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Nanophotonics

Photon‐pair generation in a lossy waveguide

Woncheol Shin, Kyungdeuk Park, Hyeongpin Kim, Dongjin Lee, Kiwon Kwon, Heedeuk Shin

Published Jan 10, 2023• 16 citations
Publisher

Abstract

Abstract An on‐chip quantum light source based on spontaneous four‐wave mixing is an essential element for developing quantum photonic integrated circuit technology, which has the advantage of no connection loss owing to the integration of the source into photonic circuits. The waveguide‐based quantum light source inevitably causes propagation loss owing to imperfections in the fabrication process, but the propagation loss effects on photon‐pair generation have not been extensively studied. In this study, propagation loss effects were examined using theoretical and experimental methods. In theory, the performance of quantum light sources, such as brightness, heralding efficiency, and coincidence‐to‐accidental ratio, strongly depend on propagation loss. We fabricate several waveguides with a moderate propagation loss of 2.2 dB/cm to investigate the loss dependence and ascertain that the brightness, heralding efficiency, and coincidence‐to‐accident ratio strongly correlate with the length of the optical waveguide. The maximum coincidence‐count brightness occurred at an optimization length of 1/ α , where α is the absorption coefficient. In contrast, the single‐count brightness shows slightly different waveguide length dependence owing to loss‐induced one‐photon states. We expect that the results obtained in this study will greatly assist in determining the proper waveguide length for photon‐pair generation according to the source’s application fields. The results will be helpful in the development of a quantum light source suitable for practical and quantum optical integrated circuits and will lead to the development of high‐fidelity quantum technologies.

