Precision channel loss estimation for quantum networks

Quantum metrology plays a crucial role in characterizing and stabilizing optical channels in a quantum network, where precise estimation of loss is critical for loss-sensitive quantum protocols. Optical fiber channels are the primary medium for linking quantum nodes, where loss estimation with optical quantum states may enhance the precision compared to classical probes. In this study, we investigate the use of two-mode squeezed vacuum (TMSV) states, an optical quantum state generated via parametric down-conversion, and their detection with photon-number-resolving detectors for characterization of transmission loss. Theoretical analysis indicates that TMSV states exhibit higher Fisher information for transmission parameter estimation than coherent states, suggesting they are superior probes that can significantly enhance measurement precision. Our experimental results confirm that these TMSV states outperform coherent states in the context of loss estimation in an optical channel, achieving up to 3.9 dB higher Fisher information for transmissivity estimation without relying on post-selection measurements. This source can effectively probe transmission losses in optical fibers up to 35 km, maintaining a measurement precision advantage over coherent probe states. These findings underscore the potential of TMSV states to advance quantum metrology and improve the robustness of quantum network applications.