  Distributed quantum information processing and communication protocols demand
the ability to generate entanglement among propagating modes. However, thermal
fluctuations can severely limit the fidelity and purity of propagating
entangled states, especially for low-frequency modes relevant for
radio-frequency (RF) signals. Here we propose nonreciprocity as a resource to
render continuous-variable entanglement of propagating modes robust against
thermal fluctuations. By utilising a cold-engineered reservoir we break the
symmetry of reciprocity in a standard two-mode squeezing interaction between a
low- and a high-frequency mode, and show that the rerouting of thermal
fluctuations allows the generation of flying entangled states with high purity.
Our approach requires only pairwise Gaussian interactions and is thus ideal for
parametric circuit QED implementations.
