  We experimentally realized a new method for transmitting quantum information
reliably through paired optical polarization-maintaining (PM) fibers. The
physical setup extends the use of a Mach-Zehnder interferometer, where noises
are canceled through interference. This method can be viewed as an improved
version of the current decohernce-free subspace (DFS) approach in fiber optics.
Furthermore, the setup can be applied bidirectionally, which means that robust
quantum communication can be achieved from both ends. To rigorously quantify
the amount of quantum information transferred, optical fibers are analyzed with
the tools developed in quantum communication theory. These results not only
suggests a practical means for protecting classical and quantum information
through optical fibers, but also provides a new physical platform for enriching
the structure of the quantum communication theory.
