How can online gamblers be sure that the casino isn't cheating? They can't — but the quantum gambling machine devised by Yi-Sheng Zhang and colleagues at the University of Science and Technology of China in Hefei could put an end to that. In their system, the casino places a particle in a quantum superposition, where it is in two 'boxes' at once until the gambler opens one, 'collapsing' the particle into either box with equal probability. The gambler bets against the casino about which box the particle will be in.
The casino then sends the gambler the unselected box for checking: the quantum rules mean that if the casino tampered with the probabilities to bias the result, the gambler has a chance of finding out.
Zhang and colleagues have demonstrated the game using photons in superpositions of polarization states. Three detectors measure the result: two read the photon's state, and the third checks for tampering.