Measurement-Based Polarization Compensation

Classical and quantum optical systems that utilize single-mode fibers in the polarization degree of freedom commonly suffer from polarization mode dispersion (PMD) and polarization drift (PD), which must be compensated to restore the information. Compensation for PMD and PD in optical fibers is particularly critical for implementing quantum networks, as they limit the transmission rate, distance, and usable spectral bandwidth of polarization-entangled qubits. Conventional solutions to these problems include massive dense wavelength division multiplexing channels and lossy dispersion compensators that use either external electric fields or mechanical force on single-mode fibers to undo the effect of birefringence of the fiber. We compensate for PMD and PD using measurement-based optimization code that finds the optimal measurement basis angles for a given state with the effect of PMD and PD taken into account. This software-based compensation allows a loss-less, device-independent, and more on-chip friendly solution. The procedure first takes a process tomography of the transmitted state after the single-mode fiber. It then calculates the maximum likelihood of the process tomography result in every polarization basis: H, V, D, A, L, R. This process is repeated with slightly different basis angles from the original basis angles until the angles that give the maximum polarization fidelity are found. This procedure is applied to optimize the CHSH violation of entangled photons distributed in optical fiber. We present and compare the measurement results with and without our compensation algorithm with both classical and quantum light sources.