We theoretically study the propagation and storage of a classical field in a Λ-type atomic medium using coherent population oscillations (CPOs). We show that the propagation eigenmodes strongly relate to the different CPO modes of the system. Light storage in such modes is discussed by introducing a "populariton" quantity, a mixture of populations and field, by analogy to the dark state polariton used in the context of electromagnetically induced transparency light storage protocol. As experimentally shown, this memory relies on populations and is then -by contrast with usual Raman coherence optical storage protocols -robust to dephasing effects.PACS numbers: 42.50. Gy, 42.25.Bs, 42.50.Md The architectures proposed to implement optical quantum information and communication protocols generally rely on quantum memories, i.e. devices able to store quantum states of light and retrieve them on demand with high fidelity and efficiency [1]. Within the last decade, much effort has been put towards their implementation in solid-state systems, ion or neutral atomic ensembles. In this context, Λ-type three-level atomic systems have received particular attention since the coherence between the ground states may have a long lifetime and can, thus, be used for storage [2,3]. In gas cells, high efficiencies were obtained in alkalimetal atoms [4] using electromagnetically induced transparency (EIT) close to [5] or far off optical resonance [6], gradient echo memories [7], or four-wave mixing [8]. Since all these methods are based on the excitation of the Raman coherence between the lower states of the system, they are sensitive to decoherence effects. Recently, it was experimentally shown that coherent population oscillations (CPOs) can be used as a storage medium for light. Experimental demonstrations were performed using metastable helium (He*) vapor at room temperature [9], as well as in cold and warm cesium [10,11]. CPOs occur in a two-level system when two detuned coherent electric fields of different amplitudes drive the same transition. When the detuning between the fields is smaller than the decay rate of the upper level, the dynamics of the saturation opens a transparency window in the absorption profile of the weak field [12][13][14]. The CPO resonance width may be decreased when the upper level decays to a long-lived shelving state, leading to an ultranarrow CPO resonance and a memory behavior [15]. Another option is to use a Λ-system where two CPOs may occur in opposite phase on the two transitions, leading to a global CPO between the two lower states [16]. This implies an ultranarrow transmission resonance for the weak field broadened by the ground states' decay rate, which can be used for storage [9][10][11]. Since it involves only populations, CPO-based light storage protocol is robust to dephasing effects, by contrast with the EIT-based protocol which involves Raman coherence. In this Letter, we theoretically explore the Λ-system option. First, we study the propagation of a weak signal field in the medium. We...