  Entanglement between a single photon and a quantum memory forms the building
blocks for quantum repeater and quantum network. Previous entanglement sources
are typically with low retrieval efficiency, which limits future larger-scale
applications. Here, we report a source of highly retrievable spinwave-photon
entanglement. Polarization entanglement is created through interaction of a
single photon with ensemble of atoms inside a low-finesse ring cavity. The
cavity is engineered to be resonant for dual spinwave modes, which thus enables
efficient retrieval of the spinwave qubit. An intrinsic retrieval efficiency up
to 76(4)% has been observed. Such a highly retrievable atom-photon entanglement
source will be very useful in future larger-scale quantum repeater and quantum
network applications.
