Quantum-entangled pions

We show that two- and three-pion states produced in the decay of neutral kaons are contextual, entangled, and Bell nonlocal in isospin space. By reinterpreting the experimental values of the different isospin amplitudes, we can determine the amount of entanglement enjoyed by these states and the extent to which they violate the noncontextuality and Bell locality inequalities. Notably, the two-pion case provides an example of a state that is more Bell nonlocal, as witnessed by the Collins-Gisin-Linden-Massar-Popescu inequality, than the maximally entangled one. The three-pion state offers a genuine multipartite test of Bell nonlocality with qutrits. This is the first instance where quantum properties like entanglement and contextuality are demonstrated to exist within the internal space of isospin.