Generation of telecom-band polarization entangled photon pairs
using spontaneous four-wave mixing in dispersion-shifted fiber
Optical Science Laboratory
Generation of entangled photon pairs in the 1.5μm band is one of the important technological challenges for realizing quantum communication over optical fiber networks. We have succeeded in generating polarization entangled photons in the wavelength band, by using spontaneous four-wave mixing (SFWM) in a loop formed with a polarization beamsplitter (PBS) and a dispersion-shifted fiber (DSF).
Fig. 1 shows the configuration. A pump pulse with a 45°linear polarization is input into the loop. The PBS divides the pump into horizontal (H) and vertical (V) polarization components. The H and V components generate signal-idler photon pairs| H >s|H >i 、and | V >s|V >i through a SFWM process while propagating in the loop in the counter-clockwise and clockwise directions, respectively. These two product states are superposed at the PBS output, the pump is then suppressed, and a polarization entangled state（| H >s|H >i 、+ | V >s|V >i ）/√2is obtained. The loop configuration makes it possible to stabilize the relative phase between two product states without any feedback control, and so our system is both simple and stable.
We confirmed the feasibility of our method experimentally. We used fiber-Bragg gratings (FBG), an arrayed waveguide grating (AWG) and bandpass filters (BPF) to suppress the pump and separate the signal and idler photons. The signal and idler photons were polarization-controlled and input into polarizers, and detected using avalanche photo diodes (APD). We undertook two-photon interference experiments, and obtained coincidence fringes with >90 % visibilities, which is shown in Fig. 2. We then observed a violation of Bell's inequality by seven standard deviations. We also confirmed the preservation of the quantum correlation between the photons even after they had been separated by 20 km optical fiber.
This result is an important step toward highly sophisticated quantum communication networks over optical fiber.
 H. Takesue and K. Inoue, Phys. Rev. A, 70, 031802(R) (2004).
Fig. 1. Configuration Fig. 2. Coincidence fringes
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