tell me that hers is heads).6 Somehow, my sister’s observation of her penny influenced the state of my penny, no matter how far away we are. The same thing happens for real with actual photons of light, when you measure not heads or tails but how the photon is spinning or the orientation of its polarization.
This shared information between entangled particles can be exploited for many kinds of quantum communication purposes. In a quantum game, entangled particles can carry information about a player’s choice in such a way that one choice will influence another. Take the Prisoner’s Dilemma game. In the classical game, the players typically choose to defect because they cannot be sure that their partner will cooperate. Overall, the pair’s best strategy is for both to remain silent—that way they’ll serve the least amount of time. But each individual prisoner’s best strategy is to rat out the other (to avoid the risk of a much longer sentence). So the best choice for the individual turns out not to be the wisest choice for the team.“We have a dilemma,” write quantum game theorists Adrian Flitney and Derek Abbott, “some form of which is responsible for much of the misery and conflict throughout the world.”7
But suppose there was a way that both players could make their decision hinge on the decision of the other. That’s what playing with entangled photons offers. As Zhou and Kuang showed, you can set up an apparatus that allows the choice of “defect” (rat out your partner) or cooperate (keep mum) to be transmitted by photons, passing through a maze of mirrors and other optical devices, ultimately to reach a detector signifying either defect or cooperate. You can shoot your photon into the maze in different ways—so that it would end up in the “defect” detector, for instance, or in the “cooperate” detector. There’s nothing tricky about that. But the maze can be set up so that the photons from the two players become entangled, with the result that both will end up cooperating. That is, you can send your choice in such a way that your photon will send a signal to cooperate only if the other one does, too.
This work shows that quantum game theory, at least in principle, could be used to alter in a deep and profound way the choices