Control by interfering quantum evolutions or 'Welcher-Weg' in history

We demonstrate that interference of evolutions (interaction 'histories') is realizable for two entangled systems, such as a two-level atom coupled to a cavity field over many different time intervals in parallel. Such interfe rence is conditional upon the detection of appropriate atomic momentum obse rvables. It allows us to fully control and manipulate any state of both ent angled systems. The sense of evolution (towards the 'past' or 'future') is also controlled by this method. In the case of a multi-mode field it allows us to protect any state of the two-level atom (qubit) from decoherence, ca used by entanglement with the field. The success probability of this contro l method can be optimized. Several schemes for realizing this method are su ggested. (C) 2000 Elsevier Science B.V. All rights reserved.