  The Schrodinger equation is solved for many free particles and their quantum
entanglement is studied via correlation analysis. Converting the Schrodinger
equation in the Madelung hydrodynamic-like form, the quantum mechanics is
extended to open quantum systems by adding Ohmic friction forces. The
dissipative evolution confirms the correlation decay over time, but a new
integral of motion is discovered, being appropriate for storing everlasting
quantum information.
