IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 16, NO. 1, JANUARY/FEBRUARY 2010 325 Frequency and Polarization Characteristics of Correlated Photon-Pair Generation Using a Silicon Wire Waveguide Ken-ichi Harada, Hiroki Takesue, Member, IEEE, Hiroshi Fukuda, Tai Tsuchizawa, Toshifumi Watanabe, Koji Yamada, Member, IEEE, Yasuhiro Tokura, and Sei-ichi Itabashi Abstract—We report the frequency and polarization character- istics of correlated photon pairs generated in a Si wire waveguide (SWW). We confirmed that the bandwidth for correlated photon- pair generation was at least >2.8 THz. Moreover, we carried out a classical four-wave mixing experiment using strong pump and idler lights to estimate the bandwidth for correlated photon-pair gener- ation. The results indicated that it is possible to generate photon pairs over a bandwidth as large as ∼12 THz. We also showed that the generation efficiencies of the signal and idler photons for the horizontal polarization mode were much higher than those for the vertical polarization mode. This is probably caused by the large efficiencies in the group indexes and the effective cross-sectional areas for the two polarization modes. Furthermore, the bandwidth for the correlated photon-pair generation in the vertical polariza- tion mode was ∼±1 THz, and this was much narrower than that for the horizontal polarization mode. The difference between the bandwidths of the two polarization modes indicates that the SWW dispersion for the vertical polarization mode is significantly larger than that for the horizontal polarization mode. We then confirmed that the noise photons generated by spontaneous Raman scatter- ing in an SWW were suppressed to below the detection limit of our setup. Index Terms—Nonlinear optics, optical Kerr effect. I. INTRODUCTION T HE GENERATION of entangled photon pairs in the 1.5-μm wavelength band has been studied intensively with a view to realize quantum information systems such as quantum key distribution [1] and quantum relay [2] over optical fiber. In this wavelength band, entangled photon-pair sources based on spontaneous parametric downconversion in periodically poled lithium niobate (PPLN) waveguides are widely used [3], [4]. However, a large group velocity mismatch between a short wavelength pump and a long wavelength photon pair induces a walk-off between them in PPLN, which results in the “tim- ing jitter” of photon pairs. This timing jitter induces visibility degradation of two-photon interference using two independent Manuscript received February 26, 2009; revised April 18, 2009. First pub- lished November 10, 2009; current version published February 5, 2010. This work was supported in part by the Japan Science and Technology Agency (JST-CREST). K. Harada, H. Takesue, and Y. Tokura are with the NTT Basic Research Laboratories, Atsugi 243-0198, Japan (e-mail: kharada@will.brl.ntt.co.jp; htakesue@will.brl.ntt.co.jp). H. Fukuda, T. Tsuchizawa, T. Watanabe, K. Yamada, and S. Itabashi are with the NTT Microsystem Integration Laboratories, Atsugi 243-0198, Japan. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSTQE.2009.2023338 PPLN waveguides. Spontaneous four-wave mixing (SFWM) in dispersion-shifted fiber (DSF) is another way to generate entan- gled photon pairs in the 1.5-μm band [5], [6]. However, DSF- based entangled photon-pair sources are adversely affected by noise photons caused by spontaneous Raman scattering (SpRS), which degrade the degree of quantum correlation. Although it is possible to avoid the noise photons generated by SpRS by soaking DSF in liquid nitrogen [6], [7], the need for cooling equipment complicates the system, and is thus undesirable. A Si wire waveguide (SWW) based on a silicon-on-insulator (SOI) structure [8] is an attractive device for generating 1.5-μm correlated [9]/entangled [10]– [12] photon pairs. An SWW is a single-mode, single-crystal Si waveguide fabricated on an SOI wafer with a Si top layer on a 3-μm SiO 2 layer. Since the effective cross-sectional area of an SWW is much smaller than that of a standard optical fiber, the former exhibits a much larger nonlinear effect than the latter [13]. Therefore, we can generate photon pairs by efficiently using SFWM in a waveguide with a length of a few centimeters. Moreover, in the SFWM process, the pump and the photon pair have similar frequencies, so group velocity matching and refractive index matching can be achieved simultaneously. As a result, the walk- off between a pump and a photon pair is negligible. Moreover, the Raman spectrum of single-crystal Si has a peak of 15.6 THz from the pump frequency with a relatively narrow width of about 100 GHz [14]. Therefore, the noise photons caused by SpRS in the Si are suppressed by selecting signal and idler frequencies that are far from the Raman peak. Thus, an SWW is suitable for the highly efficient generation of high-purity correlated photon pairs. Following the report of a correlated photon-pair generation experiment [9], our group undertook the first entanglement gen- eration experiment based on time-bin [10] and polarizations [11] using an SWW. Moreover, we recently reported a significant improvement in the quantum correlation characteristics of SWW-based correlated and entangled photon-pair sources [12]. We also obtained two-photon interference fringes of time-bin entanglement with >95% visibility, and confirmed that an SWW was capable of generating a high-purity entangled photon pair. However, the characteristics of correlated photon-pair generation in an SWW have not been studied in detail. In this paper, we report the frequency and polarization char- acteristics of correlated photon-pair generation in an SWW. We confirmed that the bandwidth for correlated photon-pair genera- tion was at least >2.8 THz. Moreover, we performed a four-wave 1077-260X/$26.00 © 2009 IEEE