title: Pulsed Sagnac polarization-entangled photon source with a PPKTP crystal at telecom wavelength.
abstract: We demonstrate pulsed polarization-entangled photons generated from a periodically poled KTiOPO(4) (PPKTP) crystal in a Sagnac interferometer configuration at telecom wavelength. Since the group-velocity-matching (GVM) condition is satisfied, the intrinsic spectral purity of the photons is much higher than in the previous scheme at around 800 nm wavelength. The combination of a Sagnac interferometer and the GVM-PPKTP crystal makes our entangled source compact, stable, highly entangled, spectrally pure and ultra-bright. The photons were detected by two superconducting nanowire single photon detectors (SNSPDs) with detection efficiencies of 70% and 68% at dark counts of less than 1 kcps. We achieved fidelities of 0.981 ± 0.0002 for |ψ(-)〉 and 0.980 ± 0.001 for |ψ(+)〉 respectively. This GVM-PPKTP-Sagnac scheme is directly applicable to quantum communication experiments at telecom wavelength, especially in free space.
authors.forename: Rui-Bo
authors.initials: RB
authors.lastname: Jin
authors.forename: Ryosuke
authors.initials: R
authors.lastname: Shimizu
authors.forename: Kentaro
authors.initials: K
authors.lastname: Wakui
authors.forename: Mikio
authors.initials: M
authors.lastname: Fujiwara
authors.forename: Taro
authors.initials: T
authors.lastname: Yamashita
authors.forename: Shigehito
authors.initials: S
authors.lastname: Miki
authors.forename: Hirotaka
authors.initials: H
authors.lastname: Terai
authors.forename: Zhen
authors.initials: Z
authors.lastname: Wang
authors.forename: Masahide
authors.initials: M
authors.lastname: Sasaki