A team of researchers at California Institute of Technology in Pasadena and at Bell Labs in Murray Hill, N.J., has reported the entanglement of ensembles of cesium atoms separated by 2.8 m and the storage of the entanglement for up to 1 µs. The investigators note that the separation distance is limited by fiber optic attenuation to approximately 2 km, which may be extended with the development of quantum repeaters, and they suggest that new trapping schemes may enable the storage time to increase to 1 s.

In the experimental setup, a 150-ns “write” pulse of 852-nm radiation, detuned by 10 MHz below an atomic transition in cesium and sufficiently weak to minimize unwanted excitations, was divided in a beamsplitter before exciting two ensembles of 100,000 cooled atoms each. This generated forward-scattered anti-Stokes fields that were filtered and combined to interfere in a second beamsplitter, entangling the ensembles. The output of the second beamsplitter was directed to a pair of single-photon detectors, with a signal at either one — but not the other — indicating entanglement.

After a delay, the scientists exposed the ensembles to a 130-ns “read” pulse of 894-nm radiation, mapping the state of the atoms to the propagating optical field. Subsequent statistical analysis of photodetection events and measurement of off-diagonal coherence verified entanglement.