Revealing inherent quantum interference and entanglement of a Dirac Fermion

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.