Novotny, C., Huber, G., Karpuk, S., Reinhardt, S., Bing, D., Schwalm, D., Wolf, A., Bernhardt, B., Hänsch, T. W., Holzwarth, R., Saathoff, G., Udem, T., Nörtershäuser, W., Ewald, G., Geppert, C., Kühl, T., Stöhlker, T., & Gwinner, G. (2009). Sub-Doppler laser spectroscopy on relativistic beams and tests of Lorentz invariance. Physical Review A, 80(2):. doi:10.1103/PhysRevA.80.022107.