Nonlocality—the entanglement of one object with another at a distance—is a powerful way to achieve quantum information processing. However, quantum mechanics is tethered by a “no-signaling” principle, that is, these correlations cannot be used to transmit information arbitrarily quickly from one point to another. In a paper in Physical Review Letters, Mafalda Almeida at the ICFO-Institut de Ciencies Fotoniques in Barcelona and colleagues at institutions in Spain, Switzerland, the UK, and Belgium follow multiplayer games that obey quantum rules to better understand how no-signaling correlations can, in and of themselves, govern the outcome.
Almeida et al. have concocted a multiplayer game they call “Guess Your Neighbor’s Input” in which players arranged in a circle each receive a bit: 0 or 1. A player then guesses the input the neighbor on her right has received, and emits a bit equal to that guess. The distribution of possible input bits is known ahead of time, but the players do not otherwise communicate.
This game clearly hints that winning it requires signaling. And indeed, Almeida et al. find that the guesses are no more successful on average when quantum correlations between the bits govern the play, than with purely classical correlations—a case where quantum information processing buys no advantage. However, surprisingly, they observe that for certain correlations governed only by the no-signalling principle, the players can outperform both quantum and classical scenarios. The results spell out for the first time where quantum nonlocality leaves off and where a stronger set of correlations can be found in multipartite situations. – David Voss