What are the odds? Experimental quantum advantage in the odd-cycle game

We report the first experimental demonstration of the odd-cycle game, where a quantum strategy provides two cooperating players a winning advantage compared to the best classical strategy. The odd-cycle game has the virtue that it can be explained in everyday terms and that the optimal classical solution is self-evident without formal mathematical proof.

Two non-communicating players are tasked to respond to queries about the colour (binary choice) of vertices in an odd cycle (circular graph with an odd number of vertices). They win a round if and only if they respond with identical (different) colours when given identical (adjacent) vertices as inputs.

Here, we describe an experiment where the players, separated by ~2m, each own a trapped-ion qubit and share an entangled state. They follow the rules of the game faithfully and without loopholes, and still manage to win the game with a probability ~26 sigma above that allowed by the best classical strategy. We explain the quantum strategy used by the players and quantify the nonlocal content of the entangled resource state that underpins this demonstration of quantum advantage; at 0.54(2), this value represents the largest nonlocal content measured for physically separate devices, free of the detection loophole, ever observed.