Unital Qubit Queue-channels: Classical Capacity and Product Decoding

Quantum queue-channels arise naturally in the context of buffering in quantum networks. It has been shown that the upper-bound on the classical capacity of an additive queue-channel has a simple expression and is achievable for the erasure channel, depolarizing [IEEE JSAIT, 1(2):432-444, Aug 2020] channel and symmetric generalized amplitude damping channel [arXiv:2107.13486]. In this paper, using a simple product (non-entangled) decoding (measurement) strategy, we show that the same upper-bound is also achievable for a large class of unital qubit queue-channels. As an intermediate result, we derive an explicit capacity achieving product decoding strategy for any i.i.d. unital qubit channel, which could be of independent interest.