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arXiv:0906.4688 · Optics Express

Continuous wave photon pair generation in silicon-on-insulator waveguides and ring resonators

Stéphane Clemmen, Kien Phan Huy, Wim Bogaerts, Roel Baets, Ph. Emplit, Serge Massar

Published Sep 2, 2009• 296 citations
arXivHTMLPDF

Abstract

Silicon waveguides are promising chi(3)-based photon pair sources. Demonstrations so far have been based on picosecond pulsed lasers. Here, we present the first investigation of photon pair generation in silicon waveguides in a continuous regime. The source is characterized by coincidence measurements. We uncover the presence of unexpected noise which had not been noticed in earlier experiments. Subsequently, we present advances towards integration of the photon pair source with other components on the chip. This is demonstrated by photon pair generation in a Sagnac loop interferometer and inside a micro-ring cavity. Comparison with the straight waveguide shows that these are promising avenues for improving the source. In particular photon pair generation in the micro-ring cavity yields a source with a spectral width of approximately 150 pm resulting in a spectral brightness increased by more than 2 orders of magnitude.

DOI
10.1364/oe.17.016558

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Silicon waveguides are promising chi(3)-based photon pair sources. Demonstrations so far have been based on picosecond pulsed lasers. Here, we present the first investigation of photon pair generation in silicon waveguides in a continuous regime. The source is characterized by coincidence measurements. We uncover the presence of unexpected noise which had not been noticed in earlier experiments. Subsequently, we present advances towards integration of the photon pair source with other components on the chip. This is demonstrated by photon pair generation in a Sagnac loop interferometer and inside a micro-ring cavity. Comparison with the straight waveguide shows that these are promising avenues for improving the source. In particular photon pair generation in the micro-ring cavity yields a source with a spectral width of approximately 150 pm resulting in a spectral brightness increased by more than 2 orders of magnitude.
The high Kerr coefficient of amorphous silicon makes it a promising material for generating photons on chip via spontaneous four wave mixing (SFWM), for applications in quantum photonics. Previous work shows high photon generation rates in a-Si waveguides, [1], and here we expand this study to investigate micro-ring resonator photon sources, which have a small footprint and can produce photons with high spectral purity, ideal for quantum interference. We compare SFWM generation rates in an a-Si micro-ring resonator to a straight waveguide, in both continuous wave and pulsed pump regimes.
Photon sources are fundamental components for any quantum photonic technology. The ability to generate high count-rate and low-noise correlated photon pairs via spontaneous parametric down-conversion using bulk crystals has been the cornerstone of modern quantum optics. However, future practical quantum technologies will require a scalable integration approach, and waveguide-based photon sources with high-count rate and low-noise characteristics will be an essential part of chip-based quantum technologies. Here, we demonstrate photon pair generation through spontaneous four-wave mixing in a silicon micro-ring resonator, reporting separately a maximum coincidence-to-accidental (CAR) ratio of 602 ± 37 (for a generation rate of 827kHz), and a maximum photon pair generation rate of 123 MHz ± 11 kHz (with a CAR value of 37). To overcome free-carrier related performance degradations we have investigated reverse biased p-i-n structures, demonstrating an improvement in the pair generation rate by a factor of up to 2 with negligible impact on CAR.
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.
Integrated quantum photonics relies critically on the purity, scalability, integrability, and flexibility of a photon source to support diverse quantum functionalities on a single chip. Here we report a chip-scale photon-pair source on the silicon-on-insulator platform that utilizes dramatic cavity-enhanced four-wave mixing in a high-Q silicon microdisk resonator. The device is able to produce high-quality photon pairs at different wavelengths with a high spectral brightness of 6.24×10(7) pairs/s/mW(2)/GHz and photon-pair correlation with a coincidence-to-accidental ratio of 1386 ± 278 while pumped with a continuous-wave laser. The superior performance, together with the structural compactness and CMOS compatibility, opens up a great avenue towards quantum silicon photonics with capability of multi-channel parallel information processing for both integrated quantum computing and long-haul quantum communication.
One of the key building blocks of quantum photonic systems is a source of indistinguishable photons. Silicon ring resonators can be used as photon pair sources by taking advantage of silicon's large third order nonlinearity with a process known as spontaneous four wave mixing. These sources are capable of producing pairs of indistinguishable photons but typically suffer from an effective 50% loss. By slightly decoupling the input waveguide from the ring, the desired photons generated in the ring can be directed to the drop port. Thus, the ratio between the coincidences from the drop port and the total number of coincidences from all ports (coincidence efficiency) can be significantly increased, with the trade-off being that the pump is less efficiently coupled into the ring. This asymmetric coupling scheme was used to demonstrate a dual-bus microring source with a coincidence efficiency of ~96%. Unfortunately, the penalty to pump power was quite large, with a required ~10x increase in the pump power to maintain the same pair generation rate.\nTo further improve upon this result, a microring source was designed with multipoint evanescent couplers to induce a strong spectral dependence in the coupling coefficients. This coupler design can be used on both sides of the ring resonator so that resonances supported by one of the couplers are suppressed by the other. This is the ideal configuration for a ring-based photon-pair source as it can only support the pump photons at the input side while only allowing the generated photons to leave through the output side. Asymmetric coupling was also used in this configuration but in a different way than in the previous source. In this case, the input side of the ring was configured to be critically coupled for maximal resonance enhancement while the output side was configured to be over coupled to minimize photon-pair loss within the ring. This device design was used to demonstrate a photon-pair source with coincidence efficiencies of 0.9981+-6.425x10^-4 and 0.9979+-1.665x10^-3 for degenerate and non-degenerate pump configuration respectively.\nIn addition to the source development, work was done to move some of the pump rejection filtering onto the photonic chip. While the fabricated device only exhibited an extinction of 50 dB, the root cause of the limited extinction was explored and a new filter design was proposed to overcome these limitations. With the combination of the microring source with its greatly improved efficiency and the movement toward reducing the required off-chip filtering, this work represents a significant advancement in the scalability of integrated quantum photonic systems which can enable high performance quantum computing and communication systems.
We report a comprehensive characterization of photon-pair generation by spontaneous four-wave mixing in a partially deuterated amorphous silicon (a-Si:D) waveguide for C-band heralded single-photon generation. In a 3.3-mm-long strip waveguide pumped with a continuous-wave laser at telecom wavelengths, we measure an on-chip pair-generation rate of 3.1 × 10 6 pairs/s, a maximum coincidence-to-accidental ratio of 66, and a heralded second-order correlation g H (2) (0)=0.058±0.002, with these values obtained at different pump powers. We also demonstrate, to the best of our knowledge, the first observation of time-energy entanglement from an amorphous silicon waveguide, with a Franson interference visibility of 77±5%. The power scaling of the singles and accidental coincidences indicates a significant noise contribution that currently limits the source performance. Our results identify a-Si:D as a promising nonlinear platform for bright, integrated photon-pair sources and future hybrid quantum photonic circuits.