An unreviewed preprint posted to arXiv proposes a unified converse bound on the two-way capacities of three canonical noisy bosonic Gaussian channels—thermal attenuation, noisy amplification, and additive Gaussian noise—when communication is subject to an energy constraint expressed as a mean transmitted-photon-number limit. The bound covers quantum capacity, entanglement-distribution capacity, private capacity, and secret-key capacity, and holds against arbitrary adaptive protocols that may exploit quantum memories between channel uses.
The authors show that their bound vanishes across the entire entanglement-breaking parameter region, consistent with the expectation that no useful quantum correlations can survive those channels, and that it recovers the known PLOB bound in the infinite-energy limit. At finite energy—the practically relevant regime—the new bound is reported to be tighter than both the Gaussian squashed-entanglement bound and the PLOB bound in certain parameter ranges. Comparing the converse with hashing lower bounds then quantifies how large the remaining gap to achievability actually is, giving researchers a concrete benchmark for assessing progress.
The work extends finite-energy converse techniques into the noise-excess regime, where quantum-limited analyses had previously left a wider gap. Because this is a preprint that has not yet undergone peer review, its claims—including the tightness comparisons and the universality of the adaptive protocol argument—should be treated as provisional until independently verified.