  This research note suggests a new way to realize a high speed direct
encryption based on quantum detection theory. The conventional cipher is
designed by a mathematical algorithm and its security is evaluated by the
complexity of the algorithm for 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 with quantum
effect in physical layer may exceed the Shannon limit of cryptography. The
quantum stream cipher by $\alpha/\eta$ or Yuen-2000 protocol (Y-00) which
operates at Gbit/sec is a typical example of such a cipher. That is, ciphertext
of mathematical cipher with a secret key is masked by quantum noise of laser
light when an eavesdropper observes optical signals as a ciphertext of the
mathematical cipher, while the legitimate receiver does not suffer the quantum
noise effect. As a result, the inherent difference of accuracy of ciphertext
between eavesdropper and legitimate receiver arises. This is a necessary
condition to exceed the Shannon limit of cryptography. In this note, we present
a new method to generate an inherent difference of accuracy of the ciphertext,
taking into account a fundamental properties of quantum detection schemes.
