  Entanglement between a single photon and a matter qubit is an indispensable
resource for quantum repeater and quantum networks. With atomic ensembles, the
entanglement creation probability is typically very low to inhibit high-order
events. In this paper, we propose and experimentally realize a scheme which
creates atom-photon entanglement with an intrinsic efficiency of 50%. We make
use of Rydberg blockade to generate two collective excitations, lying in
separate internal states. By introducing the momentum degree of freedom for the
excitations, and interfering them via Raman coupling, we entangle the two
excitations. Via retrieving one excitation, we create the entanglement between
the polarization of a single photon and the momentum of the remaining atomic
excitation, with a measured fidelity of 0.901(8). The retrieved optical field
is verified to be genuine single photons. The realized entanglement may be
employed to create entanglement between two distant nodes in a fully heralded
way and with a much higher efficiency.
