Low-loss polarization control in fiber systems for quantum computation.

Optical quantum information processing requires low loss interference of quantum light. Also, 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 low loss method to optimize interference visibility by controlling the polarizations to a crosspoint of two circular trajectories on the Poincaré sphere. Our method maximizes visibility with low optical loss by using fiber stretchers as polarization controllers on both paths of the interferometer. 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.

Generation of highly pure single-photon state at telecommunication wavelength.

Telecommunication wavelength with well-developed optical communication technologies and low losses in the waveguide are advantageous for quantum applications. However, an experimental generation of non-classical states called non-Gaussian states at the telecommunication wavelength is still underdeveloped. Here, we generate highly-pure-single-photon states, one of the most primitive non-Gaussian states, by using a heralding scheme with an optical parametric oscillator and a superconducting nano-strip photon detector. The Wigner negativity, the indicator of non-classicality, of the generated single photon state is -0.228 ± 0.004, corresponded to 85.1 ± 0.7% of single photon and the best record of the minimum value at all wavelengths. The quantum-optics-technology we establish can be easily applied to the generation of various types of quantum states, opening up the possibility of continuous-variable-quantum-information processing at the telecommunication wavelength.