  So far, unconditional security in key distribution processes has been
confined to quantum key distribution (QKD) protocols based on the no-cloning
theorem of nonorthogonal bases. Recently, a completely different approach, the
unconditionally secured classical key distribution (USCKD), has been proposed
for unconditional security in the purely classical regime. Unlike QKD, both
classical channels and orthogonal bases are key ingredients in USCKD, where
unconditional security is provided by deterministic randomness via path
superposition-based reversible unitary transformations in a coupled
Mach-Zehnder interferometer. Here, the first experimental demonstration of the
USCKD protocol is presented.
