The Dirac equation is critical to understanding the universe, and plays an
important role in technological advancements. Compared to the stationary
solution, the dynamical evolution under the Dirac Hamiltonian is less
understood, exemplified by Zitterbewegung. Although originally predicted in
relativistic quantum mechanics, Zitterbewegung can also appear in some
classical systems, which leads to the important question of whether
Zitterbewegung of Dirac Fermions is underlain by a more fundamental and
universal interference behavior without classical analogs. We here reveal such
an interference pattern in phase space, which underlies but goes beyond
Zitterbewegung, and whose nonclassicality is manifested by the negativity of
the phase-space quasiprobability distribution, and the associated
pseudospin-momentum entanglement. We confirm this discovery by numerical
simulation and an on-chip experiment, where a superconducting qubit and a
quantized microwave field respectively emulate the internal and external
degrees of freedom of a Dirac particle. The measured quasiprobability
negativities well agree with the numerical simulation. Besides being of
fundamental importance, the demonstrated nonclassical effects are useful in
quantum technology.Comment: 18 pages, 15 figure