New J. Phys. 15, 073011 (2013)

Zitterbewegung — the fast trembling motion of relativistic quantum particles — was predicted by Erwin Schrödinger as a direct consequence of the Dirac equation. But there was little hope of observing the Zitterbewegung of a free relativistic particle due to its high oscillation frequency (of the order of 1021 Hz) and amplitude close to the Compton wavelength. The quiver has been spotted in analogue situations instead: for example, the fast oscillations of quantities equivalent to the position of the particle have been tracked in experiments using trapped ions and photonic lattices.

Now Lindsay LeBlanc and colleagues have observed the Zitterbewegung of atoms directly in a Bose–Einstein condensate. The atoms in the condensate interact with two counter-propagating laser beams that couple the hyperfine states with the atomic motion. The atoms essentially behave like relativistic particles obeying the one-dimensional Dirac equation. Direct imaging of the condensate yields position and velocity data, clearly showing micrometre-amplitude oscillations with a frequency of a few kHz — the atomic Zitterbewegung, whose origin can be understood by analogy with Rabi oscillations.