Coherence Scaling in Quantum Communication Protocols

We investigate how quantum coherence scales and is redistributed in quantum communication protocols, us-ing superdense coding and quantum teleportation as paradigmatic case studies. Employing the relative entropyof coherence as a circuit-level resource measure, we show that multipartite resource states relevant to general-ized superdense coding can enable scalable communication while exhibiting only logarithmic or even constantcoherence growth, depending on their entanglement structure. In sharp contrast, quantum teleportation displaysan unavoidable, protocol-induced coherence cost that grows linearly with the number of teleported qubits andis independent of the input state. Through a stage-resolved analysis of the teleportation circuit, we separateprotocol-generated coherence from message-dependent contributions and identify a universal two-bit coherenceoffset per teleported qubit at the maximal-coherence stage. We further demonstrate explicitly that this extensiveintermediate coherence generation is fully consistent with information-theoretic bounds, including the Holevolimit, and does not correspond to an increase in accessible classical information.