Generation of High-Purity Entangled Photon Pairs Using Silicon Wire Waveguide


Ken-ichi Harada, Hiroki Takesue, Hiroshi Fukuda*, Tai Tsuchizawa*,
Toshifumi Watanabe*, Koji Yamada*, Yasuhiro Tokura, and Sei-ichi Itabashi*
Optical Science Laboratory, *Microsystem Integration Laboratories

 Recently, 1.5-µm band entangled photon pair generation using spontaneous four-wave mixing (SFWM) in a silicon wire waveguide (SWW) has been drawing attention [1]. An SWW is expected to function as a high-purity entangled photon pair generation device. By using an SWW, we obtained two-photon interference fringes of time-bin entangled photons with > 95% visibilities [2].
 An SWW is a nano-scale silicon waveguide fabricated on silicon-on-insulator wafer [3]. An SWW exhibits very large third order nonlinearity because of its extremely small effective area, and thus we can obtain efficient SFWM in a waveguide whose length is 〜1 cm.
 Figure 1 shows experimental setup. The laser light is modulated into double pulses with a repetition frequency of 100 MHz by using an intensity modulator (IM). The pulse width and interval are 90 ps and 1 ns, respectively. The double pulses are input into an SWW. The SWW used in the experiment is 460 nm wide, 200 nm thick, and 1.15 cm long. The excess loss of the SWW is 1.0 dB. Consequently, time-bin entangled photon pairs are generated through the SFWM in the SWW. The photons from the SWW are launched into a fiber Bragg grating (FBG) to suppress the pump photons, and input into an arrayed waveguide grating (AWG) to separate the signal and idler photons. The signal and idler wavelengths are ± 3.2 nm from the pump wavelength. Each photon is then launched into a 1-bit delayed interferometer fabricated using planar lightwave circuits (PLC) based on silica waveguide technology. The phase difference between two paths of the interferometer is precisely controlled by adjusting the temperature of the substrate. The photons from the PLC interferometers are received by single photon detectors (SPD) operated in a gate mode whose gate frequency is 100 MHz.
 We fixed the signal interferometer temperature and counted the coincidences while changing the idler interferometer temperature (fig. 2). The solid and dashed lines represent the measurement results obtained with nonorthogonal measurement bases for the signal photons. The visibilities of fitted curves are 96.3% (solid) and 95.2% (dashed). Thus, we successfully confirmed the generation of a high-purity entangled state in the 1.5-µm band.

[1] H. Takesue et al., Appl. Phys. Lett. 91 (2007) 201108.
[2] K. Harada et al., Opt. Express 16 (2008) 20368.
[3] T. Tsuchizawa et al., IEEE J. Sel. Top. Quantum Electron. 11 (2005) 232.

Fig. 1. Experimental setup. PC: polarization controller, FM: focusing module, BPF: band pass filter, TIA: time interval analyzer.
Fig. 2. Two-photon interference fringes.

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