Generation of Energy-Time Entangled Photon Pairs in 1.5-ƒÊm Band

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Toshimori Honjo1, Hiroki Takesue1 and Kyo Inoue2
1Optical Science Laboratory, 2Osaka University/NTT Research Professor

@The generation of entangled photon pairs in the 1.5-µm telecom band is essential for quantum communication over optical fiber networks[1, 2]. In this paper, we report the generation of 1.5-µm band energy-time entanglement using a periodically poled lithium niobate (PPLN) waveguide and a two-photon interference experiment using a planar lightwave circuit (PLC) [3].
@Figure 1 shows the experimental setup. A continuous wave light from a laser diode with a wavelength of 780 nm was used as a pump light. The light was polarization controlled and then launched into a PPLN waveguide. Frequency degenerated photon pairs were generated in the PPLN at around 1560 nm by the spontaneous parametric down conversion process. After the PPLN, the light was filtered to suppress the 780-nm pump light and was also filtered by a 1560-nm band pass filter to reduce the dispersion in the PLC interferometer. The entangled photon pairs were launched into a 3-dB fiber coupler, which separated the signal and idler with 50% probability. The signal and idler photons were launched into a planar lightwave circuit (PLC) Mach-Zehnder interferometer. The phase difference between the two paths was precisely adjusted by controlling the temperature of the interferometer and stable operation was possible. Photon detectors based on an InGaAs APD were set at each output of the Mach-Zehnder interferometer. The output signals of the photon detectors were input into a time interval analyzer (TIA) to measure the coincidence. We used this setup to perform a two-photon interference experiment. In this experiment, we fixed the temperature of the PLC interferometer for the signal and changed that for the idler, and measured the coincidence.
@Figure 2 shows the experimental results. The triangles indicate the experimentally obtained coincidence rate per detected signal photon for each temperature. The count rate of each detector was `1700 cps throughout the measurement. Although the count rates remained, we observed a deep modulation of the coincidence rate as we changed the temperature. We obtained coincidence fringes with 77.3% visibilities without subtracting the accidental coincidences. Considering both the estimated average number of photon pairs per gate of 0.12 and their Poisson distribution, the visibility of the setup was theoretically estimated to be 77.4%, which means energy-time entanglement was confirmed and very few noise photons were generated by the pump light.

[1] I. Marcikic et al., Phys. Rev. Lett. 82 (1999) 2594.
[2] H. Takesue et al., Phys. Rev. A 72 (2005) 041804(R).
[3] T. Honjo et al., CLEO/QELS 2006 (2006) JTuA5.

Fig. 1. Experimental setup. Fig.2. Coincidence fringe.

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