DECOHERENCE AND LOSS OF CONTROL

In order to affect the system-bath coupling and control, or modulate, the decoherence due to this coupling, one must dynamically modulate the system faster than the correlation time. Slower modulation will have no effect on the loss of coherence and will thus not be able to control it. Modulating the system faster than the correlation time can effectively reset the clock. Applying a modulation sequence repeatedly can thus drastically change the decoherence and impose a continued coherent evolution of the system-bath coupling [46, 94]. 

Jfl emow consider coherent control as it would apply to liquid-phase chemistry. Here, Ipoplecules of species B in solution would be subjected to laser irradiation. Since we ultimately interested in the fate of the B molecules, B is the system, and the riairiing molecules in the solution and the laser are the environment. Decoherence H Sfqcts can then arise from the collisions of the solvent with the molecule of interest, l incoherence properties of the laser that cause some loss of quantum phase... 

By definition, quantum control relies upon the unique quantum properties of light and matter, principally the wavelilce nature of both. As such, maintenance of the phase information contained in both the matter and light is central to the success of the control scenarios. Chapter 5 deals with decoherence, that is, the loss of phase information due to the influence of the external environment in reducing the system coherence. Methods of countering decoherence are also discussed. 

The uncontrollable interaction of an open quantum system with its environment leads to complete loss of the information initially stored in its quantum state. This phenomenon is commonly referred to as decoherence, loss of coherence, or loss of fidelity. The question of how it is possible to avoid the negative influence of this process is one of the most interesting issues in modem quantum mechanics, and concerns many different fields of physics, in particular the domains of coherent control [Shapiro 2003] and quantum information processing [Cirac 1995 Mplmer 1999 Sackett2000],... 

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