Fischer, M., Kolachevsky, N., Zimmermann, M., Holzwarth, R., Udem, T., Hänsch, T. W., Abgrall, M., Grunert, J., Maksimovic, I., Bize, S., Marion, H., Dos Santos, F. P., Lemonde, P., Santarelli, G., Laurent, P., Clairon, A., Salomon, C., Haas, M., Jentschura, U. D., & Keitel, C. H. (2004). New limits on the drift of fundamental constants from laboratory measurements. Physical Review Letters, 92(23):. doi:10.1103/PhysRevLett.92.230802.