A significant reduction of absorption for single gamma photons has been experimentally observed by studying Mössbauer spectra of 57 Fe in a FeCO 3 crystal. The experimental results have been explained in terms of a quantum interference effect involving nuclear level anticrossing due to the presence of a combined magnetic dipole and electric quadrupole interaction. [16] in Mössbauer spectra, gamma-microwave double resonance [13,17,18], or gamma-optical double resonance [19]. Recently, interesting proposals have been discussed to obtain lasing for gamma rays by utilizing coherent effects [20][21][22][23]. In this Letter, we report on experiments demonstrating the EIT effect at the singlephoton level via the level (anti)crossing technique. A theory of the one-photon interaction with a nucleus has been developed to describe the experimental results. The obtained results open an interesting perspective to extend coherent effects to nuclear transitions. Figure 1 represents the main results of the paper. It shows the observed Mössbauer spectrum of a single crystal of FeCO 3 at a temperature of 30.5(5) K which corresponds to a magnetic hyperfine field of B 15:1 3 T. At this field the hyperfine levels jm 1=2i and jm ÿ3=2i anticross. For the transitions connected to the anticrossing levels, a deficit of absorption of 25% is observed at the peak velocity. It means that some transparency is induced by interference, similar to EIT observed in optics.The experiments were performed by using a conventional Mössbauer setup. It includes a source of gamma radiation ( 57 CoRh), an absorber of FeCO 3 cleaved on the f1014g faces (optical thickness is of the order 10), and a detector. The absorber was mounted on a target holder which allows for a precise temperature control at the target position in the interval 4-600 K. Besides the magnetic hyperfine field, the Fe 2 nucleus in the FeCO 3 crystal [24,25] is subjected to a large axially symmetric electric field gradient (EFG) which results in a well-resolved quadrupole doublet. The level structures of the source and the absorber are shown in Fig. 2. In a magnetic field, the levels might shift to the position where their energies coincide; this situation is referred to as level crossing. But due to the presence of additional fields, the energies of levels might never be equal, and it is the case of level anticrossing.If the magnetic field is collinear with the EFG axis, the axial symmetry is preserved and the m states are eigenfunctions of the total nuclear Hamiltonian if the z axis is chosen along the symmetry axis. However, in such a mineral containing impurities and defects, one can expect a small distribution of fields which are responsible for the

We consider the propagation of a Gaussian probe pulse in an absorptive, optically dense, two-level medium if a deep, persistent hole is created in advance by another pump field in the inhomogeneously broadened absorption spectrum of this medium. Both fields are well separated in time and the lifetime of the hole is assumed to be long with respect to the delay time between the pump and probe pulses. We show that the group velocity of the Gaussian probe pulse reduces several orders in magnitude, similarly to the reduction of the group velocity for the probe field in electromagnetically induced transparency ͑EIT͒ phenomena. In contrast to EIT, the width of the transparency window can be made very wide because of the saturation broadening of the hole.

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