One of the most outstanding differences between quantum systems and our well-known environment is entanglement. More than 70 years after Einstein, Podolsky and Rosen did not believe the completeness of the laws of quantum mechanics, because their “Gedankenexperiment” EPR-pairs would be in strong conflict with observable nature. Despite up to now no experiment closed all loop holes at the same time, quantum effects are dominant for systems described by laws of quantum mechanics.

Experimentally advanced groups generate them routinely and perform very interesting experiments with them: Teleportation, quantum gates, quantum computing (Shor’s algorithm). If more than one pair is involved, pulsed systems with short pump pulses need to be the source. The high peak-power is also responsible for high rates during the potential short emission times.

In contrast to that, continuous wave (CW) excitation is perfect suited, if experiments and measurements are performed with only one pair involved in each measurement. As it is the case for QKD, multiple pairs should be avoided anyhow. Due to the nonlinear effect of Spontaneous Parametric Down-Conversion (SPDC) a pump photon is split up in 2 daughter photons. Pairs are not necessarily entangled, entanglement occurs only if two independent generation mechanisms exist that are indistinguishable.