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Journal of Lightwave Technology

Bragg-Reflection Waveguides as Practical Photon-Pair Sources for Quantum Rangefinding

Bianca Nardi, Hannah Thiel, Stefan Frick, Gregor Weihs

Published May 12, 2025• 1 citation
Publisher

Abstract

Photon pairs generated via spontaneous parametric down-conversion (SPDC) are a crucial resource in the development of quantum protocols and quantum technologies, due to the strong correlation between two photons of the same pair. Efficient pair generation is necessary for protocols that require high signal-to-noise ratios when embedded in a lossy and noisy environment. Nonlinear waveguides are well-established platforms for photon pair generation. However, scattering and material absorption can inhibit the efficiency of the pair generation and require high pump powers. Here, we present an aluminum gallium arsenide Bragg-reflection waveguide, fabricated following an optimized recipe, that shows a record brightness compared to other samples with the same structure. When pumped with a sub-mW continuous wave laser, our waveguide generates photon pairs at a rate of$340\,\rm {kHz}\,\cdot \, mW^{-1}$, and a spectral width of$100\,\rm {nm}$in the telecom band. As a direct application, we investigate the operation of our waveguide in quantum rangefinding, a quantum protocol that employs time correlations between photon pairs to measure objects' distances, while camouflaging within background. This protocol was implemented with nonlinear crystals engineered for broadband emission. Supported by simulations of signal-to-noise ratio, we introduce our low-loss and broadband waveguide as a novel platform for telecom quantum rangefinding, robust against loss and noise.

DOI
10.1109/jlt.2025.3569311

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Photon pairs generated via spontaneous parametric down-conversion (SPDC) are a crucial resource in the development of quantum protocols and quantum technologies, due to the strong correlation between two photons of the same pair. Efficient pair generation is necessary for protocols that require high signal-to-noise ratios when embedded in a lossy and noisy environment. Nonlinear waveguides are well-established platforms for photon pair generation. However, scattering and material absorption can inhibit the efficiency of the pair generation and require high pump powers. Here, we present an aluminum gallium arsenide Bragg-reflection waveguide, fabricated following an optimized recipe, that shows a record brightness compared to other samples with the same structure. When pumped with a sub-mW continuous wave laser, our waveguide generates photon pairs at a rate of340 kHz ⋅ mW−1340\,\rm {kHz}\,\cdot \, mW^{-1}340kHz⋅mW−1, and a spectral width of100 nm100\,\rm {nm}100nmin the telecom band. As a direct application, we investigate the operation of our waveguide in quantum rangefinding, a quantum protocol that employs time correlations between photon pairs to measure objects' distances, while camouflaging within background. This protocol was implemented with nonlinear crystals engineered for broadband emission. Supported by simulations of signal-to-noise ratio, we introduce our low-loss and broadband waveguide as a novel platform for telecom quantum rangefinding, robust against loss and noise.
We design and fabricate shallow-etched periodically poled lithium niobate waveguides to realize highly efficient broadband spontaneous parametric down-conversion (SPDC) on nanophotonic chips. The shallow-etched waveguide can tolerate the non-uniformities of waveguide width induced by fabrication imperfections, enabling the generation of photon pairs with high count rate and bandwidth. We demonstrate photon-pair generation with a high brightness of 11.7 GHz/mW and bandwidth of 22 THz in a 5.7-mm-long PPLN waveguide. The generated photon pairs exhibit a strong temporal correlation with a coincidence-to-accidental ratio of up to 16262±850. Our results confirm the feasibility of shallow etching in the fabrication of an efficient SPDC device on the platform of lithium niobate on an insulator, and benefit quantum information processing with a broadband photon source.
Entangled photon pairs are an important resource for quantum cryptography schemes that go beyond point-to-point communication. Semiconductor Bragg-reflection waveguides are a promising photon-pair source due to mature fabrication, integrability, large transparency window in the telecom wavelength range, integration capabilities for electro-optical devices as well as a high second-order nonlinear coefficient. To increase performance we improved the fabrication of Bragg-reflection waveguides by employing fixed-beam-moving-stage optical lithography, low pressure and low chlorine concentration etching, and resist reflow. The reduction in sidewall roughness yields a low optical loss coefficient for telecom wavelength light of alpha_reflow = 0.08(6)mm^(-1). Owing to the decreased losses, we achieved a photon pair production rate of 8800(300)(mW*s*mm)^(-1) which is 15-fold higher than in previous samples.
We demonstrate a spectrally correlated photon-pair source at telecom wavelengths (spanning across the S-, C-, and L-bands), based on type-0 spontaneous parametric downconversion (SPDC) in a fiber-coupled Zn-indiffused MgO doped periodically poled lithium niobate (PPLN) ridge waveguide. Modal analysis of the waveguide performed through numerical finite element method (FEM) simulation indicates that device temperature can be used to dramatically vary and control the emission spectrum. Efficient photon-pair generation is measured over a broad wavelength range from ∼1520 − 1580 nm [full width at half maximum (FWHM) > 45 nm] with a coincidence-to-accidental ratio (CAR) as high as ∼668 and spectral brightness ∼2.5 × 10 7 pairs/s/mW/nm. Such sources can be employed in wavelength division multiplexed (WDM) quantum key distribution (QKD) over existing fiber-optic networks.
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Joint spectral characterization of SPDC photon pairs near 2 μμμm in (Al)GaAs-on-insulator waveguides
Integrated photon-pair sources are a core component of chip-based quantum computing, communication, and metrology. Although such sources have been demonstrated at conventional telecom wavelengths, the 2 μμμm band remains comparatively less explored, despite offering advantages for free-space quantum communication, low-loss transmission in emerging fiber networks, and integration with silicon photonic platforms. In this paper, we demonstrate spontaneous parametric down-conversion (SPDC) in straight GaAs- and AlGaAs-on-insulator waveguides. This platform offers strong second-order nonlinearity and geometry-tunable dispersion, which are advantageous for efficient on-chip pair generation. Measurements of the joint spectral intensity and heralded second-order correlation function show broadband emission around 2 μμμm with strong spectral anti-correlations. To our knowledge, this is the first direct joint-spectral characterization of an integrated SPDC source in this wavelength regime.
Bragg-reflection waveguides (BRWs) fabricated from AlGaAs provide an interesting nonlinear optical platform for photon-pair generation via parametric down-conversion (PDC). In contrast to many conventional PDC sources, BRWs are made of high refractive index materials and their characteristics are very sensitive to the underlying layer structure. First, we show that the design parameters like the phase matching wavelength and the group refractive indices of the interacting modes can be reliably controlled even in the presence of fabrication tolerances. We then investigate how these characteristics can be taken advantage of when designing quantum photonic applications with BRWs. We especially concentrate on achieving a small differential group delay between the generated photons of a pair and then explore the performance of our design when realizing a Hong–Ou–Mandel interference experiment or generating spectrally multi-band polarization entangled states. Our results show that the versatility provided by engineering the dispersion in BRWs is important for employing them in different quantum optics tasks.