  The present paper is both a review on the Feynman problem, and an original
research presentation on the relations between Fermionic theories and qubits
theories, both regarded in the novel framework of operational probabilistic
theories. The most relevant results about the Feynman problem of simulating
Fermions with qubits are reviewed, and in the light of the new original results
the problem is solved. The answer is twofold. On the computational side the two
theories are equivalent, as shown by Bravyi and Kitaev (Ann. Phys. 298.1
(2002): 210-226). On the operational side the quantum theory of qubits and the
quantum theory of Fermions are different, mostly in the notion of locality,
with striking consequences on entanglement. Thus the emulation does not respect
locality, as it was suspected by Feynman (Int. J. Theor. Phys. 21.6 (1982):
467-488).
