The noisy Werner-Holevo channel and its properties

Abstract: The interest in the Werner-Holevo channel $\Lambda_{1} (\rho)=\frac{1}{2}(\text{tr}(\rho)I-\rho^T)$ has been mainly due to its abstract mathematical properties. We show that in three dimensions and with a slight modification, this channel can be realized as the rotation of qutrit states in random directions by random angles. Our modification takes the form $\Lambda_x(\rho)=(1-x)\rho+x\Lambda_1(\rho)$. Therefore and in view of the potential use of qutrits in quantum processing tasks and their realization in many different platforms, the modified Werner-Holevo channel can be used as a very simple and realistic noise model, in the same way that the depolarizing channel is for qubits. We will make a detailed study of this channel and derive its various properties. In particular, we will use the recently proposed flag extension and other techniques to derive analytical expressions and bounds for the different capacities of this channel. The role of symmetry is revealed in these derivations. We also rigorously prove that the channel $\Lambda_x$ is anti-degradable and hence has zero quantum capacity, in the region $\frac{4}{7}\leq x\leq 1.$