Inequalities and separations among assisted capacities of quantum channels
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We exhibit discrete memoryless quantum channels whose quantum capacity assisted by two-way classical communication, $Q_2$, exceeds their unassisted one-shot Holevo capacity $C_H$. These channels may be thought of as having a data input and output, along with a control input that partly influences, and a control output that partly reveals, which of a set of unitary evolutions the data undergoes en route from input to output. The channel is designed so that the data's evolution can be exactly inferred by a classically coordinated processing of 1) the control output, and 2) a reference system entangled with the control input, but not from either of these resources alone. Thus a two-way classical side channel allows the otherwise noisy evolution of the data to be corrected, greatly increasing the capacity. The same family of channels provides examples where the classical capacity assisted by classical feedback, $C_B$, and the quantum capacity assisted by classical feedback $Q_B$, both exceed $C_H$. A related channel, whose data input undergoes dephasing before interacting with the control input, has a classical capacity $C=C_H$ strictly less than its $C_2$, the classical capacity assisted by independent classical communication.