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arXiv:2506.17819 · Optics Express

Ultra-broadband spectral and polarisation entanglement using dispersion-engineered nanophotonic waveguides

Mahmoud Almassri, Mohammed F. Saleh

Published Jun 25, 2025• 1 citation
arXivHTMLPDF

Abstract

In this paper, we propose exploiting dispersion-engineered nanophotonic waveguides in generating unprecedented ultra-broadband spectral and polarisation entanglement using spontaneous four-wave mixing parametric processes. We developed a simplified theoretical quantum framework to investigate and analyze these interactions under pulse source excitations. Using aluminum gallium arsenide and thin-film lithium niobate waveguides, we anticipate obtaining photon pairs with high-dimensional frequency entanglement, characterized by Schmidt numbers as large as 10 8 , and covering the wavelength range 940–2730 nm (equivalent to a 210 THz bandwidth). Additionally, we show that Al 0.3 Ga 0.7 As waveguides with hybrid cladding can enable the generation of polarisation-entangled photon pairs with concurrence exceeding 0.93 across wavelengths from 1175 nm to 1750 nm, spanning almost across all the telecommunication bands with only approximately 27 nm window inevitable-degradation around the pump wavelength. We envisage that these introduced integrated on-chip sources will significantly advance quantum photonic technologies, enabling breakthroughs in multi-channel quantum networking and scalable quantum information systems.

DOI
10.1364/oe.562783

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In this paper, we propose exploiting dispersion-engineered nanophotonic waveguides in generating unprecedented ultra-broadband spectral and polarisation entanglement using spontaneous four-wave mixing parametric processes. We developed a simplified theoretical quantum framework to investigate and analyze these interactions under pulse source excitations. Using aluminum gallium arsenide and thin-film lithium niobate waveguides, we anticipate obtaining photon pairs with high-dimensional frequency entanglement, characterized by Schmidt numbers as large as 10 8 , and covering the wavelength range 940–2730 nm (equivalent to a 210 THz bandwidth). Additionally, we show that Al 0.3 Ga 0.7 As waveguides with hybrid cladding can enable the generation of polarisation-entangled photon pairs with concurrence exceeding 0.93 across wavelengths from 1175 nm to 1750 nm, spanning almost across all the telecommunication bands with only approximately 27 nm window inevitable-degradation around the pump wavelength. We envisage that these introduced integrated on-chip sources will significantly advance quantum photonic technologies, enabling breakthroughs in multi-channel quantum networking and scalable quantum information systems.
Quantum networks based on wavelength-multiplexed entanglement enable parallel distribution of quantum correlations, increasing channel capacity for secure communication and distributed quantum information processing. However, broadband integrated sources capable of generating multipartite entanglement beyond photon pairs remain scarce. Here we report on-chip generation of telecom-band four-photon entanglement in a periodically poled thin-film lithium niobate on insulator (LNOI) waveguide. Type-0 spontaneous parametric down-conversion provides a phase-matching bandwidth exceeding 200 nm, enabling spectrally separable generation of multi-photon entanglement across the telecom band. The generated photons are encoded in time bins for robust fiber compatibility, and a coherent interface enabling reversible conversion between time-bin and polarization degrees of freedom allows complete quantum state tomography. We measure two-photon entanglement with a brightness of 6.7 MHz/mW/nm and a fidelity of 0.874±0.0020.874 \pm 0.0020.874±0.002. At a pump power of 0.08 mW, the four-photon state exhibits a fourfold coincidence rate of 1 Hz and a fidelity of 0.74±0.010.74 \pm 0.010.74±0.01, representing a threefold improvement over previous integrated platforms. Our results establish LNOI as a scalable platform for broadband multi-photon entanglement and provide a practical route toward dense wavelength-multiplexed quantum networks.
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.
Abstract Broadband photon pairs are highly desirable for quantum metrology, quantum sensing, and quantum communication. Such sources are usually designed through type-0 phase-matching spontaneous parametric down-conversion (SPDC) that makes the photon pairs hard to separate in the frequency-degenerate case and thus limits their applications. In this paper, we design a broadband frequency-degenerate telecom-band photon pair source via the type-II SPDC in a dispersion-engineered thin-film lithium niobate waveguide, where the polarization modes of photon pairs are orthogonal and thus are easily separated deterministically. With a 5-mm-long waveguide, our design can achieve a bandwidth of 5.56 THz (44.8 nm), which is 8.6 times larger than that of the bulk lithium niobate, and the central wavelength can be flexibly adjusted. Our design is a promising approach towards high-quality integrated photon sources and may have wide applications in photonic quantum technologies.
Correlated photon-pair sources are key components for quantum computing, networking, and sensing applications. Integrated photonics has enabled chip-scale sources using nonlinear processes, producing high-rate entanglement with sub-100 microwatt power at telecom wavelengths. Many quantum systems operate in the visible or near-infrared ranges, necessitating broadband visible-telecom entangled-pair sources for connecting remote systems via entanglement swapping and teleportation. This study evaluates broadband entanglement generation through spontaneous four-wave mixing in various nonlinear integrated photonic materials, including silicon nitride, lithium niobate, aluminum gallium arsenide, indium gallium phosphide, and gallium nitride. We demonstrate how geometric dispersion engineering facilitates phase-matching for each platform and reveals unexpected results, such as robust designs to fabrication variations and a Type-1 cross-polarized phase-matching condition for III-V materials that expands the operational bandwidth. With experimentally attainable parameters, integrated photonic microresonators with optimized designs can achieve pair generation rates greater than ~1 THz/mW2^22.
The development of quantum technologies on nanophotonic platforms has seen momentous progress in the past decade. Despite that, a demonstration of time-frequency entanglement over a broad spectral width is still lacking. Here we present an efficient source of ultrabroadband entangled photon pairs on a periodically poled lithium niobate nanophotonic waveguide. Employing dispersion engineering, we demonstrate a record-high 100 THz (1.2 μm-2 μm) generation bandwidth with a high efficiency of 13 GHz/mW and excellent noise performance with the coincidence-to-accidental ratio exceeding 10^{5}. We also measure strong time-frequency entanglement with over 98% two-photon interference visibility.