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We show that Coherent Population Oscillations effect allows to burn a narrow spectral hole (26 Hz) within the homogeneous absorption line of the optical transition of an Erbium ion-doped crystal. The large dispersion of the index of refraction associated with this hole permits to achieve a group velocity as low as 2.7 m/s with a transmission of 40 %. We especially benefit from the inhomogeneous absorption broadening of the ions to tune both the transmission coefficient, from 40 % to 90 %, and the light group velocity from 2.7 m/s to 100 m/s. [5,6,7,8]. In addition to the strangeness of producing light propagating at speeds as low as few m/s, Slow Light Propagation (SLP) is at the very heart of new fundamental and applied fields of research in nonlinear and quantum optics. From the nonlinear optical side, SLP allows to strongly enhance the lightmatter interaction time. Moreover, this interaction time can be continuously tuned to produce optical buffers and variable delay lines for optical networks. From the quantum optical point of view, SLP should allow, under specific conditions, classical and quantum properties of an electromagnetic field to be mapped into an atomic system [10]. The fundamental physical idea at the origin of SLP is the creation of a very narrow spectral hole in the homogeneous absorption profile. As stipulated by Kramers-Krönig relations, this narrow spectral hole is accompanied by a strong dispersion of the index of refraction inducing a low group velocity and an increase of the transmission. These two aspects are crucial in the choice of the atomic system and the coherent interaction inducing SLP.The first direct demonstration of SLP [1,2,3,4,5] was achieved via Electromagnetically Induced Transparency (EIT) [9]. It was originally implemented by applying a secondary control field to eliminate the linear absorption of a resonant probe field through an otherwise absorbing medium. The standard scheme for EIT is a three-level Λ system, where the probe field drives the system from one of the ground states and the control field from the second ground state.

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Optical microcavities with ultralong photon storage times are of central importance for integrated nanophotonics. To date, record quality (Q) factors up to 10^{11} have been measured in millimetric-size single-crystal whispering-gallery-mode (WGM) resonators, and 10^{10} in silica or glass microresonators. We show that, by introducing slow-light effects in an active WGM microresonator, it is possible to enhance the photon lifetime by several orders of magnitude, thus circumventing both fabrication imperfections and residual absorption. The slow-light effect is obtained from coherent population oscillations in an erbium-doped fluoride glass microsphere, producing strong dispersion of the WGM (group index n_{g}∼10^{6}). As a result, a photon lifetime up to 2.5 ms at room temperature has been measured, corresponding to a Q factor of 3×10^{12} at 1530 nm. This system could yield a new type of optical memory microarray with ultralong storage times.