  A new quantum mechanical notion -- Conditional Density Matrix -- is discussed
and is applied to describe some physical processes. This notion is a natural
generalization of von Neumann density matrix for such processes as divisions of
quantum systems into subsystems and reunifications of subsystems into new joint
systems. Conditional Density Matrix assigns a quantum state to a subsystem of a
composite system under condition that another part of the composite system is
in some pure state.
