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Journal of the Optical Society of America B

Broadband integrated directional coupler for heralded single-photon characterization

Ferney Castro-Simanca, Francisco A. Domínguez-Serna, Mauricio Gómez-Robles, Rafael Salas‐Montiel, W. de la Cruz, Karina Garay-Palmett

Published Sep 16, 2025• 2 citations
Publisher

Abstract

The generation of pure single-photon states and their characterization are fundamental in quantum information processing applications like quantum computing. In this work, we design integrated optical directional couplers on a silicon nitride platform for heralded single-photon characterization. We assume the generation of such photons in silicon nitride waveguides through the spontaneous four-wave mixing process. We show that manipulating the waveguide geometry and dielectric contrast makes it possible to obtain nearly wavelength-independent directional coupler designs that perform as 50:50 beamsplitters over a broad bandwidth around the central design wavelength. Assuming two independent but identical heralded single-photon sources, we compare the visibility obtained by simulating the Hong-Ou-Mandel interference for three different 50:50 beamsplitters: an ideal one (perfectly wavelength-independent), an engineered one (nearly wavelength-independent), and an unengineered one (significantly wavelength-dependent). The purity obtained with the engineered beamsplitter closely matches the purity obtained with the ideal one, for which the interference visibility is a direct measure of the quantum state purity. This finding implies that the proposed engineered beamsplitter is suitable for characterizing heralded single-photon states, provided its bandwidth is 23 nm.

