Secure communication and information protection become more and more important in the modern society. Quantum key distribution (QKD) enables two remote legitimate users to share unconditionally secure keys. Different from the former ones, the security of these keys is based on the principles of quantum physics and could be proved. Combined with one-time pad encryption, QKD can provide us an effective way for secure communication and information protection [1, 2]. Since Bennett and Brassard proposed the famous BB84 protocol [3], QKD or quantum cryptography has become one of the most dynamic research fields. After about twenty years of development, experimental QKD has achieved significant improvements, such as the transmission distance increased from 32 cm [4] to 260 km [5], the speed increased from 200 Hz [4] to 10 GHz [6].
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