Communication channels secured from eavesdropping via transmission of photonic Bell states

This paper proposes a quantum communication scheme for sending a definite b inary sequence while confirming the security of the transmission. The schem e is very suitable for sending a ciphertext in a secret-key cryptosystem so that we can detect any eavesdropper who attempts to decipher the key. Thus we can continue to use a secret key unless we detect eavesdropping and the security of a key that is used repeatedly can be enhanced to the level of one-time-pad cryptography. In our scheme, a pair of entangled photon twins is employed as a bit carrier which is encoded in a two-term superposition o f four Bell states. Different bases are employed for encoding the binary se quence of a ciphertext and a random test bit. The photon twins are measured with a Bell state analyzer and any bit can be decoded from the resultant B ell state when the receiver is later notified of the coding basis through a classical channel. By opening the positions and the values of test bits, c iphertext can be read and eavesdropping is simultaneously detected.