Cluster state quantum computing in optical fibers

A scheme for the implementation of the cluster state model of quantum computing in optical fibers, which enables the feedforward feature, is proposed. This scheme uses the time-bin encoding of qubits. Following previously suggested methods of applying arbitrary one-qubit gates in optical fibers, two different ways for the realization of fusion gate types I and II for cluster production are proposed: a fully time-bin based encoding scheme and a combination of time-bin and polarization based encoding scheme. Also the methods of measurement in any desired bases for the purpose of the processing of cluster state computing for both these encodings are explained.Comment: 6 pages, 11 figures, submitted to the Optical Quantum-Information Science focus issue of JOSA

Fair Loss-Tolerant Quantum Coin Flipping

Coin flipping is a cryptographic primitive in which two spatially separated players, who in principle do not trust each other, wish to establish a common random bit. If we limit ourselves to classical communication, this task requires either assumptions on the computational power of the players or it requires them to send messages to each other with sufficient simultaneity to force their complete independence. Without such assumptions, all classical protocols are so that one dishonest player has complete control over the outcome. If we use quantum communication, on the other hand, protocols have been introduced that limit the maximal bias that dishonest players can produce. However, those protocols would be very difficult to implement in practice because they are susceptible to realistic losses on the quantum channel between the players or in their quantum memory and measurement apparatus. In this paper, we introduce a novel quantum protocol and we prove that it is completely impervious to loss. The protocol is fair in the sense that either player has the same probability of success in cheating attempts at biasing the outcome of the coin flip. We also give explicit and optimal cheating strategies for both players.Comment: 12 pages, 1 figure; various minor typos corrected in version

Quantum effects on the interation between atoms and radiation in cavities

La idea central de la presente tesis es poner de manifesto efectos cuánticos que aparecen en la interacción entre átomos de pocos niveles con radiación electromagnética y entre modos de radiación entre sí considerando la radiación cuantificada. Se trataron diferentes sistemas de interés en el ámbito de la elcctrodinámica cuántica de cavidades, utilizando técnicas como la de hallar los operadores relevantes en el marco de la teoría de álgebras dinámicas y también utilizando ecuación maestra para la evolución del operador densidad. Se estudió el problema de un átomo de dos niveles interactuando con dos modos de radiación en presencia de un medio no lineal. Para este sistema se encontró que la presencia del medio no lineal inhibe los efectos típicos de estos sistemas como ser los colapsos y resurrecciones de la inversión de población, y también el antiagrupamiento. También se estudió un hamiltoniano de Jaynes-Cummings generalizado que no toma en cuenta la aproximación de onda rotante y además en presencia de un campo externo. Se pudo encontrar un conjunto de operadores adecuado para estudiar este sistema que permite evaluar la evolución temporal del valor medio de cualquier cantidad de relevancia. En la presente tesis se hace una propuesta para poder utilizar un divisor dc haz como discriminador de entrelazamiento entre dos modos del campo electromagnético; además en algunos casos podemos identificar diferentes elementos de la base de Bell. También en este trabajo se hace un estudio de factibilidad de una propuesta para generar en forma condicional estados de Fock en una cavidad. Para ello se relajan aproximaciones y se tienen en cuenta las pérdidas dentro de la cavidad, encontrándose que el esquema es robusto antes condiciones reales, siendo entonces promisoria su realización experimental.The main idea of this thesis is to expose quantum effects that appear on the interaction between few level atoms and electromagnetic radiation, and between radiation modes, considering that the electromagnetic field is quantized. Different problems of interest on the field of Cavity Quantum Electrodynamics were studied, using techniques such us finding the relevant operators in the scenario of dynamical algebras theory, and also the density matrix approach. We studied the problem of a two level atom interacting with two modes of radiation in the presence of a non-linear medium. We found that, for this particular system, the nonlinear medium inhibits the decay of the population of the excited level, and the antibunching effect on the radiation field. We also studied a generalized Jaynes-Cummings hamiltonian that does not take into account the Rotating Wave Approximation, and also in the presence of an external driving field. We found a set of relevant operators suitable for studying this problem, that allows us to solve the temporal evolution of the mean value of the mean value of any quantity of interest. In this thesis, a proposal is made for using a beam-splitter as a state and entanglement detector between two modes of the electromagnetic field. We also found that in some cases we can discriminate between different elements of the Bell basis. We also tackle the problem of the factibility of a proposal for conditionally generating Fock states of the radiation in a cavity. We modified a previously proposed model, account for cavity losses, take into account levels that were adiabatiacally eliminated, and concluded that the proposal is robust under experimental conditions.Fil:Berlín, Guido. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina