  We demonstrate the first implementation of polarization encoding
measurement-device-independent quantum key distribution (MDI-QKD), which is
immune to all detector side-channel attacks. Active phase randomization of each
individual pulse is implemented to protect against attacks on imperfect
sources. By optimizing the parameters in the decoy state protocol, we show that
it is feasible to implement polarization encoding MDI-QKD over large optical
fiber distances. A 1600-bit secure key is generated between two parties
separated by 10 km of telecom fibers. Our work suggests the possibility of
building a MDI-QKD network, in which complicated and expensive detection system
is placed in a central node and users connected to it can perform confidential
communication by preparing polarization qubits with compact and low-cost
equipment. Since MDI-QKD is highly compatible with the quantum network, our
work brings the realization of quantum internet one step closer.
