Quantum correlations between two particles show nonclassical propertie
s that can be used for providing secure transmission of information. W
e present a quantum cryptographic systemin which users store particles
in a transmission center, where their quantum states are preserved us
ing quantum memories. Correlations between the particles stored by two
users are created upon request by projecting their product state onto
a fully entangled state. Our system allows for secure communication b
etween any pair of users who have particles in the same center. Unlike
other quantum cryptographic systems, it can work without quantum chan
nels and it is suitable for building a quantum cryptographic network.
We also present a modified system with many centers.