  We analyze elementary building blocks for quantum repeaters based on fiber
channels and memory stations. Implementations are considered for three
different physical platforms, for which suitable components are available:
quantum dots, trapped atoms and ions, and color centers in diamond. We evaluate
and compare the performances of basic quantum repeater links for these
platforms both for present-day, state-of-the-art experimental parameters as
well as for parameters that could in principle be reached in the future. The
ultimate goal is to experimentally explore regimes at intermediate distances,
up to a few 100 km, in which the repeater-assisted secret key transmission
rates exceed the maximal rate achievable via direct transmission. We consider
two different protocols, one of which is better adapted to the higher source
clock rate and lower memory coherence time of the quantum dot platform, while
the other circumvents the need of writing photonic quantum states into the
memories in a heralded, non-destructive fashion. The elementary building blocks
and protocols can be connected in a modular form to construct a quantum
repeater system that is potentially scalable to large distances.
