Non-local polarization alignment and control in fiber using feedback from correlated measurements of entangled photons

TL;DR

This paper introduces a rapid, feedback-based method for aligning and tracking polarization measurement bases in entangled photon experiments over fiber channels, addressing challenges from fiber birefringence.

Contribution

It presents a novel, fast, and autonomous polarization alignment technique using the Nelder-Mead method, suitable for quantum communication over optical fibers.

Findings

Successful polarization alignment over 7.1 km fiber loop

Effective dynamic tracking in drifting conditions

No need for classical pilot tones or interleaved photons

Abstract

Quantum measurements that use the entangled photons' polarization to encode quantum information require calibration and alignment of the measurement bases between spatially separate observers. Because of the changing birefringence in optical fibers arising from temperature fluctuations or external mechanical vibrations, the polarization state at the end of a fiber channel is unpredictable and time-varying. Polarization tracking and stabilization methods originally developed for classical optical communications cannot be applied to polarization-entangled photons, where the separately detected photons are statistically unpolarized, yet quantum mechanically correlated. We report here a fast method for automatic alignment and dynamic tracking of the polarization measurement bases between spatially separated detectors. The system uses the Nelder-Mead simplex method to minimize the observed coincidence rate between non-locally measured entangled photon pairs, without relying on classical wavelength-multiplexed pilot tones or temporally interleaved polarized photons. Alignment and control is demonstrated in a 7.1 km deployed fiber loop as well as in a controlled drifting scenario.