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<p>Spontaneous parametric down-conversion (SPDC) is a fundamental nonlinear optical mechanism that generates entangled photon pairs, serving as a primary source of entangled quantum states in quantum optics. Silica tapered optical fibers (TOFs), despite their weaker effective second-order nonlinearity compared with non-centrosymmetric crystals, offer a compact, low-loss, all-fiber SPDC platform due to strong optical confinement and long interaction lengths. Their effective second-order response originates from surface dipole and bulk multipole contributions induced by subwavelength symmetry breaking. In this paper, we conduct a theoretical analysis of a fully adiabatic silica TOF optimized for efficient generation of photon pairs forming entangled quantum states via SPDC, including both nonlinear contributions. The effective second-order nonlinear susceptibility can be further enhanced via functionalization with nonlinear molecular layers deposited solely on the nanofiber waist, significantly improving entangled quantum state generation without affecting taper propagation. Adiabatic control of light propagation through the two taper transitions is shown to be essential for maximizing generation efficiency. An adiabaticity criterion is introduced to establish an upper limit on the taper slope to ensure lossless coupling between the untapered fiber and the nanofiber section. The input taper is designed to remain adiabatic for the pump mode (HE<sub>21</sub> or TM<InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(_{01}\)</EquationSource> </InlineEquation>), while the output taper is optimized for efficient extraction of photon pairs propagating in the fundamental HE<InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(_{11}\)</EquationSource> </InlineEquation> mode, forming entangled quantum states. This study provides a complete theoretical model and optimized design for adiabatic tapered fibers, enabling efficient entangled quantum states generation and contributing to the development of high-performance, fully integrated all-fiber quantum-state sources for quantum communication, including quantum cryptography.</p>

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Adiabatic tapered silica optical nanofibers: optimized design for entangled quantum states generation

  • 期刊论文

  • 发表日期：

    2026年6月22日

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  • Abderrahim Azzoune,
  • Osama Mahfoudia,
  • Anis Abdennour Metidji,
  • Hocine Medjadba,
  • Hamza Gouasmia,
  • Oussama Laouedj

摘要

Spontaneous parametric down-conversion (SPDC) is a fundamental nonlinear optical mechanism that generates entangled photon pairs, serving as a primary source of entangled quantum states in quantum optics. Silica tapered optical fibers (TOFs), despite their weaker effective second-order nonlinearity compared with non-centrosymmetric crystals, offer a compact, low-loss, all-fiber SPDC platform due to strong optical confinement and long interaction lengths. Their effective second-order response originates from surface dipole and bulk multipole contributions induced by subwavelength symmetry breaking. In this paper, we conduct a theoretical analysis of a fully adiabatic silica TOF optimized for efficient generation of photon pairs forming entangled quantum states via SPDC, including both nonlinear contributions. The effective second-order nonlinear susceptibility can be further enhanced via functionalization with nonlinear molecular layers deposited solely on the nanofiber waist, significantly improving entangled quantum state generation without affecting taper propagation. Adiabatic control of light propagation through the two taper transitions is shown to be essential for maximizing generation efficiency. An adiabaticity criterion is introduced to establish an upper limit on the taper slope to ensure lossless coupling between the untapered fiber and the nanofiber section. The input taper is designed to remain adiabatic for the pump mode (HE21 or TM \(_{01}\) ), while the output taper is optimized for efficient extraction of photon pairs propagating in the fundamental HE \(_{11}\) mode, forming entangled quantum states. This study provides a complete theoretical model and optimized design for adiabatic tapered fibers, enabling efficient entangled quantum states generation and contributing to the development of high-performance, fully integrated all-fiber quantum-state sources for quantum communication, including quantum cryptography.