Quantum channels and memory effects

TL;DR

This paper surveys the theory of quantum channels, emphasizing the role of memory effects and correlated errors, which are more realistic than memoryless models, highlighting novel phenomena at the intersection of quantum information and physics.

Contribution

It provides a comprehensive overview of quantum channels with memory effects, integrating classical and quantum error models, and discussing non-Markovian dynamics in quantum systems.

Findings

Memory effects lead to new quantum information phenomena.

Correlated errors are more realistic than independent noise models.

Non-Markovian dynamics influence quantum communication capacities.

Abstract

Any physical process can be represented as a quantum channel mapping an initial state to a final state. Hence it can be characterized from the point of view of communication theory, i.e., in terms of its ability to transfer information. Quantum information provides a theoretical framework and the proper mathematical tools to accomplish this. In this context the notion of codes and communication capacities have been introduced by generalizing them from the classical Shannon theory of information transmission and error correction. The underlying assumption of this approach is to consider the channel not as acting on a single system, but on sequences of systems, which, when properly initialized allow one to overcome the noisy effects induced by the physical process under consideration. While most of the work produced so far has been focused on the case in which a given channel transformation acts identically and independently on the various elements of the sequence (memoryless configuration in jargon), correlated error models appear to be a more realistic way to approach the problem. A slightly different, yet conceptually related, notion of correlated errors applies to a single quantum system which evolves continuously in time under the influence of an external disturbance which acts on it in a non-Markovian fashion. This leads to the study of memory effects in quantum channels: a fertile ground where interesting novel phenomena emerge at the intersection of quantum information theory and other branches of physics. A survey is taken of the field of quantum channels theory while also embracing these specific and complex settings.