Hommelhoff, P., Kealhofer, C., Aghajani-Talesh, A., Sortais, Y. R. P., Foreman, S. M., & Kasevich, M. A. (2009). Extreme localization of electrons in space and time. Ultramicroscopy, 109(5), 423-429. doi:10.1016/j.ultramic.2008.10.021.