Monolithic Source of Polarization Entanglement Using Silicon Photonics


Nobuyuki Matsuda1, 3, Hanna Le Jeannic1, Hiroshi Fukuda2, 3, Tai Tsuchizawa2, 3,
Koji Yamada2, 3, and Hiroki Takesue1
1Optical Science Laboratory, 2NTT Microsystem Integration Laboratories,
3Nanophotonics Center

   Integrated photonic waveguides, which provide a highly-stable and ultra-compact optical circuitry, is a promising candidate for a platform of scalable photonic quantum information processing (QIP) system [1]. Many QIP protocols are based on photonic quantum states encoded in polarization degree of freedom. Therefore, it is essential to implement integrated subsystems to generate, manipulate and measure polarization-encoded quantum states of light. This time we realized the first monolithic source of polarization entangled photon pairs by employing the silicon photonics technology [2].
   The device consists of two silicon-wire waveguides (SWWs) connected by a silicon polarization rotator (SPR) [Fig. 1(a)]. In a SWW, a correlated pair of signal and idler photons is created following the annihilation of two pump photons via spontaneous four-wave mixing process. Here the pump, signal and idler photons are horizontally (H) polarized. The SPR has an off-axis double core structure of a Si inner core and a SiOxNy outer core. The SPR exhibits two orthogonal eigenmodes, which have different effective refractive indices and eigen-axes tilted at 45° with respect to the normal to the substrate [3]. The birefringence in the eigenmodes causes the polarization plane to rotate by an amount that depends on the length of the SPR. We obtain polarization-entangled photon pairs from the device by using optical pump pulses with + 45° linear polarization as shown in Fig. 1. The polarization-mode dispersion and the polarization-dependent loss in a SWW, which potentially degrade the purity of polarization entanglement, can be automatically cancelled out in our device. We successfully obtained a maximally entangled state from the chip with a state fidelity as high as 94 %, which is well above the classical limit.
   This work was supported by KAKENHI.

[1] A. Peruzzo et al., Science 329 (2010) 1500.
[2] N. Matsuda et al., Sci. Rep. 2 (2012) 817.
[3] H. Fukuda et al., Opt. Express 16 (2008) 2628.

Fig. 1. (a) The monolithic polarization entanglement source and (b) its working principle. V represents the vertical polarization.