Entanglement of a Photon and a Collective Atomic Excitation

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Abstract
We describe a new experimental approach to probabilistic atom-photon (signal) entanglement. Two qubit states are encoded as orthogonal collective spin excitations of an unpolarized atomic ensemble. After a programmable delay, the atomic excitation is converted into a photon (idler). Polarization states of both the signal and the idler are recorded and are found to be in violation of the Bell inequality. Atomic coherence times exceeding several microseconds are achieved by switching off all the trapping fields—including the quadrupole magnetic field of the magneto-optical trap—and zeroing out the residual ambient magnetic field.
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