Towards a New Way of Quantum Communication : Getting Around the Shannon Limit of Cryptography
Osamu Hirota, Masaki Sohma · 2011
This paper surveys a new way for research of quantum cryptography. The conventional cipher is designed by a mathematical algorithm and its security is evaluated by the complexity of the algorithm for the cryptanalysis and ability of computers. This kind of cipher cannot exceed the Shannon limit of cryptography, and it can be decrypted with probability one in principle by trying all the possible keys against the data length equal to the secret key length. A cipher based on quantum effect so called physical cipher may exceed the Shannon limit of the cryptography. The quantum stream cipher by �/� or Yuen-2000 protocol (Y- 00) which operates at Gbit/sec is a typical example of such a cipher, and may exceed the Shannon limit. In this paper, we show an existence of cipher which exceeds the Shannon limit, and discuss practical realization methods for them. First, the conditions to exceed the Shannon limit are summarized. Then we discuss the generalized secret capacity of the wiretap channel model supported by a secret key to discuss an existence of cipher exceeding the Shannon limit. The generalized secret capacities for space communication and fiber communication are given. In gen- eral, one needs quantum optimum receiver to realize such capacity. But when bandwidth limitation of users is deleted or the eavesdropper has a device limit, a cipher scheme exceeding the Shannon limit may be realized merely by the conventional optical communication devices which are affected by quantum noise at optical signal measurement.