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, ρ₂,... ρ_N with the corresponding probabilities p₁p₂,…P_N and sends them to Bob one by one. Bob then 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 that distorts the signal. The upper bound to the information that he can obtain is given by the Holevo bound.¹ We illustrate this protocol with a binary optical signal encoded into a pair of coherent states transmitted through an optical fiber. On the other hand, we can use the same channel to transmit quantum information. This involves Alice preparing a quantum state that she wants to transmit to Bob as faithfully as possible through a decoherent 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 state from Alice to Bob. The rate of information transfer is in this case governed by a quantity called the coherent information.²

© 1998 Optical Society of America