Quantum dialogue mediated by EPR-type entangled coherent states

Abstract

Talking with each other on the telephone is convenient but insecure because the conversation content can be eavesdropped perfectly. Quantum dialogue protocols have thus been devised to enable two parties to talk with a reasonable level of security suited to urgent situations without the need of a prior quantum key distribution. Existing protocols use either discrete-variable or continuous-variable entangled states each of which has its own pros and cons. Here we employ Einstein–Podolky–Rosen-type entangled coherent states with fixed and large enough amplitudes which are intermediate between discrete- and continuous-variable states. The outstanding pros is the possibility of unambiguous and efficient identification of a given entangled coherent state which is necessary for the decoding process. Single-mode gates required for the encoding process are executable as well, possibly with the assistance of additional resources. Two types of control methods are introduced to protect the quantum dialogue from outsider’s attacks. The information leakage problem is also discussed showing that it hardly influences the protocol security for a long enough dialogue.