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Optics Letters

High-extinction CROW filters for scalable quantum photonics

Rakesh Ranjan Kumar, Hon Ki Tsang

Published Dec 4, 2020• 34 citations
Publisher

Abstract

We report an integrated tunable-bandwidth optical filter with a passband to stop-band ratio of over 96 dB using a single silicon chip with an ultra-compact footprint. The integrated filter is used in filtering out the pump photons in non-degenerate spontaneous four-wave mixing (SFWM), which is used for producing correlated photon pairs at different wavelengths. SFWM occurs in a long silicon waveguide, and two cascaded second-order coupled-resonator optical waveguide (CROW) filters were used to spectrally remove the pump photons. The tunable bandwidth of the filter is useful to adjust the coherence time of the quantum correlated photons and may find applications in large-scale integrated quantum photonic circuits.

DOI
10.1364/ol.409784

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We report an integrated tunable-bandwidth optical filter with a passband to stop-band ratio of over 96 dB using a single silicon chip with an ultra-compact footprint. The integrated filter is used in filtering out the pump photons in non-degenerate spontaneous four-wave mixing (SFWM), which is used for producing correlated photon pairs at different wavelengths. SFWM occurs in a long silicon waveguide, and two cascaded second-order coupled-resonator optical waveguide (CROW) filters were used to spectrally remove the pump photons. The tunable bandwidth of the filter is useful to adjust the coherence time of the quantum correlated photons and may find applications in large-scale integrated quantum photonic circuits.
We describe the use of cascaded second-order coupled-resonator optical waveguide (CROW) tunable filters to achieve one of the highest reported measured extinction ratios of $ {\gt} {110}\;{\rm dB}$>110dB. The CROW filters were used to remove the pump photons in spontaneous four-wave mixing (SFWM) in a silicon waveguide. The SFWM generated quantum-correlated photons that could be measured after the cascaded CROW filters. The CROW filters offer a compact footprint for use in monolithic quantum photonic circuits.
Integrated photonic circuits have become an attractive platform for the quantum information processing, paving the way for quantum information management with scalable device. In this context, silicon photonics represents the most mature technology to implement the quantum system functionalities, due to its large scalability and compatibility with CMOS technology. Efficient photon-pairs sources based on Spontaneous Four-Wave Mixing (SFWM) and high-performance photodetector have been already demonstrated. The efficient detection of photon-pairs requires a pump filter at the photodetector, preserving the signal-idler pair. Thus, filters with high Extinction Ratio (ER), low Insertion Loss (IL) and narrow rejection Bandwidth (BW) are needed. Here, we propose the design of an ultra-high-performance rejection filter, based on a silicon dual-loaded single input/output Mach-Zehnder Interferometer (MZI), with one branch coupled to a Ring Resonator (RR) and the other to three serially coupled RRs forming a Coupled Optical Resonator Waveguide (CROW). Very high performance (ER = 150.55 dB, IL = 0.104 dB, BW = 0.243 nm), within a footprint of 60 µm × 160 µm, has been calculated, demonstrating its suitability for an efficient suppression of the pump signal. The filter response is also thermo-optically tuneable in a 6 MHz range, with a reconfigurability time of about 8 µs.
In this work, we design and fabricate a telecom band quantum light source (QLS) on a silicon photonic chip, which integrates a piece of a long silicon waveguide as the nonlinear medium for spontaneous four-wave mixing (SFWM) and five narrow FSR-free bandpass filters based on a grating-assisted contra-directional coupler (GACDC). Two optical filtering functions of the silicon integrated QLS have been demonstrated. First, the QLS supports two tunable outputs of photon pair generations by four GACDC filters. A wavelength tunable range of 6 nm is demonstrated. Second, one GACDC bandpass filter is designed as an on-chip pump filter before the silicon waveguide. The performances of the QLSs with and without the on-chip pump filter are measured and compared. It shows that the on-chip pump filter has the effect to enhance the performance of the QLS by suppressing the Raman noise photons generated when a pump light propagated in optical fibers before it is injected into the chip. These results show that FSR-free filters would play important roles in developing silicon integrated QLSs.
We demonstrate the generation of quantum-correlated photon pairs combined with the spectral filtering of the pump field by more than 95 dB on a single silicon chip using electrically tunable ring resonators and passive Bragg reflectors. Moreover, we perform the demultiplexing and routing of signal and idler photons after transferring them via an optical fiber to a second identical chip. Nonclassical two-photon temporal correlations with a coincidence-to-accidental ratio of 50 are measured without further off-chip filtering. Our system, fabricated with high yield and reproducibility in a CMOS-compatible process, paves the way toward large-scale quantum photonic circuits by allowing sources and detectors of single photons to be integrated on the same chip.