7 research outputs found
Design of Strongly Modulating Pulses to Implement Precise Effective Hamiltonians for Quantum Information Processing
We describe a method for improving coherent control through the use of
detailed knowledge of the system's Hamiltonian. Precise unitary transformations
were obtained by strongly modulating the system's dynamics to average out
unwanted evolution. With the aid of numerical search methods, pulsed
irradiation schemes are obtained that perform accurate, arbitrary, selective
gates on multi-qubit systems. Compared to low power selective pulses, which
cannot average out all unwanted evolution, these pulses are substantially
shorter in time, thereby reducing the effects of relaxation. Liquid-state NMR
techniques on homonuclear spin systems are used to demonstrate the accuracy of
these gates both in simulation and experiment. Simulations of the coherent
evolution of a 3-qubit system show that the control sequences faithfully
implement the unitary operations, typically yielding gate fidelities on the
order of 0.999 and, for some sequences, up to 0.9997. The experimentally
determined density matrices resulting from the application of different control
sequences on a 3-spin system have overlaps of up to 0.99 with the expected
states, confirming the quality of the experimental implementation.Comment: RevTeX3, 11 pages including 2 tables and 5 figures; Journal of
Chemical Physics, in pres
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