Polarization Tracking and Active Compensation Using Classical Headers in Quantum Wrapper Networking

Abstract

Quantum wrapper networking (QWN) is an emerging quantum networking protocol that wraps qubits in classical header bits to enable switching/routing, monitoring, and control without detecting the quantum signal. In this work, we encode header bits with two nonorthogonal polarization references to track and actively compensate for the changing birefringence of a 48 km deployed fiber link. Our method is analytical and deterministic, using motorized waveplates and a variable phase retarder to accurately and stably compensate the channel. We verify successful compensation by measuring the polarization stability of single photon qubits and the visibility of entangled photon pairs under both slow birefringence drift due to environmental fluctuations and large sudden changes designed to emulate those that occur during packet switching and rerouting over different fiber paths. For large, sudden changes, our compensator recovers the Stokes vector of single photons to within 10 degrees of the target state on the Poincar\'e sphere and restores two-photon interference visibilities to better than 79% on a deployed fiber link. Additionally, experiments monitoring long-term compensation over 44 hours show that visibilities remain above 84.5% with compensation active and degrade to below the quantum threshold of 70.7% within 4 hours of the compensator being turned off. These results add a polarization-control layer to QWN and illustrate that information-carrying headers can enable deterministic physical-layer compensation in the quantum channel over long-distance deployed fiber links.