  Bit sifting is an important step in the post-processing of Quantum Key
Distribution (QKD) whose function is to sift out the undetected original keys.
The communication traffic of bit sifting has essential impact on the net secure
key rate of a practical QKD system, and it is facing unprecedented challenges
with the fast increase of the repetition frequency of quantum channel. In this
paper, we present an efficient bit sifting scheme whose core is a lossless
source coding algorithm. Both theoretical analysis and experimental results
demonstrate that the performance of our scheme is approaching the Shannon
limit. Our scheme can greatly decrease the communication traffic of the
post-processing of a QKD system, which means it can decrease the secure key
consumption for classical channel authentication and increase the net secure
key rate of the QKD system. Meanwhile, it can relieve the storage pressure of
the system greatly, especially the device at Alice side. Some recommendations
on the application of our scheme to some representative practical QKD systems
are also provided.
