Optical frequency measurement of the 1S–3S two-photon transition in hydrogen O. Arnoult, F. Nez, L. Julien and F. Biraben Eur. Phys. J. D, 60 2 (2010) 243-256 Published online: 28 September 2010 DOI: 10.1140/epjd/e2010-00249-6