  Quantum cryptography has attracted much attention in recent years. In most
existing quantum cryptographic protocols, players usually need the full quantum
power of generating, manipulating or measuring quantum states. Semiquantum
cryptography was proposed to deal with the issue that some players require only
partial quantum power, such as preparing or measuring quantum states in the
classical basis, which simplifies the implementations of quantum cryptography.
However, the efficiency of the existing semiquantum cryptographic protocols was
relatively low from a practical point of view. In this paper, we devise some
new semiquantum key distribution (SQKD) protocols which highly improve the
efficiency of the most well-known SQKD protocols [Phys. Rev. Lett. 99, 140501
(2007) & Phys. Rev. A 79, 052312 (2009)]. By letting players select their
actions asymmetrically, the efficiency of our new protocols can be made
asymptotically close to 100%. Besides, one of our proposed protocols also
utilizes the discarded X-SIFT bits in the original SQKD protocol, which further
improves the efficiency of SQKD. We prove that the proposed SQKD protocols are
completely robust against the most general attack.
