Reliability Function of Classical-Quantum Channels

TL;DR

This paper establishes the reliability function for classical-quantum channels, proving a lower bound that confirms a long-standing conjecture and matches known upper bounds at high rates, advancing quantum information theory.

Contribution

It proves a lower bound for the reliability function using quantum Renyi information, resolving Holevo's conjecture and determining the function at high rates.

Findings

Proved a lower bound matching the upper bound at high rates.

Resolved Holevo's conjecture from 2000.

Determined the reliability function for classical-quantum channels.

Abstract

We study the reliability function of general classical-quantum channels, which describes the optimal exponent of the decay of decoding error when the communication rate is below the capacity. As the main result, we prove a lower bound, in terms of the quantum Renyi information in Petz's form, for the reliability function. This resolves Holevo's conjecture proposed in 2000, a long-standing open problem in quantum information theory. It turns out that the obtained lower bound matches the upper bound derived by Dalai in 2013, when the communication rate is above a critical value. Thus, we have determined the reliability function in this high-rate case. Our approach relies on Renes' breakthrough made in 2022, which relates classical-quantum channel coding to that of privacy amplification, as well as our new characterization of the channel Renyi information.