Semi-quantum Key Distribution Robust Against Combined Collective Noise

This paper first proposes a new coding function for the six-qubit decoherence-free states that can resist both types of collective noise (i.e., dephasing and rotation noise) simultaneously. Subsequently, based on the coding function, a semi-quantum key distribution (SQKD) protocol is designed such that a sender with strong quantum capabilities can send a key to a classical receiver who can merely perform classical operations. This is the first SQKD protocol that can resist the combined collective noise. Analyses show that this protocol is secure and effective.