We perform a proof-of-principle demonstration of the
measurement-device-independent quantum key distribution (MDI-QKD) protocol
using weak coherent states and polarization-encoded qubits over two optical
fiber links of 8.5 km each. Each link was independently stabilized against
polarization drifts using a full-polarization control system employing two
wavelength-multiplexed control channels. A linear-optics-based polarization
Bell-state analyzer was built into the intermediate station, Charlie, which is
connected to both Alice and Bob via the optical fiber links. Using
decoy-states, a lower bound for the secret-key generation rate of 1.04x10^-6
bits/pulse is computed