  Quantum key distribution can provide unconditionally secure key exchange for
remote users in theory. In practice, however, in most quantum key distribution
systems, quantum hackers might steal the secure keys by listening to the side
channels in the source, such as the photon frequency spectrum, emission time,
propagation direction, spatial angular momentum, and so on. It is hard to
prevent such kinds of attacks because side channels may exist in any of the
encoding space whether the designers take care of or not. Here we report an
experimental realization of a side-channel-free quantum key distribution
protocol which is not only measurement-device-independent, but also immune to
all side-channel attacks in the source. We achieve a secure key rate of 4.80e-7
per pulse through 50 km fiber spools.
