The European Space Agency (ESA) has supported since 2002 several studies in the field of quantum communications for space systems. As a result of these studies, a European research consortium submitted the mission proposal Space-QUEST (“QUantum Entanglement for Space experimenTs”) to the European Life and Physical Sciences in Space Programme of ESA, aiming at a quantum communications space-to-ground experiment from the International Space Station ISS (rated as “outstanding” within ESA’s ELIPS-2 AO-2004-054). To bring quantum entanglement for the first time in the space environment will open a new range of fundamental physics experiments.
From a fundamental point of view, the important question is whether there are limits on the distance between two entangled quantum systems. Primarily, such experiments would allow to expand the scale for testing the validity of quantum physics theory by several orders of magnitude in distance i.e. beyond the capabilities of purely earth-based laboratories. Earth-based photonic propagation in quantum experiments using glass fibres is limited to some 100 km with present-day technology. The long distances accessible when going into space are therefore essential with respect to both fundamental and application aspects. Space provides a unique "lab"-environment for entanglement: In the case of photons, the space environment allows much larger propagation distances compared to purely earth-bound free space experiments. Due to the lack of atmosphere and due to the fact that space links do not encounter the problem of obscured line-of-sight by unwanted objects or due to the curvature of the Earth. On the long run, experiments on quantum entanglement in a space might even provide the basis for fundamental tests of the interplay between gravitation and quantum physics.
Moreover, quantum mechanics is also the basis for emerging technologies of quantum information science,
presently one of the most active research felds in physics. Today's most prominent application is quantum key distribution (QKD), 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 of quantum communication on a global scale, a task impossible on ground with current optical fber and photon-detector technology. Currently, quantum communication on ground is limited to the order of some 100 of kilometers. Bringing quantum communication into space is the only way to overcome this limit with state-of-the-art technology. Another area of applications is in metrology, where quantum clock synchronization and quantum positioning are studied. Furthermore, sources of quantum states in space may have applications in the new field of quantum astronomy.
The European Space Agency (ESA) is encouraging and actively supporting the teaming-up of European scientists who share a common interest in performing experiments using infrastructure in a space environment including the International Space Station or ground facilities. The Topical Team for supporting the Space-QUEST founded in summer 2007 comprise researchers from academia who are already actively involved in space experiments, as well as partners who are presently not yet involved in space research. The objective is to support the Space-QUEST proposal within ESA member states.