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　particles　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　agree　well　with　the　numerical　simulation.　Besides　being　of　fundamental　importance,　the　demonstrated　nonclassical　effects　are　useful　in　quantum　technology.