  Over decades quantum cryptography has been intensively studied for
unconditionally secured data transmission in a quantum regime. Due to the
quantum loopholes caused by imperfect single photon detectors and/or lossy
quantum channels, however, the quantum cryptography is practically inefficient
and even vulnerable to eavesdropping. Here, a method of unconditionally secured
key distribution potentially compatible with current fiber-optic communications
networks is proposed in a classical regime for high-speed optical backbone
networks. The unconditional security is due to the quantum superposition-caused
measurement indistinguishability of a paired transmission channel and its
unitary transformation resulting in deterministic randomness corresponding to
the no-cloning theorem in a quantum regime.
