Zero Error Correctibility and Phase Retrievability for Twirling Channels

Abstract

A twirling channel is a quantum channel induced by a continuous unitary representation $\pi = \sum_{i}^{\oplus} m_i\pi_i$, where $\pi_i$ are inequivalent irreducible representations. Motivated by a recent work \cite{Twirling} on minimal mixed unitary rank of $\Phi_{\pi}$, we explore the connections of the independence number, zero error capacity, quantum codes, orthogonality index and phase retrievability of the quantum channel $\Phi_{\pi}$ with the irreducible representation multiplicities $m_i$, the irreducible representation dimensions $\dim H_{\pi_i}$. In particular we show that the independence number of $\Phi_{\pi}$ is the sum of the multiplicities, the orthogonal index of $\Phi_{\pi}$ is exactly the sum of those representation dimensions, and the zero-error capacity is equal to $\log (\sum_{i=1}^{d}m_i)$. We also present a lower bound for the phase retrievability in terms of the minimal length of phase retrievable frames for $C^n$.