Direct observation of zitterbewegung in a Bose–Einstein condensate
Zitterbewegung, a force-free trembling motion ﬁrst predicted for relativistic fermions like electrons, was an unexpected consequence of the Dirac equation’s uniﬁcation of quantum mechanics and special relativity. Though the oscillatory motion’s large frequency and small amplitude have precluded its measurement with electrons, zitterbewegung is observable via quantum simulation. We engineered an environment for 87Rb Bose–Einstein condensates where the constituent atoms behaved like relativistic particles subject to the one-dimensional Dirac equation. With direct imaging, we observed the submicrometre trembling motion of these clouds, demonstrating the utility of neutral ultracold quantum gases for simulating Dirac particles.