Hybrid Quantum Encryption with an Ultrafast rGO/Ni All-Optical Logic Gate

We report the development of a Mach–Zehnder interferometer (MZI) based XOR all-optical modulator prototype employing ultrafast photonic structures fabricated with nickel-doped reduced graphene oxide (rGO/Ni). This two-dimensional material exhibits high electron mobility, ultrafast carrier dynamics, and a tunable refractive index, enabling strong third-order nonlinear optical responses desirable for quantum communication and encryption. The rGO/Ni monolayers were synthesized via the Langmuir–Blodgett technique, and their nonlinear optical properties were systematically characterized using a modified Z-scan method incorporating thermal loading modulation. This approach allowed direct separation of electronic and thermal contributions to the nonlinear refractive index. Experiments using a Ti:Sapphire femtosecond laser (800 nm, 76 MHz) revealed pronounced nonlinear behavior dominated by saturable absorption. The rGO/Ni films exhibited a positive electronic nonlinear refractive index (n₂ ≈ +16.15 cm²/GW), substantially higher than that of pristine rGO, alongside a strong negative thermal component (n₂ ≈ –68.51 cm²/GW). Since the electronic ��2 is positive and the thermal ��2 is negative, it is particularly attractive to highlight that the effective nonlinear response can be tuned by controlling the intensity and timescale of the excitation, allowing the material to transition between a self-focusing and a self-defocusing regime. This property introduces an additional degree of freedom for designing highly adaptive optical modulators, allowing for tuning the interferometric phase without resorting to external architectures or additional electric fields. These results demonstrate that nickel doping effectively tunes the interplay between electronic and thermal nonlinearities in reduced graphene oxide, providing a pathway to optimize its performance in ultrafast photonic modulators. Integration of rGO/Ni into an MZI XOR platform presents a promising approach for hybrid quantum encryption, combining classical logic-level control with quantum coherence preservation.