  A workflow for evaluation of entanglement source quality is proposed. Based
on quantum state density matrices obtained from theoretical models and
experimental data, we make an estimate of a potential performance of a quantum
entanglement source in quantum key distribution protocols. This workflow is
showcased for continuously pumped spontaneous parametric down-conversion (SPDC)
source, where it highlights the trade-off between entangled pair generation
rate and entanglement quality caused by multiphoton nature of the generated
quantum states. We employ this characterization technique to show that secure
key rate of down-converted photon pairs is limited to 0.029 bits per detection
window due to intrinsic multiphoton contributions. We also report that there
exists one optimum gain for continuous-wave down-conversion sources. We find a
bound for secure key rate extracted from SPDC sources and make a comparison
with perfectly single-pair quantum states, such as those produced by quantum
dots.
