A critical requirement for diverse applications in quantum information science is the capability to disseminate quantum resources over complex quantum networks^{1, 2}. For example, the coherent distribution of entangled quantum states together with quantum memory (for storing the states) can enable scalable architectures for quantum computation³, communication⁴ and metrology⁵. Here we report observations of entanglement between two atomic ensembles located in distinct, spatially separated set-ups. Quantum interference in the detection of a photon emitted by one of the samples projects the otherwise independent ensembles into an entangled state with one joint excitation stored remotely in 10⁵ atoms at each site⁶. After a programmable delay, we confirm entanglement by mapping the state of the atoms to optical fields and measuring mutual coherences and photon statistics for these fields. We thereby determine a quantitative lower bound for the entanglement of the joint state of the ensembles. Our observations represent significant progress in the ability to distribute and store entangled quantum states.

1. Norman Bridge Laboratory of Physics 12-33, California Institute of Technology, Pasadena, California 91125, USA
2. Bell Labs, Lucent Technologies, Room 1D-428, 600-700 Mountain Avenue, Murray Hill, New Jersey 07974, USA

Correspondence to: H. J. Kimble¹ Correspondence and requests for materials should be addressed to H.J.K. (Email: hjkimble@caltech.edu).

Received 31 August 2005; Accepted 19 October 2005

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