A fundamental test and an application of quantum entanglement

This work describes two experiments that are based on correlation measurements between entangled photons, spatially separated by 144 km between the Canary Islands, La Palma and Tenerife. The first of which contributes to the debate of whether or not quantum mechanical predictions can be described within a local realistic frame, a question that plays a fundamental role in the foundation of quantum mechanics ever since the famous Einstein-Podolsky-Rosen (EPR) 'paradox'. The experiment presented is a test of the CHSH form of Bell's inequality, simultaneously closing two out of three possible 'loopholes' for local realism that can arise in an experimental Bell test. These two loopholes are the locality loophole and the freedom-of-choice loophole. The latter has not been addressed experimentally so far and was closed for the first time in our experiment by space-like separating the setting choice from the photon pair emission. Within the second experiment described entanglement based quantum key distribution (QKD) was demonstrated, probably one of the most mature applications in the field of quantum information and quantum communication. It has recently been emphasized that entanglement based quantum key distribution systems can tolerate higher channel losses compared to systems based on weak coherent laser pulses. This is in particular the case when the entangled photon source is located symmetrically between the two receiver stations, called Alice and Bob. We experimentally studied this important advantage by implementing different entanglement based QKD setups on the 144 km free-space, operated at two-photon attenuations of 35 dB, 58 dB and 71 dB, respectively. (author)