Quantum communication uses the laws of quantum physics to encode and decode information and hence provides much more efficient ways of communication than are classically possible. Additionally employing quantum entanglement offers access to a new domain of communication protocols, such as Quantum Dense Coding, Quantum Teleportation or Quantum Cryptography. Photonic realizations allow implementing quantum communication over long distances. To achieve quantum networking on a global scale, photonic free-space links via satellites are a promising future trend.
Entanglement-based quantum communication schemes rely on an efficient source of high-quality quantum-entangled photons. Over the last decade extensive effort has been put into the development and enhancement of highly efficient sources. Using spontaneous parametric down conversion (SPDC) in nonlinear crystals turned out to be the most successful method of creating polarization-entangled photon pairs. To date, periodically poled crystals allow collinear setups of very high brightness.
We present a highly efficient source of polarization-entangled photons whose performance in pair-production rate, phase stability and compactness by far exceeds that of previous approaches. The superior brightness and quality of entanglement of our source enabled us to cope with high link attenuation, thus allowing to bridge long free-space distances. We performed a proof-of-concept experiment between the Canary Islands of La Palma and Tenerife making use of the European Space Agency's (ESA) optical ground station (OGS). Therefore we used space hardware within a quantum communication free-space experiment. We successfully emulated conditions that are expected for a LEO-satellite down link and we faithfully transmitted a maximally quantum-entangled state over a long-distance free-space link of 144km. These results pave the way for first quantum communication experiments in space.