An upper bound on quantum capacity of unital channels

TL;DR

This paper establishes an upper bound on the quantum capacity of unital channels using output 2-norm analysis, showing that exceeding this bound results in high decoding error, with applications to quantum expanders.

Contribution

It introduces a new upper bound on quantum capacity for unital channels based on regularized output 2-norm and demonstrates its near tightness for quantum expander channels.

Findings

Upper bound on quantum capacity in terms of output 2-norm

Exceeding the bound leads to large decoding errors

Quantum expander channels have nearly matching bounds

Abstract

We analyze the quantum capacity of a unital quantum channel, using ideas from the proof of near-optimality of Petz recovery map [Barnum and Knill 2000] and give an upper bound on the quantum capacity in terms of regularized output $2$-norm of the channel. We also show that any code attempting to exceed this upper bound must incur large error in decoding, which can be viewed as a weaker version of the strong converse results for quantum capacity. As an application, we find nearly matching upper and lower bounds (up to an additive constant) on the quantum capacity of quantum expander channels. Using these techniques, we further conclude that the `mixture of random unitaries' channels arising in the construction of quantum expanders in [Hastings 2007] show a trend in multiplicativity of output $2$-norm similar to that exhibited in [Montanaro 2013] for output $\infty$-norm of random quantum channels.