S. Elham Mousavigharalari

arXiv2026

Operational Collapse Region in Repeaterless Loss-Dephasing Quantum Channels

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.

quant-ph↗

arXiv2026

Characterizing Phase Fragility via Algorithmically Prepared Ancillas in Repeated-Interaction Models

We study how a phase parameter $ϕ$, encoded through a single-qubit $H\,φ\,H$ gate sequence, is reflected in the quantum Fisher information (QFI) under realistic noisy dynamics. Within a collision-model framework, a probe qubit interacts sequentially with algorithmically prepared reservoir ancillas, leading to a $ϕ$-dependent steady state from which $\mathcal{F}_ϕ$ can be evaluated in closed form. In parallel, we perform pulse-resolved open-system simulations of the same gate sequence, using Gaussian-driven control motivated by transmon hardware, to obtain the corresponding pre-measurement density matrix. Despite the distinct physical descriptions, both approaches yield QFI profiles with similar phase dependence on the encoded phase. A quantitative comparison using profile-level similarity metrics further shows that the two descriptions identify the same phase-sensitive regions, although their absolute QFI contrasts differ. This consistency indicates that the steady state of a probe qubit interacting with algorithmically prepared ancillas can capture key features of the phase response observed in noisy device-level implementations. The underlying physical mechanism is the persistence of finite steady coherence in the asymptotic probe state, which retains the phase imprint of the prepared ancillas. Beyond conceptual insight, the steady-state framework provides a model-based, tomography-free diagnostic route for characterizing phase sensitivity. Possible uses in biased-noise error correction, hardware-aware compilation, and pulse-level optimization are therefore presented as future outlook directions.

quant-ph↗