  To reduce the influence of random channel polarization variation, especially
fast polarization perturbation,for continuous-variable quantum key distribution
(CV-QKD) systems, a simple and fast polarization tracking algorithm is proposed
and experimentally demonstrated. The algorithm is implemented by an orthogonal
pilot tone scheme, one of the pilot tones is used for estimating the
polarization rotation angle, and the other one is used for compensating the
polarization perturbation introduced phase noise. Meanwhile, residual effects
are compensated precisely with the help of a real-valued FIR filter. In this
case, the excess noise introduced by polarization perturbation is effectively
suppressed. Experimental results show that the polarization scrambling rate
12.57 krad/s can be tracked by using the proposed algorithm, and a good
estimated parameters performance is achieved. Compared to conventional
polarization tracking algorithms such as the constant modulus algorithm (CMA),
experimental results show that the polarization tracking apability of the
proposed algorithm is significantly improved. Furthermore, much faster
polarization tracking performance is evaluated by digital simulations, and the
simulation results show that about 188.50 Mrad/s can be tracked by the proposed
algorithm. Thus, our method provides effective technology for the practical
application of fiber-based CV-QKD.
