  We report a free-space entanglement-based quantum key distribution
experiment, implementing the biased basis protocol between two sites which are
15.3 km apart. Photon pairs from a polarization-entangled source are
distributed through two 7.8-km free-space optical links. An optimal bias 20:80
between the X and Z basis is used. A post-processing scheme with finite-key
analysis is applied to extract the final secure key. After three-hour
continuous operation at night, a 4293-bit secure key is obtained, with a final
key rate of 0.124 bit per raw key bit which increases the final key rate by
14.8% comparing to the standard BB84 case. Our results experimentally
demonstrate that the efficient BB84 protocol, which increases key generation
efficiency by biasing Alice and Bob's basis choices, is potentially useful for
the ground-satellite quantum communication.
