  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 $-0.228\pm0.004$, corresponded to $85.1\pm0.7\%$ 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 telecommunication
wavelengths.
