Quantum Key Distribution With Limited Classical Bob

Recently, quantum key distribution (QKD) with classical Bob has been suggested and proven completely robust by Boyer et al. [Phys. Rev. Lett. 99 (2007) 140501] in which quantum Alice has a fully-edged quantum power, while classical Bob is restricted to preparing and measuring a particle in the computational basis, reflecting or reordering the particles. In this paper, we describe two related but simpler semi-quantum key distribution protocol that exempts classical Bob from measurement (hereafter called limited classical Bob for convenience) and, prove them completely robust. As limited classical Bob can deterministically prepare the qubits, we exploit this feature to construct a semi-quantum secure direct communication protocol, which is the direct communication of secret messages without first producing a shared secret key.