Low-loss polarization control in fiber systems for quantum computation

Optical quantum information processing exploits interference of quantum light. However, when the interferometer is composed of optical fibers, degradation of interference visibility due to the finite polarization extinction ratio becomes a problem. Here we propose a method to optimize interference visibility by controlling the polarizations to a crosspoint of two circular trajectories on the Poincar\'{e} sphere. Our method maximizes visibility with low optical loss, which is essential for quantum light, by using fiber stretchers as polarization controllers. We also experimentally demonstrate our method, where the visibility was maintained basically above 99.9% for three hours using fiber stretchers with an optical loss of 0.02 dB (0.5%). Our method makes fiber systems promising for practical fault-tolerant optical quantum computers.