Quantum communication exploits superposition and entanglement to enable secure and efficient information transfer. This dissertation investigates two aspects of quantum communication. First, we introduce quantum ring states, a class of non-Gaussian mixed states generated through phase modulation and transmission over lossy thermal bosonic channels. These states admit tractable achievable-rate expressions that approach the quantum capacity in the low-power regime and improve covert communication performance. Second, we develop the theoretical foundations of Quantum-Memory-Free (QMF) Quantum Secure Direct Communication (QSDC), an alternative to QKD-based secure communication. We establish privacy amplification theorems for coded sequences and characterize information leakage arising from non-ideal keys and retransmissions, enabling practical and resource-efficient QMF-QSDC protocols.