Broadband Polarization Compensation with Link Segment Reconstruction for Quantum Optical Links

In polarization-encoded quantum communication, optical links often induce wavelength-dependent polarization transformations that degrade signal fidelity. When a polarization compensator is embedded between two segments of a link, it cannot simply invert the end-to-end transformation. Furthermore, standard three-wave-plate controllers often fail to provide universal control when operating away from their design wavelengths due to retardance dispersion. This paper addresses these challenges by developing a robust, broadband compensation framework.

The researchers introduce a four-wave plate (Q-Q-Q-H) compensator that acts as a preconditioned system, allowing for the synthesis of arbitrary SO(3) polarization rotations even with non-ideal wave plates. To handle the middle-link placement, they developed an eight-Stokes vector protocol that performs tomography on the two link segments surrounding the compensator. By measuring the output Stokes vectors for four known compensator settings, the system reconstructs the Mueller matrices for both the input-side and output-side link segments, enabling the precise calculation of the required compensation settings.

The four-plate sequence successfully suppressed polarization-induced excess quantum bit error rate (QBER) to sub-percent levels at wavelengths more than 100 nm away from the design wavelength, significantly outperforming standard three-plate configurations. The researchers validated this approach across various link components, including fiber spools and optical switches. Additionally, they demonstrated that by using two auxiliary wavelengths to track temperature-driven drift, they could maintain an excess QBER below 1% in a non-interruptive manner, supporting the viability of wavelength-division-multiplexed (WDM) quantum networks.

This work provides a deterministic, flexible framework for polarization stabilization in complex quantum networks. By enabling compensator placement at intermediate nodes and ensuring robustness against wavelength-dependent retardance errors, this method facilitates the integration of quantum communication into existing fiber infrastructure where multiple channels must coexist without constant recalibration.