We present a detailed experimental analysis of a free-propagating light pulse prepared in a "Schrödinger kitten" state, which is defined as a quantum superposition of "classical" coherent states with small amplitudes. This kitten state is generated by subtracting one photon from a squeezed vacuum beam, and it clearly presents a negative Wigner function. The predicted influence of the experimental parameters is in excellent agreement with the experimental results. The amplitude of the coherent states can be amplified to transform our "Schrödinger kittens" into bigger Schrödinger cats, providing an essential tool for quantum information processing.

Recent developments of quantum information science [1] critically rely on entanglement, an intriguing aspect of quantum mechanics where parts of a composite system can exhibit correlations stronger than any classical counterpart [2]. In particular, scalable quantum networks require capabilities to create, store, and distribute entanglement among distant matter nodes via photonic channels [3]. Atomic ensembles can play the role of such nodes [4]. So far, in the photon counting regime, heralded entanglement between atomic ensembles has been successfully demonstrated via probabilistic protocols [5,6]. However, an inherent drawback of this approach is the compromise between the amount of entanglement and its preparation probability, leading intrinsically to low count rate for high entanglement. Here we report a protocol where entanglement between two atomic ensembles is created by coherent mapping of an entangled state of light. By splitting a single-photon [7,8,9] and subsequent state transfer, we separate the generation of entanglement and its storage [10]. After a programmable delay, the stored entanglement is mapped back into photonic modes with overall efficiency of 17%. Improvements of single-photon sources [11] together with our protocol will enable "on-demand" entanglement of atomic ensembles, a powerful resource for quantum networking.In the quest to achieve quantum networks over long distances [3], an area of considerable activity has been the interaction of light with atomic ensembles comprised of a large collection of identical atoms [4,12,13]. In the regime of continuous variables, a particularly notable advance has been the teleportation of quantum states between light and matter [14]. For discrete variables with photons taken one by one, important achievements include the efficient mapping of collective atomic excitations to single photons [15,16,17,18,19], the realization of entanglement between a pair of distant ensembles [5,20] In all these cases, progress has relied upon probabilistic schemes following the measurement-induced approach developed in the seminal paper by Duan, Lukin, Cirac and Zoller [4] (DLCZ ) and subsequent extensions. For the DLCZ protocol, heralded entanglement is generated by detecting a single photon emitted indistinguishably by one of two ensembles. Intrinsically, the probability p to prepare entanglement with only 1 excitation shared between two ensembles is related to the quality of entanglement, since the likelihood for contamination of the entangled state by processes involving 2 excitations scales as p [20], and results in low success probability for each trial. Although the degree of stored entanglement can approach unity for the (rare) successful trials [20], the condition p ≪ 1 dictates reductions in count rate and compromises in the quality of the resulting entangled state (e.g., as p → 0, processes such as stray light scattering and detector dark counts become increasingly important). Furthermore, for finite memory time, subsequent connection of entanglement become...