Experimental proof that a qubit is more than a quantum coin
Esteban S. Gómez, Santiago A. Gomez, P. González, Gustavo Cañas, Johanna F. Barra, A. Delgado, Guilherme B. Xavier, Adán Cabello, Matthias Kleinmann, Tamás Vértesi, G. Lima · arXiv (Cornell University) · 2016
The qubit, the simplest system and unit of information, is sometimes compared to a quantum coin: a coin with infinitely many sets of two sides which can only be tossed when specifying the desired set. However, this picture fails to capture the richness of the world, where qubit measurements are not necessarily binary. Consequently, when pairs of entangled qubits are measured locally, theory predicts correlations that can neither be explained as produced by locally tossing classical coins nor explained by locally tossing coins. However, this last prediction was unconfirmed until now since it requires the observation of a very small effect on an almost perfectly entangled qubit-qubit state. Here we present an experiment on pairs of entangled photonic qubits violating by more than 8 standard deviations a new Bell-like correlation inequality, which is valid only for binary measurements. By also providing device-independent evidences that our system is best described by two qubits, we prove that measurements on a qubit can be fundamentally non-binary. Since non-binary measurements on a qubit cannot be sharp, this constitutes the first device-independent certification of a genuine generalized measurement.