Symbol-Oriented Quantum Communication via Temporal-Mode Multiplexing

We introduce and analyze a quantum-assisted classical communication protocol that encodes symbols from a finite alphabet onto temporal modes using the LM05 operation as a building block. The protocol applies a bit-flip gate independently to temporal slots corresponding to message symbols, yielding a tensor product of LM05 operations on parallel channels. This tensor product structure enables collective attacks not covered by standard LM05 security proofs. We derive a theoretical upper bound on the success probability for complete recovery under random guessing, accounting for the receiver's 50\% guessing ability on lost photons, and emphasize that this bound assumes perfect loss identification. We characterize the intended transmission, derive an asymptotic collective-attack bound for the symbol-set mode, and identify open challenges for composable security. The protocol is not a standalone Quantum Secure Direct Communication scheme, as the classical ordering information requires encryption. For a 53-symbol alphabet, the optimistic 1\% success bound occurs at about 0.8 kilometers under zero QBER and reduces to about 0.6 kilometers for QBER equals 0.01; practical constraints severely limit performance.