Yuen's Criticisms on Security of Quantum Key Distribution and Onward

Quantum Key Distribution (QKD) has been attracting researchers that it would provide provable security to distribute secret keys since its birth in 1984. Since 2005, the trace distance between an ideal quantum state and an actually distributed state has been employed to evaluate its security level, and the trace distance was given an interpretation that it would be a maximum failure probability in distributing perfectly secure keys. However, in 2009, H. P. Yuen criticized that the trace distance would not have such an interpretation. Since then, O. Hirota, K. Kato, and T. Iwakoshi have been warning to make people pay attention to Yuen's criticisms. In 2015, T. Iwakoshi precisely explained why Yuen has been correct. In 2016, Yuen himself published a paper to explain the potentially unsolved problems in QKD. This study precisely explains the most important problems given in Yuen's paper, and gives recent topics around QKD and other quantum cryptographic protocols.