  A ternary ring of operators (TRO) in finite dimensions is a diagonal sum of
spaces of rectangular matrices. TRO as operator space corresponds to quantum
channels that are diagonal sums of partial traces, which we call TRO channels.
TRO channels admits simple, single-letter capacity formula. Using operator
space and complex interpolation techniques, we give perturbative capacities
estimates for a wider class of quantum channels by comparison to TRO channels.
Our estimates applies mainly for quantum and private capacity and also strong
converse rates. The examples includes random unitary from group representations
which in general are non-degradable channels.
