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International Journal of Quantum Information

REDUCED ENTANGLEMENT FOR QUANTUM GAMES

LI ZHANG, Tad Hogg

Published Sep 1, 2003• 4 citations
Publisher

Abstract

Quantum generalizations of conventional games exploit entangled states to improve performance. With many players, quantum games can require entangling many states. Such entanglement is difficult to implement, especially if the states must be communicated over some distance. To simplify possible implementations, we examine some quantum versions of social dilemma games and show their use of entanglement can be substantially reduced by randomly replacing some of the entangled states by unentangled ones. For the example of public goods games, we identify a unique Nash equilibrium invariant with respect to the amount of this replacement. We also show players obtain no advantage from adding more entanglement to states which they control. With many players, a fairly small number of entangled states can give nearly as good performance as using the full number of such states.

DOI
10.1142/s0219749903000255

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Quantum generalizations of conventional games exploit entangled states to improve performance. With many players, quantum games can require entangling many states. Such entanglement is difficult to implement, especially if the states must be communicated over some distance. To simplify possible implementations, we examine some quantum versions of social dilemma games and show their use of entanglement can be substantially reduced by randomly replacing some of the entangled states by unentangled ones. For the example of public goods games, we identify a unique Nash equilibrium invariant with respect to the amount of this replacement. We also show players obtain no advantage from adding more entanglement to states which they control. With many players, a fairly small number of entangled states can give nearly as good performance as using the full number of such states.
Quantum generalizations of conventional games broaden the range of available strategies, which can help improve outcomes for the participants. With many players, such quantum games can involve entanglement among many states which is difficult to implement, especially if the states must be communicated over some distance. This paper describes a quantum mechanism for the economically significant nnn-player public goods game that requires only two-particle entanglement and is thus much easier to implement than more general quantum mechanisms. In spite of the large temptation to free ride on the efforts of others in this game, two-particle entanglement is sufficient to give near optimal expected payoff when players use a simple mixed strategy for which no player can benefit by making different choices. This mechanism can also address some heterogeneous preferences among the players.
A well-known model in game theory, the Volunteer's Dilemma describes a group of nnn players who decide whether to volunteer for a collective benefit at a personal cost, or to abstain and risk forfeiting the benefit altogether. A quantum version of this dilemma, developed within the Eisert-Wilkens-Lewenstein framework, allows each player to manipulate one qubit of a shared entangled state, leading to symmetric Nash equilibria with higher expected payoffs than in the classical game. Existing analyses, however, assume maximal entanglement, which is experimentally demanding for hardware-constrained quantum systems. Within the same framework, we introduce a generalized Quantum Volunteer's Dilemma with a tunable entanglement parameter γγγ and study the extent to which equilibrium behavior depends on the level of entanglement. We derive explicit conditions relating γγγ, the number of players, and the players' strategies under which symmetric Nash equilibria exist, focusing on two canonical strategy profiles: one for 2≤n≤92\leq n\leq 92≤n≤9, and one for even nnn. We find that maximal entanglement is not required to sustain symmetric equilibria. Instead, equilibrium behavior persists above a threshold value, which we compute analytically in both cases. We also demonstrate that the threshold value directly depends on system size.
A quantum version of the Minority game for an arbitrary number of agents is studied. When the number of agents is odd, quantizing the game produces no advantage to the players, however, for an even number of agents new Nash equilibria appear that have no classical analogue. The new Nash equilibria provide far preferable expected payoffs to the players compared to the equivalent classical game. The effect on the Nash equilibrium payoff of reducing the degree of initial entanglement between the players' qubits or of introducing various forms of decoherence is discussed. As the number of players increases the multipartite GHZ state becomes increasingly fragile, as indicated by the smaller error probability required to reduce the Nash equilibrium payoff to the classical level.
A quantum version of the Minority game for an arbitrary number of agents is studied. When the number of agents is odd, quantizing the game produces no advantage to the players, however, for an even number of agents new Nash equilibria appear that have no classical analogue. The new Nash equilibria provide far preferable expected payoffs to the players compared to the equivalent classical game. The effect on the Nash equilibrium payoff of reducing the degree of entanglement, or of introducing decoherence into the model, is indicated.
A version of the Monty Hall problem is presented where the players are permitted to select quantum strategies. If the initial state involves no entanglement the Nash equilibrium in the quantum game offers the players nothing more than that obtained with a classical mixed strategy. However, if the initial state involves entanglement of the qutrits of the two players, it is advantageous for one player to have access to a quantum strategy while the other does not. Where both players have access to quantum strategies there is no Nash equilibrium in pure strategies, however, there is a Nash equilibrium in quantum mixed strategies that gives the same average payoff as the classical game.