DOI
10.1515/nanoph-2022-0582

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Abstract An on‐chip quantum light source based on spontaneous four‐wave mixing is an essential element for developing quantum photonic integrated circuit technology, which has the advantage of no connection loss owing to the integration of the source into photonic circuits. The waveguide‐based quantum light source inevitably causes propagation loss owing to imperfections in the fabrication process, but the propagation loss effects on photon‐pair generation have not been extensively studied. In this study, propagation loss effects were examined using theoretical and experimental methods. In theory, the performance of quantum light sources, such as brightness, heralding efficiency, and coincidence‐to‐accidental ratio, strongly depend on propagation loss. We fabricate several waveguides with a moderate propagation loss of 2.2 dB/cm to investigate the loss dependence and ascertain that the brightness, heralding efficiency, and coincidence‐to‐accident ratio strongly correlate with the length of the optical waveguide. The maximum coincidence‐count brightness occurred at an optimization length of 1/ α , where α is the absorption coefficient. In contrast, the single‐count brightness shows slightly different waveguide length dependence owing to loss‐induced one‐photon states. We expect that the results obtained in this study will greatly assist in determining the proper waveguide length for photon‐pair generation according to the source’s application fields. The results will be helpful in the development of a quantum light source suitable for practical and quantum optical integrated circuits and will lead to the development of high‐fidelity quantum technologies.
Abstract We present a semi-classical numerical model to estimate the impact of linear scattering loss and nonlinear absorption losses on the biphoton flux and their quantum correlations generated via spontaneous four-wave mixing in silicon nanowaveguides. The counter-intuitive observed enhancement of pair correlations with increasing loss is attributed to the dominant effect of reduced accidental counts from multiphoton generation, with a corresponding trade-off for the source brightness. Silicon nanowaveguides are shown to be capable of generating highly correlated paired photons with coincidental-to-accidental ratio as high as ∼3400 and spectral brightness 2.8 × 10 4 pairs s−1 GHz−1 mW−1, even in the presence of linear propagation loss of 1 dB cm−1 and nonlinear losses (two-photon absorption and free-carrier absorption), over a length of 1 cm with an input pump power of 10 mW. Loss and the corresponding Langevin noise are modeled using distributed beam splitters along the waveguide length to encapsulate the phenomenological coupling to the background reservoir (vacuum fluctuations). The proposed numerical model is more general compared to previous analytical models, particularly for including dispersion and wavelength-dependent losses, and more accurate for noise estimation in the high-photon-flux regime such as optical parametric amplifiers and squeezed state generation.
We investigate theoretically the spontaneous four-wave mixing (FWM) process that occurs in optical waveguides, as a source of quantum correlated photon-pairs. We consider that the waveguide used to implement the spontaneous FWM process presents a high value of nonlinear parameter, γ = 93.4 W-1m-1, and a non-negligible value of loss coefficient, α = 133.3 dB/m. Moreover, the theoretical model also consider the Raman scattering that inevitably accompanies the FWM process, and generates time-uncorrelated (noise) photon pairs. We use the coincident-to-accidental ratio (CAR) as a figure of merit of the photon pair source, and we were able to observe a CAR of the order of 65 in a high loss regime. After, we use the time-correlated photon pairs generated by the spontaneous FWM process to implement a heralded single photon source at waveguide output. In this scenario, the detection of one photon of the pair heralds the presence of the other photon. The quality of the source was studied by the evaluation of the second order coherence function for one photon of the pair conditioned by the detection of its twin photon. We observe that the presence of a high loss coefficient tends to improve the quality of the photon source, when compared with the lossless regime, even considering the Raman noise photons. We obtain a value for the conditional second order coherence function of the order of 0.11 in absence of loss, and a value of 0.03 for a loss coefficient of 133.3 dB/m.
We demonstrate a source of correlated photon pairs which will have applications in future integrated quantum photonic circuits. The source utilizes spontaneous four-wave mixing (SFWM) in a dispersion-engineered nanowaveguide made of AlGaAs, which has merits of negligible two-photon absorption and low spontaneous Raman scattering (SpRS). We observe a coincidence-to-accidental (CAR) ratio up to 177, mainly limited by propagation losses. Experimental results agree well with theoretical predictions of the SFWM photon pair generation and the SpRS noise photon generation. We also study the effects from the SpRS, propagation losses, and waveguide lengths on the quality of our source.
Integrated silicon photonics has played an important role in advancing the applications of quantum information and quantum science. However, it is challenging to integrate all components with state-of-the-art performance using only a homogeneous platform. Here, by combining high nonlinearity and low losses in a heterogeneous silicon platform, we efficiently generate high-quality photon pairs through spontaneous four-wave mixing in a hydrogenated amorphous silicon waveguide and route them off-chip through a low-loss silicon nitride waveguide. A record high coincidence-to-accidental ratio value of 1632.6 (±260.4) is achieved in this heterogeneous design with a photon pair generation rate of 1.94 MHz. We also showcase a wide range of multichannel photon sources with a coincidence-to-accidental ratio consistently around 200. Lastly, we measure heralded single-photons with the lowest g H (2) (0) of 0.1085 ± 0.0014. Our results demonstrate the heterogeneous silicon platform as an ideal platform for the efficient generation of photon pairs and off-chip routing with low losses. It also paves the way for a future hybrid photonic integrated circuit by collecting distinct features from different materials.
Integrated Silicon photonics has played an important role in advancing the applications of quantum information and quantum science. However, due to different material properties, it is challenging to integrate all components with excellent performance based on homogeneous material. Here, by combining high nonlinearity and low losses in a heterogeneous silicon platform, we efficiently generate high-quality photon pairs through spontaneous four-wave mixing in hydrogenated amorphous silicon waveguide and route them off-chip through low loss silicon nitride waveguide. A record high coincidence- to- accidental rate value of 1632.6 (±\pm± 260.4) is achieved in this heterogeneous design with a photon pair generation rate of 1.94 MHz. We also showcase a wide range of multi-channel photon sources with coincidence- to- accidental rate consistently at 200. Lastly, we measure heralded single-photons with a lowest gH(2)(0)g^{(2)}_H(0)gH(2)​(0) of 0.1085 ±\pm± 0.0014. Our results demonstrate the heterogeneous silicon platform as an ideal platform for efficient generation of photon pairs and routing them off-chip with low losses. It also paves a way for the future hybrid photonic integrated circuit by collecting distinct features from different materials.
Photon-pair sources are critical building blocks for photonic quantum systems. Leveraging Kerr nonlinearity and cavity-enhanced spontaneous four-wave mixing, chip-scale photon-pair sources can be created using microresonators built on photonic integrated circuit. For practical applications, a high microresonator quality factor Q is mandatory to magnify photon-pair sources' brightness and reduce their linewidth. The former is proportional to Q^{4}, while the latter is inversely proportional to Q. Here, we demonstrate an integrated, microresonator-based, narrowband photon-pair source. The integrated microresonator, made of silicon nitride and fabricated using a standard CMOS foundry process, features ultralow loss down to 0.03 dB/cm and intrinsic Q factor exceeding 10^{7}. The photon-pair source has brightness of 1.17×10^{9} Hz/mW^{2}/GHz and linewidth of 25.9 MHz, both of which are record values for silicon-photonics-based quantum light source. It further enables a heralded single-photon source with heralded second-order correlation g_{h}^{(2)}(0)=0.0037(5), as well as an energy-time entanglement source with a raw visibility of 0.973(9). Our work evidences the global potential of ultralow-loss integrated photonics to create novel quantum light sources and circuits, catalyzing efficient, compact, and robust interfaces to quantum communication and networks.