Hyperfine Interactions 151/152: 93–104, 2003. © 2004 Kluwer Academic Publishers. Printed in the Netherlands. 93 Slowing Down of Gamma Photons R. COUSSEMENT 1,2 , S. GHEYSEN 1 , I. SERDONS 1 , R. CALLENS 1 , K. VYVEY 1 , R. N. SHAKHMURATOV 1,3 , J. ODEURS 1 , P. MANDEL 2 , Y. ROSTOVTSEV 4 and O. KOCHAROVSKAYA 4 1 Instituut voor Kern-en Stralingsfysica, Katholieke Universiteit Leuven, Celestijnenlaan 200 D, B-3001 Leuven, Belgium 2 Université Libre de Bruxelles, Optique Nonlinéaire Théorique, Campus Plaine CP 231, Bld du Triomphe, B-1050 Bruxelles, Belgium 3 Kazan Physical-Technical Institute, Russian Academy of Sciences, 10/7 Sibirsky Trakt Street, Kazan 420029, Russia 4 Department of Physics and Institute for Quantum Studies, Texas A&M University, College Station, TX 77843-4242, USA On the occasion of the 80th birthday of Hendrik de Waard Abstract. By a transformation to a rotating reference system, a nuclear level mixing scheme is equivalent to a scheme with pure quadrupole splitting and a strong RF-drive between quadrupole split lines. It is shown that this scheme is similar to the one used in optics in which electromagneti- cally induced transparency (EIT) and low group velocity have been demonstrated. One can therefore consider the slowing down of gamma photons. However, with gamma photons one always works with single photons and the question remains whether the concept of group velocity can be applied. Key words: nuclear level mixing, electromagnetic induced transparency (EIT) and low group velo- city. 1. Introduction Electromagnetically induced transparency and the slowing down of pulses of pho- tons in the optical domain, are still subjects of intense research in the field of quantum optics [1–4]. Coherence and interference are the basic ingredients and strong driving lasers are an important tool. For gamma rays, several experiments in which the coherence and the interferences are explored have been reported (see, e.g., [5–17]). However, the slowing down of gamma photons is not reported yet. This can be explained by the lack of a gamma-ray laser. So, the scheme in the optical domain cannot be straightforwardly extended. In this paper, an alternative method will be proposed that gives the opportunity to check whether single gamma photons can be slowed down. At first, the equivalence between nuclear level mixing and the use of driving fields will be shown. In a basis that rotates about the hyperfine axis, the nuclear