  Recently, new physics for unconditional security in a classical key
distribution (USCKD) in a frame of a double Mach-Zehnder interferometer has
been proposed and demonstrated as a proof of principle, where the unconditional
security is unaffected by the no-cloning theorem of quantum key distribution
protocols. Due to environmental phase fluctuations caused by temperature
variations, atmospheric turbulences, or mechanical vibrations, active phase
locking seems to be necessary for the two-channel transmission layout of USCKD.
Here, the two-channel layout of USCKD is demonstrated to be an environmental
noise-immune protocol especially for free space optical links, where the
transmission distance is potentially unlimited if random phase noises are
considered.
