  We develop a model for practical, entanglement-based long-distance quantum
key distribution employing entanglement swapping as a key building block.
Relying only on existing off-the-shelf technology, we show how to optimize
resources so as to maximize secret key distribution rates. The tools comprise
lossy transmission links, such as telecom optical fibers or free space,
parametric down-conversion sources of entangled photon pairs, and threshold
detectors that are inefficient and have dark counts. Our analysis provides the
optimal trade-off between detector efficiency and dark counts, which are
usually competing, as well as the optimal source brightness that maximizes the
secret key rate for specified distances (i.e. loss) between sender and
receiver.
