Integrated photon pair source for SOI-based quantum optics

Photon pair sources are an important building block for optics based Quantum Information Processing (QIP) such as Quantum Key Distribution (QKD) and Linear Optics Quantum Computing (LOQC) [1]. Beyond the demand for reliable and cheap photon pair sources for QKD, LOQC also requires interferometers that are mechanically stable and loss-free for which integrated photonics is particularly attractive [2]. In this context, photon pair generation (PPG) has been demonstrated via the third order nonlinear interaction between a silicon straight waveguide (Si-w) and a pulsed beam [3]. Such a source cannot be directly followed by other integrated components as photon pairs would be generated throughout the silicon chip. The solution we propose to suppress the pump beam consists of a Sagnac Loop Interferometer (SLI) in which photon pairs are generated and pump beam destructively interferes with itself, thereby allowing photon pairs to be generated in a localised part of the silicon chip. Furthermore, our study has been performed in a continuous instead of a pulsed regime, thereby avoiding any synchronization between detectors and laser and allowing for much cheaper laser sources. Our experimental setup is presented in Fig. 1. The experiment is performed at different pump power both in the a Si-w and the SLI. Results are presented in Fig. 2. The flux of generated pairs and the Signal-to-noise ratio are limited by the outcoupling loss and detector inefficiency. The photon flux is less than expected because the emitted spectrum is around 8nm (FWHM) broad instead of the expected 70nm. The performance of the SLI is not yet perfect: the pump suppression efficiency decreases with the pump power (from −22dB to −15dB) due to unexpected nonlinear behaviour in the directional coupler that changes the coupling ratio; and the Signal-to-Noise ratio is lower than in the Si-w which might be due to PPG in the broad (800nm) waveguide before the SLI.