Novel Robust Quantum Key Distribution Protocol with Symmetry

Quantum key distribution (QKD) has been proved unconditionally secure. But noise and imperfection of realistic devices are major difficulties for implementing quantum key distribution. Particularly, the birefringence in optical fiber corresponds to an insuppressible collective unitary transformation on the polarization space. Obviously does not change the permutation symmetry of multiphoton state. By encoding on three 3-photon states with different permutation symmetries, we propose a novel polarization-based protocol, which is robust against an arbitrary degree of collective noise. A clear characteristic of this protocol is that entanglement is absent in principle.