Quantum entanglement enhances the capacity of bosonic channels with memory

The bosonic quantum channels have recently attracted a growing interest, motivated by the hope that they
open a tractable approach to the generally hard problem of evaluating quantum channel capacities. These
studies, however, have always been restricted to memoryless channels. Here, it is shown that the classical
capacity of a bosonic Gaussian channel with memory can be significantly enhanced if entangled symbols are
used instead of product symbols. For example, the capacity of a photonic channel with 70%-correlated thermal
noise of one-third the shot noise is enhanced by about 11% when using 3.8-dB entangled light with a modu-
lation variance equal to the shot noise.