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development of space-based and ground-based transmitter and receiver con-
cepts for quantum communication hardware. On the space-terminal side, we
have started to investigate the possibility of incorporating an entangled pho-
ton source onboard (existing) laser communication satellite terminals [96]. On
the ground station side, we have performed the first proof-of-concept tests
to demonstrate the feasibility of adapting existing optical ground stations for
satellite-ground quantum communication [97]. It may not be too long until
the first space-based quantum communication experiment with entangled
photons will take off.
3.3 Conclusion and Outlook
Quantum communication has come the long way from purely fundamental
considerations on the nature of quantum physics to the implementation of
novel concepts and technologies of information processing. Somewhat as a
surprise, the role of quantum entanglement has also become more and more
important for the applications. It is now a relevant resource for most advanced
quantum communication schemes and even for novel quantum computing
architectures such as the one-way quantum computer [98,99]. Main future ex-
perimental challenges in the field of quantum communication and quantum
computation certainly include the development of more reliable and more ef-
ficient sources for entanglement. For example, space-based experiments will
require a compact, robust, and efficient source for entangled pairs, while quan-
tum computing schemes and multiparty quantum communication schemes
will benefit from high-fidelity sources of multiparticle entangled states. Also,
improving the interface between photonic qubits and stationary systems such
as atoms or solids will eventually allow the realization of quantum memories,
which are the major building blocks in any quantum communication network.
We have presented a collection of recent advances in the field of
entanglement-based quantum communication. It is fascinating to observe
how the last years have paved the way for key quantum technologies such as
a quantum repeater and even the realization of a satellite-based global quan-
tum communication network. The quantum physics community is about to
reach a stage in which the developed concepts and techniques of quantum
communication, which started from curiosity about fundamental aspects of
the nature of quantum physics, will evolve into technologies and commercial
products on an industrial level. Quantum cryptography has already reached
this stage. We are confident that other technologies will soon follow.
Acknowledgments
We acknowledge discussions and collaborations with our scientific colleagues
and friends. Our work was supported by the Austrian Science Foundation un-
der Project SFB1506; the European Commission under projects IST-QuComm,
RamboQ and SECOQC; the Austrian Research Center Seibersdorf; the
