Figure 8: Two-way capacities of basic qubit channels.
From: Fundamental limits of repeaterless quantum communications
(a) Two-way capacity of the depolarizing channel 
with arbitrary probability p. It is contained in the shadowed region specified by the bounds in equation (36). We also depict the best-known bound based on the squashed entanglement54 (dashed). (b) Two-way capacity of the amplitude damping channel 
for arbitrary damping probability p. It is contained in the shadowed area identified by the lower bound (LB) of equation (48) and the upper bound (UB) of equation (49). We also depict the bound of equation (47) (upper solid line), which is good only at high dampings; and the bound 
of ref. 54 (dotted line), which is computed from the entanglement-assisted classical capacity CA. Finally, note the separation of the two-way capacity 
from the unassisted quantum capacity 
(dashed line).
