Quantum informational properties of the Landau-Streater channel
We study the Landau--Streater quantum channel $Φ: \mathcal{B}(\mathcal{H}_d) \mapsto \mathcal{B}(\mathcal{H}_d)$, whose Kraus operators are proportional to the irreducible unitary representation of $SU(2)$ generators of dimension $d$. We establish $SU(2)$ covariance for all $d$ and $U(3)$ covariance for $d=3$. Using the theory of angular momentum, we explicitly find the spectrum and the minimal output entropy of $Φ$. Negative eigenvalues in the spectrum of $Φ$ indicate that the channel cannot be obtained as a result of Hermitian Markovian quantum dynamics. Degradability and antidegradability of the Landau--Streater channel is fully analyzed. We calculate classical and entanglement-assisted capacities of $Φ$. Quantum capacity of $Φ$ vanishes if $d=2,3$ and is strictly positive if $d \geqslant 4$. We show that the channel $Φ\otimes Φ$ does not annihilate entanglement and preserves entanglement of some states with Schmidt rank $2$ if $d \geqslant 3$.