  Most traditional applications of quantum cryptography are point-to-point
communications, in which only two users can exchange keys. In this letter, we
present a network scheme that enable quantum key distribution between
multi-user with wavelength addressing. Considering the current state of
wavelength division multiplexing technique, dozens or hundreds of users can be
connected to such a network and directly exchange keys with each other. With
the scheme, a 4-user demonstration network was built up and key exchanges were
performed.
