  Gradual loss of the entanglement of a twin beam containing around 25 photon
pairs with the increasing external noise is experimentally investigated. The
entanglement is quantified by the non-classicality depths and the
non-classicality counting parameters related to several non-classicality
criteria. The reduction of intensity moments of the analyzed multi-mode twin
beams to single-mode ones allows to determine the negativity as another
quantifier of the entanglement. Both the raw photocount histograms and the
reconstructed photon-number distributions are analyzed in parallel.
