Air-Stable Bright Entangled Photon-Pair Source from Graphene-Encapsulated van der Waals Ferroelectric NbOI2

Area of Science:

• Nonlinear photonics
• Quantum optics
• Materials science

Background:

• Van der Waals (vdW) ferroelectrics offer large second-order nonlinear susceptibility (χ(2)) for miniaturized spontaneous parametric down-conversion (SPDC) sources.
• Practical vdW SPDC is limited by low brightness and poor air stability under continuous irradiation, leading to material degradation.

Purpose of the Study:

• To demonstrate a bright and air-stable vdW ferroelectric SPDC source.
• To enhance the performance and stability of NbOI2 for integrated quantum photonic applications.

Main Methods:

• Utilized graphene encapsulation to protect ferroelectric NbOI2 from ambient exposure and improve heat dissipation.
• Investigated the performance of graphene-encapsulated NbOI2 for SPDC, including photon-pair generation rate and brightness.
• Generated polarization-entangled photon pairs from a 90°-twisted bilayer NbOI2 structure.

Main Results:

• Achieved an absolute photon-pair generation rate of 258 Hz and a normalized brightness of 19,900 Hz mW⁻¹ mm⁻¹.
• Demonstrated robust environmental protection and suppression of pump-induced degradation via graphene encapsulation.
• Generated polarization-entangled photon pairs with 94% fidelity to a maximally entangled Bell state.

Conclusions:

• Graphene-encapsulated NbOI2 provides a practical and stable vdW ferroelectric platform for SPDC.
• This approach paves the way for advanced integrated quantum photonic sources.
• The enhanced stability and brightness are crucial for real-world quantum technology applications.