  Entanglement distribution is key to the success of secure communication
schemes based on quantum mechanics, and there is a strong need for an ultimate
architecture able to overcome the limitations of recent proposals such as those
based on entanglement percolation or quantum repeaters. In this work we provide
broad theoretical background for the development of such technologies. In
particular, we investigate the question of whether entanglement distribution is
more efficient if some amount of entanglement -- or some amount of correlations
in general -- is available prior to the transmission stage of the protocol. We
show that in the presence of noise the answer to this question strongly depends
on the type of noise and on the way how entanglement is quantified. On the one
hand, subadditive entanglement measures do not show advantage of preshared
correlations if entanglement is established via combinations of single-qubit
Pauli channels. On the other hand, based on the superadditivity conjecture of
distillable entanglement, we provide evidence that this phenomenon occurs for
this measure. These results strongly suggest that sending one half of some pure
entangled state down a noisy channel is the best strategy for any subadditive
entanglement quantifier, thus paving the way to a unified approach for
entanglement distribution which does not depend on the nature of noise. We also
provide general bounds for entanglement distribution involving quantum discord,
and present a counter-intuitive phenomenon of the advantage of arbitrarily
little entangled states over maximally entangled ones, which may also occur for
quantum channels relevant in experiments.
