IQC Math and CS seminar featuring Peixue Wu

Additivity of quantum capacities in simple non-degradable quantum channels

by Peixue Wu 

Quantum channel capacities give the fundamental performance limits for information flow over a communication channel. However, the prevalence of superadditivity is a major obstacle to understanding capacities, both quantitatively and conceptually.  Examples of additivity, while rare, provide key insight into the origins of nonadditivity and enable our best upper bounds on capacities. Degradable channels, which have additive coherent information, are some of the only channels for which we can calculate the quantum capacity. In this paper, we introduce two families of non-degradable channels whose coherent information is nonetheless additive, making their quantum capacities straightforward to evaluate.

First, we show that channels that can “outperform” their environment in a sense weaker than degradability can  exhibit strong or weak additive coherent information. Second, we explore a complementary construction that modifies a channel so that it maintains additivity of coherent information but loses the aforementioned “outperforming” property. We illustrate how structural constraints can guarantee strong and weak additivity, and how weakening these constraints causes strong additivity to fail while weak additivity may persist.

Location

• QNC 1201

• Online on Zoom

  • Meeting ID: 952 5312 7571

  • Passcode: 915909