The development of large-scale quantum networks requires reliable quantum channels, the quality of which can be quantified by the framework of quantum process tomography. In this work, we leverage ancilla-assisted process tomography and Bayesian inference to probe a 1.6 km deployed fiber-optic link. We send one of two polarization-entangled photons from Alice in one building to Bob in another, exploiting the local qubit as an ancilla system to characterize the corresponding quantum channel. Monitoring over a 24 h period returns a steady process fidelity of 95.1(1)%, while controllable spectral filtering with passbands from 0.025-4.38 THz finds fidelities that first increase, then level off with bandwidth, suggesting both stable operation with time and minimal polarization mode dispersion. To our knowledge, these results represent the first AAPT of a deployed quantum link, revealing a valuable tool for in situ analysis of entanglement-based quantum networks.
@article{arxiv.2410.00892,
title = {Deployed quantum link characterization via Bayesian ancilla-assisted process tomography},
author = {Arefur Rahman and Noah I. Wasserbeck and Zachary Goisman and Rhea P. Fernandes and Brian T. Kirby and Muneer Alshowkan and Chris Kurtz and Joseph M. Lukens},
journal= {arXiv preprint arXiv:2410.00892},
year = {2025}
}