the incident electrons if one uses spin-sensitive electron counters. In this case the joint probability cannot be expressed as a product of single-terminal probabilities.

Given the general setup to detect entanglement an important issue is to understand how to generate it. This has been discussed in several papers. Most of the existing proposals are based on the generation of Bell states by means of electron-electron interaction. This can be achieved through superconducting correlations [@footnote] in hybrid normal - superconducting [@loss00; @loss01; @lesovik01; @samuelsson03] and superconductor - carbon nanotubes systems [@bena01; @bouchiat02], quantum dots in the Coulomb blockade regime [@saraga03] or Kondo-like impurities [@costa01]. Then, by using energy or spin filters, the two electrons forming the Bell state are separated. The entanglement can be created in the spin or in the orbital [@samuelsson03] degrees of freedom. Very recently, as it is also discussed in Section \[ns\], it was shown that in a mesoscopic multi-terminal conductor entanglement can be produced also in absence of electron interaction [@beenakker03]. Besides electrons, it is possible to produce entangled states with Cooper pairs in superconducting nanocircuits [@plastina01] or by coupling a mesoscopic Josephson junctions with superconducting resonators [@buisson00; @marquardt01; @you02; @plastina03].

Since Bell’s work [@bell64], it 