIE Proceedings on Quantum Communications and Quantum Imaging (2003)