  Quantum communication is an important branch of quantum information science,
promising unconditional security to classical communication and providing the
building block of a future large-scale quantum network. Noise in realistic
quantum communication channels imposes fundamental limits on the communication
rates of various quantum communication tasks. It is therefore crucial to
identify or bound the quantum capacities of a quantum channel. Here, we
consider Gaussian channels that model energy loss and thermal noise errors in
realistic optical and microwave communication channels and study their various
quantum capacities in the energy-constrained scenario. We provide improved
lower bounds to various energy-constrained quantum capacities of these
fundamental channels and show that higher communication rates can be attained
than previously believed. Specifically, we show that one can boost the
transmission rates of quantum information and private classical information by
using a correlated multi-mode thermal state instead of the single-mode thermal
state of the same energy.
