Effectively classical quantum states for open systems

Notions of robust and 'classical' states for an open quantum system are int roduced and discussed in the framework of the isometric-sweeping decomposit ion. Using the predictability sieve proposed by Zurek [Phys. Rev. D 24 (198 1) 1516; 26 (1982) 1862], 'quasi-classical' states are defined. A number of examples illustrating how the 'quasi-classical' states correspond to class ical points in phase space connected with the measuring apparatus are prese nted. (C) 2000 Published by Elsevier Science B.V.