  We report on an experimental demonstration of the modified Ekert 91 protocol
of quantum key distribution using a hybrid entanglement source with two
different degrees of freedoms, a 1550 nm time-bin qubit and 810 nm polarization
qubit. The violation of the Clauser-Horne-Shimony-Holt inequality could be
demonstrated for the entanglement between the polarization qubit in free space
and the time-bin qubit through 20 km fiber transmission. The secure key rate in
our system is estimated 70-150 bps.