DOI
10.1364/josab.572117

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The generation of pure single-photon states and their characterization are fundamental in quantum information processing applications like quantum computing. In this work, we design integrated optical directional couplers on a silicon nitride platform for heralded single-photon characterization. We assume the generation of such photons in silicon nitride waveguides through the spontaneous four-wave mixing process. We show that manipulating the waveguide geometry and dielectric contrast makes it possible to obtain nearly wavelength-independent directional coupler designs that perform as 50:50 beamsplitters over a broad bandwidth around the central design wavelength. Assuming two independent but identical heralded single-photon sources, we compare the visibility obtained by simulating the Hong-Ou-Mandel interference for three different 50:50 beamsplitters: an ideal one (perfectly wavelength-independent), an engineered one (nearly wavelength-independent), and an unengineered one (significantly wavelength-dependent). The purity obtained with the engineered beamsplitter closely matches the purity obtained with the ideal one, for which the interference visibility is a direct measure of the quantum state purity. This finding implies that the proposed engineered beamsplitter is suitable for characterizing heralded single-photon states, provided its bandwidth is 23 nm.
Heralded single photon sources generated through spontaneous four wave mixing in silicon microring resonators are highly demanding for large-scale integrated quantum photonics. However, the spectral purity of conventional microring resonators is limited to 93%, which impacts the quality of quantum interference within photonic circuits. The wavelength dependent coupling between ring and bus waveguides plays a significant role in the generation/heralding of single photons from the resonator. In this letter, we have presented a detailed investigation of the dispersive nature of relatively long directional couplers in order to design a microring resonator in all-pass configuration for chip-scale generation of single photons with maximum purity. In our studies, we have shown that a photon purity level close to 99% (λ ∼ 1550 nm) can be achieved by the use of a dispersive directional coupler in a race-track microring resonator in all-pass configuration, without affecting the photon heralding rate. The design parameters of the device are optimized such that it can be fabricated in commercial CMOS foundries.
Integrated silicon nitride photonics has emerged as a key enabling technology for quantum technologies. Among the essential devices, beam splitters in the form of integrated directional couplers are widely used in circuits such as Hong-Ou-Mandel (HOM) interferometry. However, the wavelength-dependent nature of these couplers limits their use in experiments involving broadband photons. In this work, we experimentally demonstrate a wavelength-flattened directional coupler on the silicon nitride platform, exhibiting a nearly wavelength-flattened splitting ratio over the measured telecom wavelength range. This integrated photonic circuit is compatible with broadband quantum photonic circuits, including HOM interferometry, and supports the development of integrated single- and photon-pair sources.
Heralded single photon sources generated through spontaneous four wave mixing in silicon microring resonators are highly demanding for large-scale integrated quantum photonics. However, the spectral purity of conventional microring resonators is limited to 93%, which impacts the quality of quantum interference within photonic circuits. The wavelength dependent coupling between ring and bus waveguides plays a significant role in the generation/heralding of single photons from the resonator. In this letter, we have presented a detailed investigation of the dispersive nature of relatively long directional couplers in order to design a microring resonator in all-pass configuration for chip-scale generation of single photons with maximum purity. In our studies, we have shown that a photon purity level close to 99% (λ ∼ 1550 nm) can be achieved by the use of a dispersive directional coupler in a race-track microring resonator in all-pass configuration, without affecting the photon heralding rate. The design parameters of the device are optimized such that it can be fabricated in commercial CMOS foundries.
We demonstrate theoretically the direct (filter-free) generation of spectrally uncorrelated photon pairs, and consequently high-purity heralded single photons at telecommunication wavelengths, in SOI (silicon-on-insulator) nanowaveguides via spontaneous four-wave mixing (SFWM). The inherent strong waveguide dispersion in such high-index contrast structures is employed for discrete-band phase-matching in order to suppress the pair spectral correlations, and to produce photons in the fundamental spatial mode for easy in-out coupling to single-mode optical fibers. Additionally, by optimizing the source parameters like waveguide length, dispersion and pump bandwidth, the group-velocity of generated photons is also engineered to further eliminate spectral correlations. Extensive numerical mode simulations for various waveguide widths are used to design intrinsically pure photon sources via joint spectral amplitude (JSA) engineering. Purity$>9595% is shown to be achievable in a silica-clad SOI waveguide with a cross-section 220 × 600 nm95^{2}fortypicalvaluesoflengthandpumpbandwidth.Thewavelengthoftheseheraldedsinglephotonscanbetunedintherangeof1266–1363nm(for typical values of length and pump bandwidth. The wavelength of these heralded single photons can be tuned in the range of 1266–1363 nm (fortypicalvaluesoflengthandpumpbandwidth.Thewavelengthoftheseheraldedsinglephotonscanbetunedintherangeof1266–1363nm(\sim$O-band) by varying the pump wavelength. Considering the high accuracy provided by silicon nanofabrication techniques, the feasibility of generating single photons across the wavelength range 1333–1425 nm (E-band) and 1437–1510 nm (S-band) for slightly different waveguide widths (625 nm & 650 nm) is also demonstrated. These filter-free integrated photon sources could be important for large-scale linear optical quantum computation (LOQC) and long-distance fiber-optic quantum key distribution (QKD) networks.
While integrated photonics is a robust platform for quantum information processing, architectures for photonic quantum computing place stringent demands on high quality information carriers. Sources of single photons that are highly indistinguishable and pure, that are either near-deterministic or heralded with high efficiency, and that are suitable for mass-manufacture, have been elusive. Here, we demonstrate on-chip photon sources that simultaneously meet each of these requirements. Our photon sources are fabricated in silicon using mature processes, and exploit a dual-mode pump-delayed excitation scheme to engineer the emission of spectrally pure photon pairs through inter-modal spontaneous four-wave mixing in low-loss spiralled multi-mode waveguides. We simultaneously measure a spectral purity of 0.9904 ± 0.0006, a mutual indistinguishability of 0.987 ± 0.002, and >90% intrinsic heralding efficiency. We measure on-chip quantum interference with a visibility of 0.96 ± 0.02 between heralded photons from different sources.
We propose integrated photonic sources based on spontaneous four-wave mixing (SFWM) in dispersion engineered silicon nitride (SiN) nanowaveguides capable of generating spectrally-pure polarization-entangled photon-pairs bridging the visible-NIR and telecom bands. A simple silica-clad SiN waveguide design with height 640 nm and width in the range 860–960 nm can generate polarization-entangled photon pairs that simultaneously offers high heralded single photon purity of$>9999% and concurrence upto 0.99, with signal and idler lying in the range99\sim736–740nmand736–740 nm and736–740nmand\sim1460–1560nm,respectively,employingapulsedpump(BW1460–1560 nm, respectively, employing a pulsed pump (BW1460–1560nm,respectively,employingapulsedpump(BW\gtrsim0.25–3THz)ofwavelengthnear0.25–3 THz) of wavelength near0.25–3THz)ofwavelengthnear\sim1−1-1−\mum.Slightlydifferentwaveguideheightof620nmandcorrespondingwaveguidewidthsintherange1040–1180nm,offeringthesamepurityandconcurrencestandards,canalsoaddressvariousquantummemoriesdemonstratedatthevisible−NIRwavelengthsofm. Slightly different waveguide height of 620 nm and corresponding waveguide widths in the range 1040–1180 nm, offering the same purity and concurrence standards, can also address various quantum memories demonstrated at the visible-NIR wavelengths ofm.Slightlydifferentwaveguideheightof620nmandcorrespondingwaveguidewidthsintherange1040–1180nm,offeringthesamepurityandconcurrencestandards,canalsoaddressvariousquantummemoriesdemonstratedatthevisible−NIRwavelengthsof\sim$775–800 nm with a telecom band interface. Such CMOS-compatible on-chip sources of spectrally-pure polarization-entangled photon pairs can potentially be employed in a scalable manner for various optical quantum technologies.