Optimality of Sequential Decoding for the Bosonic Compound Channel

Recent efforts in the design of communication systems have in addition to data rates embraced a focus on other metrics, such as round-trip latency. Data transmission over a channel is typically preceded by so-called pilot symbols, which let the receiver learn the channel conditions. Together with appropriate feedback, this allows for transmission at high data rates. Trading data rates for communication latency, a communication link may however avoid feedback and instead operate under channel uncertainty, in which case the accurate model is the compound channel. Further reductions of latency may be harnessed from quantum receiver design. In this work we focus in particular on the application of the sequential decoding approach to the bosonic compound channel with unknown phase rotations, loss parameters and thermal noise values. We show that sequential decoding is optimal.