  Quantum Key Distribution (QKD) allows unconditionally secure communication
based on the laws of quantum mechanics rather then assumptions about
computational hardness. Optimizing the operation parameters of a given QKD
implementation is indispensable in order to achieve high secure key rates. So
far, there exists no model that accurately describes entanglement-based QKD
with continuous-wave pump lasers. For the first time, we analyze the underlying
mechanisms for QKD with temporally uniform pair-creation probabilities and
develop a simple but accurate model to calculate optimal trade-offs for maximal
secure key rates. In particular, we find an optimization strategy of the source
brightness for given losses and detection-time resolution. All experimental
parameters utilized by the model can be inferred directly in standard QKD
implementations, and no additional assessment of device performance is
required. Comparison with experimental data shows the validity of our model.
Our results yield a tool to determine optimal operation parameters for already
existing QKD systems, to plan a full QKD implementation from scratch, and to
determine fundamental key rate and distance limits of given connections.
