Characterization of spectral entanglement of spontaneous parametric-down conversion biphotons in femtosecond pulsed regime

We verified the operational approach based on the direct measurement of the entanglement degree for bipartite systems. In particular spectral distributions of single counts and coincidence for pure biphoton states generated by a train of short pump pulses have been measured and the entanglement quantifier has been calculated. The approach gives an upper bound of entanglement stored in total biphoton states, which can reach extremely high values up to 104–105.