Hybrid waveguide scheme for silicon-based quantum photonic circuits with quantum light sources

Silicon quantum photonic circuit is an important way to realize the integration of photonic quantum information functions. In telecom band, silicon waveguides have very high third-order optical nonlinear coefficient. They are promising candidates for realizing four-wave mixing (FWM) based quantum light sources operating at room temperature. On the other hand, a variety of silicon photonic devices with different functions provide rich means for on-chip manipulation of photonic quantum states.

In recent years, the combination of silicon waveguides and superconducting nanowire single photon detectors makes it possible to analyze and measure photonic quantum states on a silicon photonic chip. What's more, the fabrication of silicon photonic quantum circuit is compatible with the technology of microelectronics. Hence, it has great potential to extend the integration scale and to introduce new control methods, which will promote the development and application of photonic quantum information functions.

Although the generation, manipulation and detection of photonic quantum states have been realized on silicon photonic chip respectively, new problems would be introduced when these functions are integrated on the same chip. For example, on a chip with both quantum light sources and photonic circuits for quantum state manipulation, the pump lights of quantum light sources would generate noise photons in the silicon waveguide out of the quantum light sources. It will be more serious with the development of the chip scale, which will integrate more and more quantum light sources and complicated photonic circuits.

Researchers have proposed several methods to solve this problem. For example, on-chip optical filters, such as Mach-Zehnder interferometers and ring resonators, are designed before and after quantum light sources. They remove noise photons generated by the pump light before the sources and reduce the power level of residual pump lights after the sources. Ring resonators also can be used as the quantum light sources, by which the requirement of pump light power can be greatly reduced thanks to the pump light enhancement in the resonator. In these schemes, the filtering or resonant wavelengths of these components should be controlled carefully to match the pump wavelengths, usually by thermal-optic phase shifters. For a quantum photonic circuit with multiple quantum light sources, the difficulties of its design and application would be increased greatly, since the components for each source should be controlled separately.

The research team led by Prof. Wei Zhang from Tsinghua University proposed a novel scheme of silicon quantum photonic circuits to overcome this problem in their paper at Photonics Research, Vol. 8, Issue 3, 2020 (Lingjie Yu, Chenzhi Yuan, Renduo Qi, Yidong Huang, Wei Zhang. Hybrid waveguide scheme for silicon-based quantum photonic circuits with quantum light sources[J]. Photonics Research, 2020, 8(3): 03000235).

This scheme utilizes two types of silicon waveguides, strip waveguide and shallow-ridge waveguide. The strip waveguides are used as nonlinear media for the FWM based quantum light sources, while the shallow-ridge waveguides are used to realize the circuits for pump light transmission and photonic quantum state manipulation. The impact of noise photons generated by the pump lights out of the quantum light sources can be overcome utilizing the spectral difference of spontaneous FWMs in these two waveguides, which is due to the difference of their dispersion characteristics. In this work, the principle of this scheme is demonstrated theoretically and experimentally. And its potential is shown by the theoretical analysis on two silicon quantum photonic circuit designs.

Prof. Zhang believes that the proposed scheme overcomes the impact of noise photons introduced by pump lights without optical filters/resonators and their control units. It greatly simplifies the design and application of quantum photonic circuits, and provides a simple way to develop circuits with large scale and complicated quantum information functions. At present, this scheme has been applied to several quantum photonic circuit designs of his research team.

This scheme uses silicon strip waveguides to realize quantum light sources, and uses silicon shallow-ridge waveguides to realize optical circuits for pump light transmission and quantum state manipulation. Photons generated in strip waveguides by SFWM have wider spectrum, hence, the impact of noise photons generated in shallow-ridge waveguides can be overcome by proper optical filtering.