  Quantum key distribution over probabilistic quantum repeaters is addressed.
We compare, under practical assumptions, two such schemes in terms of their
secure key generation rates per quantum memory. The two schemes under
investigation are the one proposed by Duan et al. in [Nat. 414, 413 (2001)] and
that of Sangouard et al. in [Phys. Rev. A 76, 050301 (2007)]. We consider
various sources of imperfection in both protocols, such as nonzero
double-photon probabilities at the sources, dark counts in detectors, and
inefficiencies in the channel, photodetectors and memories. We also consider
memory decay and dephasing processes in our analysis. For the latter system, we
determine the maximum value of the double-photon probability beyond which
secretkey distillation is not possible. We also find the crossover distance
beyond which the repeater schemes outperform the non-repeater ones. We finally
compare the two protocols in terms of their achievable secret key generation
rates at their optimal settings
