15,338 research outputs found
Quantum repeaters based on entanglement purification
We study the use of entanglement purification for quantum communication over
long distances. For distances much longer than the coherence length of a
corresponding noisy quantum channel, the fidelity of transmission is usually so
low that standard purification methods are not applicable. It is however
possible to divide the channel into shorter segments that are purified
separately and then connected by the method of entanglement swapping. This
method can be much more efficient than schemes based on quantum error
correction, as it makes explicit use of two-way classical communication. An
important question is how the noise, introduced by imperfect local operations
(that constitute the protocols of purification and the entanglement swapping),
accumulates in such a compound channel, and how it can be kept below a certain
noise level. To treat this problem, we first study the applicability and the
efficiency of entanglement purification protocols in the situation of imperfect
local operations. We then present a scheme that allows entanglement
purification over arbitrary long channels and tolerates errors on the per-cent
level. It requires a polynomial overhead in time, and an overhead in local
resources that grows only logarithmically with the length of the channel.Comment: 19 pages, 16 figure
Sequential decisions in allocation problems
In the context of cooperative TU-games, and given an order of players, we consider the problem of distributing the worth of the grand coalition as a sequential decision problem. In each step of the process, upper and lower bounds for the payoff of the players are required related to successive reduced games. Sequentially compatible payoffs are defined as those allocation vectors that meet these recursive bounds. The core of the game is reinterpreted as a set of sequentially compatible payoffs when the Davis-Maschler reduced game is considered (Th.1). Independently of the reduction, the core turns out to be the intersection of the family of the sets of sequentially compatible payoffs corresponding to the different possible orderings (Th.2), so it is in some sense order-independent. Finally, we analyze advantageous properties for the first player.core, reduced game, sequential allocation, tu-game
Learning from failure
We study decentralized learning in organizations. Decentralization is captured through a symmetry constraint on agents’ strategies. Among such attainable strategies, we solve for optimal and equilibrium strategies. We model the organization as a repeated game with imperfectly observable actions. A fixed but unknown subset of action profiles are successes and all other action profiles are failures. The game is played until either there is a success or the time horizon is reached. For any time horizon, including infinity, we demonstrate existence of optimal attainable strategies and show that they are Nash equilibria. For some time horizons, we can solve explicitly for the optimal attainable strategies and show uniqueness. The solution connects the learning behavior of agents to the fundamentals that characterize the organization: Agents in the organization respond more slowly to failure as the future becomes more important, the size of the organization increases and the probability of success decreases.Game theory
Gradualism and Irreversibility.
This paper considers a class of two-player dynamic games in which each player controls a one-dimensional variable which we interpret as a level of cooperation. In the base model, there is an irreversibility constraint stating that this variable can never be reduced, only increased. It otherwise satisfies the usual discounted repeated game assumptions.GAMES ; PUBLIC GOODS ; COOPERATION
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