Nature Commun. (2012)

Quantum correlations challenge our classical notion of locality — measuring one particle influences the measurement outcome on its spatially separated pair. Ognyan Oreshkov and colleagues now reveal more quantum mechanical spookiness: quantum correlations that defy causal order.

The framework developed by Oreshkov et al. assumes no definite causal structure, but considers two laboratories inside which the laws of quantum mechanics hold. The inhabitants of these laboratories are each given a particle and start playing a game, trying to guess each other's random choice. They can communicate their choice only by sending their particle to the other lab. Causal order restricts this communication, forbidding bidirectional signalling, therefore the success probability of the game is bounded.

However, it turns out that correlations can exist beyond those predicted by standard quantum mechanics. These do not obey causal order, allowing the players to improve their probability of success. Interestingly, this bears similarity to the violation of Bell's inequality — that is, violation of local realism. Moreover, causal order seems to emerge in the classical limit. Whether these exotic quantum correlations can be found in nature is not yet clear.