Entangled quantum particles are correlated in ways that are classically impossible. Although these correlations can’t be used to send definitive messages because quantum events are themselves unpredictable, they can reduce the amount of communication needed to transmit information. In Physical Review X, researchers show explicitly how partners can use a shared quantum resource to improve their odds in the card game bridge, where communication is restricted. The demonstration is the first example of the quantum enhancement of information transfer applied to a real-world situation.
After cards are dealt to four bridge players, competing pairs must communicate the strength of their hidden cards using verbal bids with an extremely limited vocabulary. A team led by Mohamed Bourennane, of Stockholm University in Sweden, imagines that the players also receive one particle of an entangled pair to help them communicate. Since it is well known that such particles cannot be used to send messages, the researchers argue that such sharing would not violate the rules.
The scientists analyze one particular situation that can arise late in bidding, in which one player (“Bob”) must decide if the high cards held by his partner (“Alice”) fill the gaps in his own hand so that the pair could win enough “tricks” to earn a bonus. To make his decision, Bob needs only one bit of information about Alice’s hand, but he can’t tell her which of two possible questions he needs her to answer. The researchers devise a scheme to combine measurements on the entangled pairs with her coded bid to give partial answers to both questions. In theory, the quantum scheme increases his chances of guessing her cards from to ; the team demonstrated for a version of this idealized task using entangled photons. As a result of their findings, the researchers suggest that authorities may need to add new rules to cover quantum signaling. – Don Monroe