  Quantum communication enables the implementation of tasks that are
unachievable with classical resources. However, losses on the communication
channel preclude the direct long-distance transmission of quantum information
in many relevant scenarios. In principle quantum repeaters allow one to
overcome losses. However, realistic hardware parameters make long-distance
quantum communication a challenge in practice. For instance, in many protocols
an entangled pair is generated that needs to wait in quantum memory until the
generation of an additional pair. During this waiting time the first pair
decoheres, impacting the quality of the final entanglement produced. At the
cost of a lower rate, this effect can be mitigated by imposing a cut-off
condition. For instance, a maximum storage time for entanglement after which it
is discarded. In this work, we optimize the cut-offs for quantum repeater
chains. First, we develop an algorithm for computing the probability
distribution of the waiting time and fidelity of entanglement produced by
repeater chain protocols which include a cut-off. Then, we use the algorithm to
optimize cut-offs in order to maximize secret-key rate between the end nodes of
the repeater chain. We find that the use of the optimal cut-off extends the
parameter regime for which secret key can be generated and moreover
significantly increases the secret-key rate for a large range of parameters.
