  As with classical information, error-correcting codes enable reliable
transmission of quantum information through noisy or lossy channels. In
contrast to the classical theory, imperfect quantum channels exhibit a strong
kind of synergy: there exist pairs of discrete memoryless quantum channels,
each of zero quantum capacity, which acquire positive quantum capacity when
used together. Here we show that this "superactivation" phenomenon also occurs
in the more realistic setting of optical channels with attenuation and Gaussian
noise. This paves the way for its experimental realization and application in
real-world communications systems.
