Quantum information science involves the storage, manipulation and communication of information encoded in quantum systems, where the phenomena of superposition and entanglement can provide enhancements over what is possible classically1,2. Large-scale quantum information processors require stable and addressable quantum memories, usually in the form of fixed quantum bits (qubits), and a means of transferring and entangling the quantum information between memories that may be separated by macroscopic or even geographic distances. Atomic systems are excellent quantum memories, because appropriate internal electronic states can coherently store qubits over very long timescales. Photons, on the other hand, are the natural platform for the distribution of quantum information between remote qubits, given their ability to traverse large distances with little perturbation. Recently, there has been considerable progress in coupling small samples of atomic gases through photonic channels2,3, including the entanglement between light and atoms4,5 and the observation of entanglement signatures between remotely located atomic ensembles6,7,8. In contrast to atomic ensembles, single-atom quantum memories allow the implementation of conditional quantum gates through photonic channels2,9, a key requirement for quantum computing. Along these lines, individual atoms have been coupled to photons in cavities2,10,11,12, and trapped atoms have been linked to emitted photons in free space13,14,15,16,17. Here we demonstrate the entanglement of two fixed single-atom quantum memories separated by one metre. Two remotely located trapped atomic ions each emit a single photon, and the interference and detection of these photons signals the entanglement of the atomic qubits. We characterize the entangled pair by directly measuring qubit correlations with near-perfect detection efficiency. Although this entanglement method is probabilistic, it is still in principle useful for subsequent quantum operations and scalable quantum information applications18,19,20.
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This work is supported by the National Security Agency and the Disruptive Technology Office under Army Research Office contract, and the National Science Foundation Information Technology Research (ITR) and Physics at the Information Frontier (PIF) programmes.
Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests.
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Moehring, D., Maunz, P., Olmschenk, S. et al. Entanglement of single-atom quantum bits at a distance. Nature 449, 68–71 (2007). https://doi.org/10.1038/nature06118
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