Enhanced quantum capacity thresholds from symmetry

TL;DR

This paper significantly improves quantum capacity thresholds for key noise models by extending a representation theoretic framework to optimize coherent information, revealing new bounds in quantum communication.

Contribution

It generalizes a recent framework to the full symmetric subspace, enabling better capacity threshold estimates for depolarizing and Pauli channels.

Findings

First improvement in 18 years for depolarizing channel capacity thresholds.

Achieved a larger increase beyond the hashing bound than all previous efforts.

Identified exponential Kraus operator annihilation leading to decreased environment entropy.

Abstract

The quantum capacity captures the value of a quantum channel for transmitting quantum information, establishing the fundamental limits on quantum communication. In spite of its central role in quantum information theory, the quantum capacity of most channels is unknown, with wide gaps between the best upper and lower bounds. Even deciding whether a channel has nonzero capacity -- finding its capacity threshold -- is difficult. In this paper we report significant increases in the capacity thresholds of two prototypical noise models: the depolarizing channel and Pauli channels. In the case of the depolarizing channel, this is the first improvement in 18 years, giving a bigger increase beyond the hashing bound than all previous improvements combined. Our starting point is the representation theoretic framework recently proposed by Bhalerao and Leditzky (2025) to compute coherent information for special permutation invariant states. We generalize their framework to the full symmetric subspace, which allow us to optimize coherent information over rank two states in that space. A representation theoretic calculation shows that exponentially many Kraus operators of the channel annihilate the symmetric space, corresponding to a massive decrease in environment entropy for states on the symmetric space compared to the maximally mixed state. This explains the enhanced coherent information as a manifestation of degeneracy for the resulting codes.