Abstract
We study the Landau--Streater quantum channel Φ: B(H_d) ↦ B(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 4. We show that the channel Φ ⊗ Φ does not annihilate entanglement and preserves entanglement of some states with Schmidt rank 2 if d 3.