  Entangled states of pseudoscalar mesons represent a very interesting tool for
studying foundations of quantum mechanics, e.g. for testing Bell inequalities.
Recently, they also emerged as a test bench for quantum information protocols.
On the other hand, from a quantum information point of view, the
characterization of the distance between two quantum states is a topic of the
utmost importance. In this letter, with the purpose of providing a useful tool
for further investigations, we address the problem of which distance allows a
better discrimination between density matrices appearing in pseudoscalar
phenomenology.
