Quantum-limited amplification without instability

Distributed quantum information processing and communication protocols demand the ability to generate entanglement among propagating modes of high purity. 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 utilizing 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 re-routing of the 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.

arXiv:2208.00024 (2022)