Explorations of quantum decoherence phenomena

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Physics, 2002.Includes bibliographical references (p. 77-80).This thesis describes the experimental exploration of quantum decoherence using discrete and continuous-time decoherence maps. The experimental methodology uses liquid-state nuclear magnetic resonance spectroscopy techniques. Initially, a brief discussion of coherent control methods is given. Then, a detailed discussion of the decoherent control methods is presented. These methods describe how strong measurements can be emulated in an ensemble system by using pulsed magnetic field gradients, and how NMR decoupling techniques can be used to implement partial trace operations. Next, using quantum erasers we explore the stability of three-particle systems under different entangling interactions. With a two-spin system we illustrate the essential features of quantum erasers. The extension to three-spins allows us to use the pair of orthogonal decoherent operations used in quantum erasers to probe the two classes of entanglement in three-particle systems: the GHZ state and the W state. Finally, we develop a decoherence model of a decohering two-level system coupled to an environment with a few degrees of freedom. The couplings are of the [sigma]z [sigma]z type and only induce coherence damping. By introducing a stochastic evolution on the environment, the resulting randomization of the environment phases causes loss of information over the environment degrees of freedom and decohers the system. Control parameters in the stochastic driving of the environment were used to vary the rates of decoherence on the system, thereby allowing the establishment of a scaling law that related control parameters to decay rates.by Grum Teklemariam.Ph.D