Abstract

There are two basic uses of a quantum communication channel. On the one hand, the channel can be used to transmit classical information from the sender Alice to the receiver Bob. Alice encodes her message into quantum states ρ₁,ρ_2,…ρ_Nwith the corresponding probabilities ρ₁,ρ_2,…ρ_N and sends them to Bob one by one. Bob I hen has to perform suitable measurements to uncover the nature of Alice’s message. The action of the channel is described as the interaction of the quantum system with the environment which distorts the signal. The upper bound to the information that he can obtain is given by the Holevo bound. We illustrate this protocol with a binary optical signal encoded into a pair of coherent states transmitted through an optical fibre. On the other hand, we can use the same channel to transmit quantum information. This involves Alice preparing a quantum state which she wants to transmit to Bob as faithfully as possible through a decohering quantum channel. This is related to Alice preparing a bipartite quantum system and sending one of the subsystems to Bob through the channel. Once they share a certain amount of entanglement, they can use the standard teleportation protocol to reliably send a stale from Alice to Bob. The rate of information transfer is in this case governed by "the coherent information" [1]. We analyse a general quantum channel, and describe how entanglement changes due to interaction with the environment [2]. We obtain relationships involving changes in the mutual information, entanglement and coherent information and relate the two separate uses of the quantum channel. obtaining a relationship between the Holevo bound and the amount of entanglement transmitted through a quantum channel. The channel might also represent actions of an eavesdropper so we can put bounds on the eavesdropping efficiency. We apply these results to a pair of entangled optical modes when one of them is transmitted through a decoherent channel.

© 1998 IEEE