High-Fidelity Photon Pair Entanglement in GaAs Photonic Crystal Nanocavities via Spontaneous Parametric Downconversion

This study investigates photon pair entanglement generation in GaAs photonic crystal nanocavity resonators via spontaneous parametric downconversion within the context of quantum optics and nanophotonic device physics, an area of growing scientific importance given its implications for integrated quantum photonic circuits for quantum key distribution and quantum computing interconnects. Using time-correlated single-photon counting with Hong-Ou-Mandel interference and quantum state tomography for entanglement fidelity characterization, we examine second-order chi(2) nonlinear optical interaction in GaAs generating correlated signal-idler photon pairs within the cavity mode volume in 8 photonic crystal cavity designs (4 nanobeam, 4 L3) with Q-factor range 10^4 to 10^6 drawn from dilution refrigerator cryogenic environment at 4K under continuous-wave pump laser excitation. Results indicate that optimized L3 cavity with Q = 4.8 x 10^5 and mode volume 0.7 (lambda/n)^3 achieves photon pair entanglement fidelity of 0.942 with two-photon interference visibility of 94.8% (p < 0.001), with 0.942 entanglement fidelity as the primary quantitative benchmark. Concordance between primary and confirmatory measurement approaches exceeded 93%, validating the analytical framework. These findings contribute empirically to quantum optics and nanophotonic device physics and carry actionable implications for the design of programs and policies targeting integrated quantum photonic circuits for quantum key distribution and quantum computing interconnects.