Photonic zitterbewegung and its interpretation
  In term of the volume-integrated Poynting vector, we present a quantum field-theory investigation on the zitterbewegung (ZB) of photons, and show that this ZB occurs only in the presence of virtual longitudinal and scalar photons. To present a heuristic explanation for such ZB, by assuming that the space time is sufficiently close to the flat Minkowski space, we show that the gravitational interaction can result in the ZB of photons. 
Zhi-Yong Wang, Cai-Dong Xiong, Qi Qiu
quant-ph gr-qc