The existence of unconditionally secure quantum bit commitment (QBC) is excluded by the Mayers-Lo-Chau no-go theorem. Here we look for the second-best: a QBC protocol that can defeat certain quantum attacks. By breaking the knowledge symmetry between the participants with quantum algorithm, a QBC protocol is proposed and is proven to be secure against a major kind of coherent attacks-the dummy attack, in which the participant makes an empty promise instead of committing to a specific bit. Therefore it surpasses previous QBC protocols which are secure against individual attacks only.
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