Among the optical degrees of freedom, the orbital angular momentum of light [1] provides unique properties [2], including mechanical torque action with applications for light manipulation [3], enhanced sensitivity in imaging techniques [4] and potential high-density information coding for optical communication systems [5]. Recent years have also seen a tremendous interest in exploiting orbital angular momentum at the single-photon level in quantum information technologies [6,7]. In this endeavor, here we demonstrate the implementation of a quantum memory [8] for quantum bits encoded in this optical degree of freedom. We generate various qubits with computer-controlled holograms, store and retrieve them on demand using the dynamic electromagneticallyinduced transparency protocol. We further analyse the retrieved states by quantum tomography and thereby demonstrate fidelities exceeding the classical benchmark, confirming the quantum functioning of our storage process. Our results provide an essential capability for future networks [9] exploring the promises of orbital angular momentum of photons for quantum information applications.A most studied family of beams carrying orbital angular momentum (OAM) are the Laguerre-Gaussian (LG) modes, which exhibit a helical phase structure and carry an OAM that can take any integer value. Arising from the solution of the paraxial wave equation in cylindrical coordinates, these modes define an unbounded basis for transverse modes. Owing to this infinite dimensionality, the orbital angular momentum of photons raised intense theoretical and experimental efforts related to its use for encoding and processing quantum information [2].Following the pioneering work demonstrating entanglement in this degree of freedom for photons [6], great advances have been obtained in the experimental control of OAM state superpositions and their use in a variety of protocols. These advances include quantum cryptography [10], bit commitment [11], experimental quantum coin tossing [12], and more recently, the demonstration of very high-dimensional entanglement [13,14]. Beyond their fundamental significance, these groundbreaking experiments testify the potential of the orbital angular momentum of light as quantum information carrier, holding much promise for an enhanced information coding density and processing capabilities. This promise can be * Electronic address: julien.laurat@upmc.fr extended to applications such as quantum networks, including quantum repeaters over long distances [9]. For such OAM-based implementation, spatially multimode light-mater interfaces will be required.Therefore, the ability to store OAM superpositions at the single-photon level in matter systems is of crucial importance for future developments. In recent years, significant progresses in this direction have been achieved by demonstrating the entanglement of OAM states between a photon and an ensemble of cold atoms [15] or the reversible mapping of bright light beams carrying OAM [16,17]. Moreover, the preservation of th...

We report the experimental observation of slow-light and coherent storage in a setting where light is tightly confined in the transverse directions. By interfacing a tapered optical nanofiber with a cold atomic ensemble, electromagnetically induced transparency is observed and light pulses at the single-photon level are stored in and retrieved from the atomic medium with an overall efficiency of (10 ± 0.5)%. Collapses and revivals can be additionally controlled by an applied magnetic field. Our results based on subdiffraction-limited optical mode interacting with atoms via the strong evanescent field demonstrate an alternative to free-space focusing and a novel capability for information storage in an all-fibered quantum network.PACS numbers: 03.67.Hk, 42.50.Gy, 42.50.Ex, 42.81.Qb Over the recent years, the physical implementation of quantum interfaces between light and matter has triggered a very active research, with unique applications to quantum optics and quantum information networks [1,2]. Within this context, a promising approach consists in coupling light with atomic ensembles [3,4]. Reversible quantum memories have been realized in a variety of ensemble-based systems, e.g. doped crystals and free-space collection of alkali atoms [5]. Significant advances have been made, including the demonstration of entanglement between remote memories and the development of first rudimentary capabilities for quantum repeater architectures [6][7][8][9]. However, free-space focusing as used in these seminal works is limiting the coupling one can obtain and the connectivity to fiber networks.Interfacing guided light with atoms has therefore been foreseen as a promising alternative, enabling longer interaction length, large optical depth and potential nonlinear interactions at low power level [2]. A first possible implementation consists in encasing a vapor into the hollow core of a photonic-crystal fiber, confining thus atoms and photons in the waveguide. Slow-light, alloptical switching and few-photon modulation have been demonstrated [10][11][12]. Recently, single-photon-level Raman memory has been realized with larger core fibers, with storage limited to the 10 ns time-scale [13]. Another approach can be based on an even tighter confinement of light in a nanoscale waveguide leading to a large evanescent field that can interact with atoms located in the vicinity. This situation can be ideally realized with optical nanofibers exhibiting subwavelength diameter [14]. Using a nanofiber in a hot Rubidium vapor, nonlinear interactions and low-power saturation have been reported [15][16][17], albeit with very short transit time of hot atoms in the evanescent field and large broadening.In this new avenue of research, the unique prospects of combining cold atoms with nanofibers have triggered vast theoretical and experimental efforts. Pioneering works investigated the interaction of a small number of atoms with the guided mode, including fluorescence coupling and surface interactions [18][19][20], and the dipole trapping of...