Curator's Take
This article pinpoints a “operational collapse region” where loss‑dephasing fibers still harbor genuine entanglement yet standard coincidence‑based schemes cannot extract any quantum advantage, highlighting a gap between theoretical resourcefulness and practical usability that has been largely overlooked in repeaterless quantum‑network designs. By showing that the width of this inaccessible window varies non‑monotonically with phase‑noise rates, it challenges the common assumption that simply reducing dephasing will always improve link performance—a nuance especially relevant as telecom C‑band fibers become the backbone of emerging quantum‑internet prototypes. The findings give engineers concrete criteria for choosing detection architectures and error‑mitigation strategies before committing to costly repeater deployment, while reminding the community that entanglement metrics alone may be insufficient to gauge real‑world protocol success.
— Mark Eatherly
Summary
The distribution of entangled photon pairs over standard optical fiber is a fundamental requirement for the realization of the quantum internet. However, real-world deployment is severely bottlenecked by the interplay of amplitude damping (photon loss) and phase noise (birefringence). In this paper, we numerically investigate the degradation of dual-rail polarization entanglement in telecom C-band fiber links. We demonstrate a critical disparity between the physical survival of quantum correlations and their practical utility in standard communication protocols. By evaluating the unconditional logarithmic negativity against the post-selected teleportation fidelity, we identify a distinct ``operational collapse region'' -- a distance window where the channel retains true quantum entanglement, yet standard coincidence-based detection architectures fail to provide any advantage over classical strategies. Furthermore, we reveal that the width of this inaccessible region exhibits a non-monotonic dependence on the phase noise rate, implying that simply minimizing fiber dephasing does not necessarily optimize the operational efficiency of the network. These findings provide vital guidelines for the design of practical quantum communication links.